NCAR/TN-301+STRNCAR TECHNICAL NOTE

March 1988

INTERCOMPARISON OF

NMC AND ECMWF GLOBAL ANALYSES:1980-1986

KEVIN E. TRENBERTH

JERRY G. OLSON

.D

E

L,

an

LatitudeJUL 86

CLIMATE AND GLOBAL DYNAMICS DIVISION

NATIONAL CENTER FOR ATMOSPHERIC RESEARCHBOULDER, COLORADO

-

II-

Table of ContentsPreface . . . . . . . . . . . . . . . . . . .

Acknowledgments ...............

1. Introduction . . .. . . . .. . . . . .. .

2. The data sets ...........

3. The intercomparison of data sets .......

3.1 Time series of zonal mean differences . . .

3.2 Meridional cross sections ... ...

3.3 Geographical distributions .......

3.4 Poleward eddy heat and momentum fluxes

3.5 Surface differences over the oceans . ...

4. A case study in June 1985 .........

4.1 The South Pacific low .........

4.2 The South Atlantic low .........

4.3 The Antarctic Peninsula ........

5. Discussion and conclusions ..........

5.1 Divergent wind ............

5.2 Relative humidity ..........

5.3 Surface fields . ...........

5.4 The Southern Hemisphere .....

5.5 Heat and momentum fluxes ......

5.6 Concluding remarks . ........

References . . . . . . . . . .. . . . . . . .

Appendix I Acronyms .............

v

. .. . . . . V1·. .. . . . 1·

... . . . . 4

. 5

.· · · · · · 6

... . . . .20

...... .32

.... . . . . 48

....... 58

....... 68

....... 68

....... 72

....... 72

....... 73

....... 73

....... 74

....... 75

....... 76

....... 77

....... 77

... . . . . 79

. . . . . . . 81

111

Preface

This report is the third in a series of three as part of a project to comprehensively

evaluate data sets of global analyses from the U.S. National Meteorological Center (NMC)

and the European Centre for Medium Range Weather Forecasts (ECMWF). Our partic-

ular purpose is to establish the climate record from global analyses for use in compiling

climatologies which may then be used to validate the National Center for Atmospheric

Research (NCAR) Community Climate Model or as a base for examining anomalies such

as occur in association with El Ninio-Southern Oscillation events.

In order to make optimal use of these data sets, it is necessary to know of any

problems that might exist and the effects of any changes in the operational system that

produced the analyses. These issues were extensively documented in the first two Technical

Notes which separately evaluated the NMC and ECMWF data sets of global analyses. This

third Note describes results from a detailed intercomparison of the two data sets and a

case study which compares the analyses with each other and with observations. In this

way we provide a measure of the accuracy of individual and mean analyses and the relative

biases in the two data sets, and a documentation of how these have changed with time. It

therefore provides a measure of how much confidence should be placed in the fields and,

in particular, in resulting general circulation statistics.

The main results shown are monthly mean and root mean square (RMS) differences

between the analyses. Geopotential heights or sea level pressures, wind components, and

relative humidity are the main state variables examined, but we also consider the eddy

poleward heat and momentum fluxes. Both the mean and RMS differences have been

zonally averaged as a function of latitude and plotted as time series from 1980 to 1986 at

single levels and as cross sections. The differences have also been mapped geographically

for each month. At the surface, separate fields are presented for the oceans as a whole,

and for each ocean separately, to show how well the fields important for programs such

as the World Ocean Circulation Experiment (WOCE) and the Tropical Oceans Global

Atmosphere (TOGA) Program are known.

v

Results show fairly widespread agreement between the analyses from the two centers

over the NH extratropics. However, in general, the quality of the analyses, as revealed by

their differences, is much less in the tropics and SH. This is reflected in much greater

differences in wind fields south of 20 0 N, with RMS differences in both components often

exceeding 5 m s - ' above ^500 mb throughout most of this region. It is further revealed

in the much greater differences in geopotential height south of .30°S. In part, this is due

to the poorer traditional observational synoptic network in the SH. But it also appears, at

least in part, to be due to problems in assimilating observations that are available. Both the

mean and RMS differences over and around Antarctica reveal major problems in that area

at both centers, and especially at NMC prior to May 1986. In the tropics, there are major

disagreements in the divergent wind field and associated vertical motions. These fields

have become more intense and realistic with time but still appear to be poorly known. The

relative humidity field, however, is the poorest known and has undergone major changes

with time at both centers. The possible reasons for these results are discussed and some

implications and recommendations are given in the conclusions section.

Acknowledgments

This research was partially supported by the Tropical Oceans Global Atmosphere

Project Office under grant NA86AANRG0100. We thank Grant Branstator for helpful

comments.

vi

1. Introduction

There is a compelling need for increasingly accurate knowledge about the state of

the atmosphere, both at any instant, for weather forecast-type studies, and for longer-term

means. A reliable climatology of the general circulation of the atmosphere is required for

many purposes, ranging from everyday use of mean conditions for planning air transporta-

tion routes to the validation of climate models. However, accurate means on shorter,

say monthly, time scales are also essential along with the basic climatology in order to

monitor ubiquitous climate variations and determine anomalies in the circulation, such as

those associated with the El Niniio-Southern Oscillation phenomenon. The latter can cause

widespread devastation through excessive rains in some parts of the world while droughts

pervade other parts with pronounced economic and political consequences.

One goal of our current research is to determine climatologies of the global atmo-

spheric circulation and establish how' reliable they are and their suitability for various

uses. A second goal is to examine the extent to which the monthly anomalies, defined

as departures from the mean climatology, can be reliably determined. We are interested

in not only the mean state variables of the atmosphere, but also the second moments,

such as variances, and covariances that determine the fluxes of quantities of interest. In

order to address these goals we have comprehensively evaluated two sets of global daily

analyses from the U.S. National Meteorological Center (NMC) and the European Centre

for Medium Range Weather Forecasts (ECMWF) for the period 1979-1986. This is the

third of a three-part project. In Trenberth and Olson (1988a) we reported on the NMC

data set and in Trenberth and Olson (1988b) the ECMWF analyses were examined.

At both centers, the analyses are produced using a four-dimensional data assimila-

tion system that uses a set of first guess fields as the base for integrating the observations

into an analysis. At NMC the analyses are produced under what is referred to as the

Global Data Assimilation System (GDAS). The first guess at both centers is either a 6

or 12 hour forecast using a numerical weather prediction (NWP) model from a previous

analysis and it therefore carries the information from all the previous analyses foreward

1

in time. However, it also means that the first guess is dependent upon the veracity of

the NWP model and it can be biased as a consequence. The subsequent analysis proce-

dure itself is based upon some sort of statistically optimum interpolation scheme which

makes use of the statistical mean errors expected in both the first guess and in the data.

Then the results are initialized using nonlinear normal mode initialization (NNMI) which

is a procedure designed, using the normal modes of the model equations, to ensure that

the fields are dynamically consistent with each other while appropriately emphasizing the

relatively slow meteorologically significant components and damping the spurious gravity

waves. Such a procedure greatly influences the vertical motion field and associated diver-

gent wind components, with the result that the latter have undergone spurious changes

with time as the initialization procedures have been improved.

In Trenberth and Olson (1988a), which dealt with the NMC data set, and in Tren-

berth and Olson (1988b), which considered the ECMWF analyses, we primarily focused

on evaluating each data set alone. We compiled a detailed chronology of the changes in

the operational systems at each center that produced the analyses and assessed the impact

of each change on the analyses. We documented any missing data and, through a series

of tests for the internal consistency of the data set, mainly through examination of daily

time series of various statistics, we compiled lists of "bad" analyses. These are days when

something clearly went wrong in the operational analysis/forecast cycle and/or erroneous

observations greatly impacted the analyses. Such analyses generally should not be used, in

compilations of climatological statistics, in forecast experiments, or for any other purpose.

Results of these studies showed that missing data was a problem in the NMC set

with more than 13% of the analyses missing in 1982 and 1983. Relatively few (only four

from 1980-86) analyses are missing in the ECMWF set. In addition, bad analyses are more

common in the NMC set and we classified an additional 6.6% of the analyses in 1984, for

instance, as bad. At ECMWF bad analyses occur only in the earlier years.

We have also shown that many of the changes in the operational systems used

to produce the analyses at both centers had a substantial impact on the analyses and

resulted in either spurious trends or discontinuities in several quantities. The most sensitive

2

quantities are the divergent wind component and associated vertical motion fields, and the

moisture fields.

In this third part of the project, we attempt to obtain a measure of the accuracy of

the analyses and the relative biases in the two data sets by systematically comparing the

NMC and ECMWF global fields. We will focus on the differences between the analyses and,

accordingly, this does not provide an absolute measure of accuracy. It is also important to

recognize that a decrease in the differences between the analyses does not necessarily mean

improved accuracy. This is because changes that improve NWP model forecast performance

at one center are likely to be adopted at the other center with resulting parallel changes

in the analyses. In particular, the relative humidity (RH) fields are evidently dominated

by input from the NWP model at the expense of observations, so that values are apt to

change whenever the model is altered.

The interpretation of the differences is complicated by the spurious discontinuities

due to changes in the two analysis systems, but even such a relative measure of how well

we really know the state of the atmosphere at any time is a useful guide on how much

faith to put in the resulting climatologies. Further, it shows where more effort is needed

in enhancing the observations and in improving data analysis-assimilation methods.

It is possible to attempt to define a more fundamental measure of accuracy by

comparing the analyses with observations. This has also been done to a limited extent but

it is necessary to recognize that station observations are also flawed. In particular, many

station records have data missing which can bias mean statistics (Trenberth, 1987a). A

not uncommon cutoff criterion for computing statistics has been to require a minimum of

10 observations per month. Kidson and Trenberth (1988) examined the effects of missing

data on estimates of monthly mean general circulation statistics and found, for the zonal

wind at 300 mb for example, that 11 randomly distributed observations in a month would

result in standard errors in the monthly mean wind of up to 4.1 m s-.

In this report we intercompare the NMC and ECMWF analyses at selected levels.

A previous comparison of the ECMWF analyses with those from the Geophysical Fluid

3

Dynamics Laboratory (GFDL) for parts of the year of the First GARP Global Experiment

(FGGE) has been reported by Lau (1984, 1985). We have therefore not included the FGGE

year (December 1978-November 1979) in this comparison, in part also because for that

year the two analyses are on different grids. The comparison focuses on the 1980-86 period

when both sets of analyses are available on a 2 ° grid.

The comparison was first done on a daily basis by differencing individual analyses

and producing time series of various summary statistics for each month. Such series were

helpful in identifying the bad days in the two previous studies. For this note, we have

summarized all the statistics into either monthly means or time series of 15-day means.

Horizontal maps and cross sections will be shown for individual months to illustrate the

complete fields associated with the time series difference fields. In addition we report on a

case study of one major difference in June 1985 in which both analyses are compared with

the original available data.

A list of acronyms used in this report is given in Appendix I.

2. The data sets

The data sets consist of twice daily, at 0000 and 1200 GMT, global analyses at

multiple levels of the fields of geopotential height z, temperature T, zonal and meridional

components of wind u and v, and relative humidity RH. Other parameters were not

considered. For the ECMWF data set there are only seven levels in the vertical at 1000,

850, 700, 500, 300, 200 and 100 mb, so that the intercomparison was limited to these levels.

For 1980-86 both data sets are available on the same 2 ° latitude-longitude grid and it

was on this grid that the fields were differenced.

Trenberth and Olson (1988a and b) describe many more details about each data

set including lists of the missing days, days determined to be bad, documentation of all

the changes in the analysis systems and the impacts of these changes on the analyses. We

have excluded the bad days from the intercomparison, except where noted in Section 3.5.

4

3. The intercomparison of data sets

Fields have been compared on a daily basis and results summarized in several ways.

For each month from 1980 to 1986 the mean and root mean square (RMS) temporal differ-

ences have been computed at each grid point. The mean difference shows the systematic

component which may arise through a bias in one or other analysis system. The RMS

differences include the mean differences but also include the time dependent uncertainties

in the individual analyses. Generally, we have produced horizontal maps at selected levels

in the vertical to reveal the geographic distribution of the differences and zonal mean cross

sections of the two differences as a means of concisely summarizing the results.

In addition to the monthly means, for each latitude we have computed zonal mean

RMS and mean differences averaged over non-overlapping 15-day periods. These have been

plotted as time series as a function of latitude, but in order to cut down on clutter, it was

found desirable to further smooth the results using a (1 - 2 - 1) binomial smoother, so

that the time series are more representative of running monthly means.

The main detailed comparison was between the following fields: z, u and v at 1000,

500 and 200 mb, and RH at 850, 700 and 300 mb. T is related to z through the hydrostatic

equation and was not compared directly. However, in order to gain an appreciation of the

impacts of differences in the primary state variables, time series were constructed of daily

values of [v*T*] and [u'v*], where the bracket refers to the zonal mean and the ( )*

the departure from the zonal mean. These quantities represent the poleward heat and

momentum fluxes by the eddies and are probably the most important second moment

general circulation statistics. Comparisons will be shown of [v*T*] at 700 mb, and [u*v*]

at 200 mb, both near the respective level at which the poleward flux of each quantity

reaches its maximum.

For some purposes, the more familiar sea level pressure field is preferred over the

1000 mb height. In particular, in order to examine how well the surface fields agree over

the oceans, we have converted the 1000 mb heights to sea level pressures making use of a

fairly accurate conversion factor that depends latitudinally on temperature. The oceanic

5

surface fields are of particular interest to scientists involved in the Tropical Oceans Global

Atmosphere (TOGA) program for assessing how well surface fluxes of sensible and latent

heat and momentum (wind stress) are determined. We have therefore compared the surface

pressure and wind fields over the oceans, both as a whole and individually.

For the most part, we do not present fields of the state variables themselves and

only show the differences. The mean statistics for each month of the year are given for the

ECMWF data in Trenberth and Olson (1988b).

3.1 Time series of zonal mean differences

In order to provide an overall measure of the differences between the analyses and

how they have changed with time, we first present a series of latitude-time sections. These

are 1(1 - 2 - 1) smoothed 15-day averages in time (roughly monthly means) and zonal

means of the differences as a function of latitude. For each 15-day period, values were set

to missing only if there were less than 7 out of 30 good analyses available and all missing

cases are due to lost NMC data. All differences are computed as NMC-ECMWF and

both the mean and RMS differences are presented for selected levels. On all plots negative

values are dashed and stippling has been used to highlight the larger differences. Periods

with bad or missing data have been left blank. Note that in most cases, the contour levels

are uneven and have been chosen to specifically show different thresholds of the differences.

When uneven contours are used, the complete list of all contours plotted is shown on the

side of each plot.

Several of the fields are related, most notably the geopotential heights and winds

through the geostrophic relation. Consequently, we will defer most discussion until after

the related fields have been presented. We will comment on the differences and, wherever

possible, suggest causes for the differences and attribute them to one or other center. The

background for this is in the extensive documentation of the changes made at each center

and the impacts these had on the analyses, as described by Trenberth and Olson (1988a,

1988b). Reference to both of these Technical Notes is implied in much of the following

discussion.

6

Figure 1 shows the 1000 mb z differences, the 200 mb z differences are given in Fig. 2

and the corresponding u differences in Figs. 3 and 4. The top panel in each figure shows

the zonal mean differences and the lower panel shows the zonal mean RMS differences.

At 1000 mb the mean z differences are mostly less than 5 m after 1980 north of 35°S

and these correspond to RMS z differences of 10-20 m and for u of less than 4 m s- 1. The

exceptions, as will be seen from the geographic distribution shown later in Figs. 19-22, are

mainly associated with orography (e.g., near 350 N in Fig. 1) and result from differences in

the way the two centers extrapolate below the ground. However, this does not account for

the large differences from 30-65°S although it is a factor in the enormous differences over

Antarctica in Figs. 1 and 3. There is some reduction in mean differences in Fig. 1 over

Antarctica after changes were made to the NMC GDAS system on 28 May 1986, but the

reduction in RMS differences elsewhere was small.

A major part of the huge differences over the Southern Hemisphere (SH) can be

traced to problems at NMC. Trenberth and Christy (1985) compared sea level pressures

from NMC with those from Australian analyses. NMC pressures were too high until late

1981 from 35-65°S, both relative to the Australian and ECMWF analyses (see Fig. 1). They

were corrected by introducing the Australian PAOBs (Pseudo Australian Observations)

and some other changes. More insight on the nature of the problems at both centers in

the SH is shown by the case study reported in Section 4.

At 200 mb (Figs. 2 and 4) problems in handling orography cannot be blamed for

the differences which remain huge over Antarctica and which are generally higher south

of 20°S than over the Northern Hemisphere (NH). There is an annual cycle to the RMS u

differences from 20-40°N in association with the wintertime development of the strong NH

subtropical jet. But in the tropics and SH RMS u differences exceed 5 m s - 1 year round

although with notable reduction in late 1985 and again in late 1986 (Fig. 4).

At NMC there were chronic problems with analyses over Antarctica until changes

were made in May 1986. The impact of that change is documented by Bonner et al. (1986)

who confirmed that the analyzed heights at NMC had been much too high over Antarctica.

7

[ZINM - [Z]EcMWF

SMOOTHEb

(M)

1000 MB

1979 1980 1981 1982 1983 1984 1985 1 o5

15-DAY AVERAGES

RMS DIFF. (ZNMC/ECMWF

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1000 MB

12.1 11.E

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· . ~ O.%~ ';:.: : - - ...

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. . . . . . .

...... . .......... i...

(

1979 1980 1981 1982 1983 1984 1985 1986

15-DAY AVERAGES

Fig. 1. Differences in z at 1000 mb as a function of latitude and smoothed 15-day

averages in time. The contour interval is uneven as given at left. Negative

contours are dashed. Top: [Z]NMC - IZ]ECMWF- Values greater than ±20

m are stippled. Bottom: RMS z differences. Values greater than 20 and 50

m are stippled in different densities.

8

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Fig. 2. Differences in z at 200 mb as a function of latitude and time.Top: [Z]NMC - [Z]ECMWF. Values greater than ±50 m are stippled.Bottom: RMS z differences. Values greater than 20 and 50 m are stippledin different densities.

9

[U]NMC - [U]ECMWF

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90N

60N

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1

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605

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(M/S)1000 MB

15-DAY AVERAGES

RMS DIFF. (UNMC/ECMWF

SMOOTHED

M/S)

1000 MB

15-DAY AVERAGES

Fig. 3. Differences in u at 1000 mb as a function of latitude and time.

Top: [u]NMC - [U]ECMWF. Values greater than ±2 m s- 1 are stippled.

Bottom: RMS u differences. Values greater than 4 m s-1 are stippled.

10

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1979 1980 198T'" 1 98

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_ . . . .I . .

Fig. 4. Differences in u at 200 mb as a function of latitude and time.

Top: [u]NMC - [U]]ECMWF Values greater than ±5 m s- 1 are stippled.

Bottom: RMS u differences. Values greater than 5 and 10 m s- 1 are stippled

in different densities.

11

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A comparison of both NMC (not shown) and ECMWF (see Trenberth and Olson, 1988b)

analyzed heights with station data at the South Pole (Amundsen-Scott) and McMurdo

Sound has been carried out for days when station data existed. For NMC at 200 mb,

monthly mean RMS differences between station and analyzed values averaged s50 m at

McMurdo and ~80 m at the South Pole during 1979. But from 1980 to 1982 NMC values

were usually biased high with RMS differences typically >100 m and at times >200 m.

From 1983 to May 1986 at the South Pole both mean and RMS differences increased to

>300 m in the winter months and resulted in a small polar high rather than the polar

vortex shown by the observations. As a result there was a spurious easterly component

to the zonal winds of >10 m s -1 near 80°S in the winter half year. After May 1986, the

NMC RMS differences at 200 mb from NMC diminished to -50 m at both stations.

Trenberth and Olson (1988b) present results of the comparison and discuss the

difficulties in Antarctic analyses at ECMWF. They note huge negative biases in z (<-200 m

at 200 mb) compared with station values over Antarctica in spring months of 1980 and

1981. There is a marked improvement in 1982 and a further gradual improvement with

time, so that by 1986 the monthly mean RMS differences are 40 to 50 m. The main

difficulty at both NMC and ECMWF is suspected to be communication problems with the

result that data are not received for input to the analyses, although this is compounded

by a total absence of data at times.

Figures 5 and 6 show the differences in the v field at 1000 and 200 mb. The RMS

values for v are dominated by the rotational component and consequently tend to be quite

similar to those for u (Figs. 3 and 4) with biggest uncertainties revealed over the SH.

However the zonal mean v, given by [v], is zero for the rotational component and thus the

top panels of Figs. 5 and 6 show the differences in the lower and upper branches of the

mean meridional circulation corresponding entirely to the divergent wind component. In

the tropics these reveal the Hadley circulation differences as seen in the initialized fields

from the two centers. Trenberth and Olson (1988a and 1988b) noted that this quantity

was extremely sensitive to changes in the model and data assimilation and there were

major discontinuities in the records at both centers. Fig. 7, adapted from Trenberth and

12

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Fig. 5. Differences in v at 1000 mb as a function of latitude and time.Top: [V]NMC - [V]ECMWF. Values greater than ±1 m s - 1 are stippled.Bottom: RMS v differences. Values greater than 4 m s - 1 are stippled.

13

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1985T - -I I 198

M/S)

200 MB

1979 1980 1981 I l8z 1o3 i 9o4 1io I ou

15-DAY AVERAGES

Fig. 6. Differences in v at 200 mb as a function of latitude and time.

Top: [V]NMC - [V]ECMWF. Values greater than ±1 m s- 1 are stippled.

Bottom: RMS v differences. Values greater than 5 and 10 m s-1 are stippled

in different densities.

14

CONTOURS:-5.0-2.0

-0.5 HL t

0 -. 429 .0'701.02.0

::::~:.::' ,,: . ::::: :::::.::::~:;~:.:,'' 1 C·'''',::::..:', .,

· : ... ,-*. ,,~:, 'i'I (ft1d';

'^'ry ^'..(M I\^\ ^*>\u\~

oi··Iiiii . .S-i r : ~ o `

6- "~~~~~~~~~~~~-l

I I I~~~~~~~Jl'rL( ,X ,4Wb51 ,+ j * & %4 -

VW ·-I , to - :61 Y 9 · CC4 to .no r - I WC , I 6&oLjkg--

QIaT"s&6;

.I,,..0-

(t:

1986

3

z

0tC

_

2

0

-1

-2

-3

REGIONAL MEAN OF v

15-DAY PERIODS1 JRN 79 OZ - 19 DEC 86 12Z

Fig. 7. Time series of 15-day averages of regional means of [v] averaged over 20°Nto 20°S at 200 mb. The NMC values are in solid and the ECMWF valuesare dashed. Values are in m s- 1.

15

(M/S) 200 MB

Olson (1988a and 1988b), shows the temporal changes for the 200 mb [v] averaged from

20°N to 20°S and thus depicts the upper branch of the main winter hemisphere Hadley

cell. Changes in [v] were especially pronounced at ECMWF, and this is also reflected in

Figs. 15 and 16, presented later.

Differences in the upper branch of the Hadley cell are best illustrated by Fig. 6 and

become more extensive with time as the meridional circulations were increased at both

centers with the introduction of diabatic effects into the NNMI. They became largest after

1 May 1985 when ECMWF invoked other improvements associated with the introduction

of shallow convection into their model. The differences lessened again after 28 May 1986

when shallow convection and related changes were implemented at NMC.

It is noteworthy that there is a strong annual cycle to the differences in the top

panels of Figs. 5 and 6 and this is directly linked to the pronounced annual cycle in the

Hadley cells themselves. Also of note is that the differences commonly exceed 1 m s - 1

which is comparable to the strength of the Hadley circulation itself. Consequently, it

should be recognized that there will be major uncertainties in any diagnostic studies,

such as analyses of atmospheric energetics, which depend upon the verisimilitude of the

divergent wind and associated vertical motion fields.

Figures 8, 9 and 10 present differences in RH at 850, 700 and 300 mb. RH is another

quantity that is extremely sensitive to changes in the NWP model and data assimilation

systems at both centers and, although the changes typically have an impact at all three

levels at the same time, the direction and magnitude of the impact at each level is often

quite different. Trenberth et al. (1987) and Trenberth and Olson (1988b) discuss the

changes in RH at ECMWF in detail. A number of changes occurred in 1980 and 1981

that are reflected in Figs. 8-10. However the most notable change occurred on 1 May

1985 at ECMWF with the introduction of shallow convection in the model that resulted

in a 22% drop in the analyzed tropical RH at 700 mb. This is reflected in Fig. 9. A

similar change occurred at 300 mb (Fig. 10), and an opposite change occurred at 850 mb

(Fig. 8). Then the introduction of shallow convection in the NWP model at NMC in May

1986 brought the tropical differences back to more modest levels. At 300 mb (Fig. 10)

16

[RH]NMC - [RH]ECMWF

SMOOTHED. . . I I . . . . .

(7o)

850 MB

CONTOURS:-30-20

'1020304050

,L'-14 '1

- l r. 8 ;. .. C'? 0

. A.-'~'/ "' :::::.'':: _ '," ,

Y IL ) )�) Ir ( I I r Ir · r'

I -(r-- (II ((( U· '

r

(rI I , \ r �.rr r I

Ccr rI (,r

IrrI

Ir ·�·r

,r ·�, , I

I 'I (II ,I I II r ( Ir( .I r It ·'·'·�·�--I I .·.·..-..-.-. ;;-rI )· ··.. ·.......'('�� :·:·:·:·):·:·:·:·:·:·:��I �·.·;·.·�i ;·.·:·;·:·;(�

1.·I:�;Lai:E·�Sj......;':r

�·;�·�·�� IrrI

'.rrI

I ,

I r.·.\ tI r I ('·· '·� ') ·- · i"rI ( rr,L;a 1 r?·j .·: C

' ' I i ·, .. I · I ·, - . q-

1979 1980 1981 1982 1983 1984

15-DAY AVERAGES

RMS DIFF. (RHNMC/ECMWF %)

SMOOTHED

1985 1986

850 MB

1979 1980 1981 1982 1983 1984 1985 1986

15-DAY AVERAGES

Fig. 8. Differences in RH at 850 mb as a function of latitude and time.Top: [RH]NMC - [RH]ECMWF. Values greater than ±20% m are stippled.Bottom: RMS RH differences. Values greater than 30 and 40% are stippledin different densities.

17

90N

60N

30N -

,)

-DO-

305 -

605 -

90S i

90N

60N

30N

,)

3 O

30S

605

905

I . I- '. I 1 . - I . . . L... . .1I-I I " , , , . I I . , I ,

........._ _ ' _%.t r ' ' .. r. I

0A

I.,:

I

-.-. :-

. I

[RH]NMC - [RH]ECMWF

SMOOTHED 700 MB

CONTOURS:-30-20-10

1020304050

- 0 , ; 'H "

-21*. ,6 -" . ,,

·. 0^U^ .

-214.76,

U

1979 1980I , .. I .- . .

1981 1982 1983 1984 1985 1986

15-DAY AVERAGES

RMS DIFF. (RHNMC/ECMWF %)

SMOOTHED 700 MB

1979 1980 1981 1982 1983 1984 1985 1986

15-DAY AVERAGES

Fig. 9. Differences in RH at 700 mb as a function of latitude and time.Top: [RHNMC - [RH]ECMWF. Values greater than ±20% m are stippled.Bottom: RMS RH differences. Values greater than 30 and 40% are stippledin different densities.

18

^; .

.o

, ,. : ',

.- *,o

.... ."

' * '

,. * ' ,;

' .', *%', I

* vv

. , , ° '

..':, .'::]:-.

*, ., , . .·

; ., , 5 .

90N

60N -

30N -

0 -

305 -

605 -

905

90N

60N

30N

0O

305

605

90S

. I I - I -- .. . i . I I I 1 2 , , I . . I .

I1

Or�r·1Lr, r)I r r r rr r

r"· r c��:_ � .ciir ·

I I I I . I . . .. I ,, " Q 4..

I

,:·r'·'·'·'·'·'·�·� �:::::::::·i4 �·;·:·:·:·:r

;·'·'·'·'/

r Ir (

I·

·r

Co

SON

60N

30N

305

60S

90S

[RH]NMC - [RH]ECMWF

SMOOTHED

(%)

300 MB

15-DAY AVERAGES

RMS DIFF. (RHNMC/ECMwF

SMOOTHED

90N

60N

30N

)

O

3

305

605

905

300 MB

15-DAY AVERAGES

Fig. 10. Differences in RH at 300 mb as a function of latitude and time.Top: [RH]NMC - [RH]ECMWF. Values greater than ±20% m are stippled.Bottom: RMS RH differences. Values greater than 30 and 40% are stippledin different densities.

19

--

differences are also very large at high latitudes.

It is emphasized that the huge changes in analyzed RH at both centers have resulted

primarily from changes in the NWP model, not in the available data or data assimilation.

It is apparent that the analyzed fields in the tropics and subtropics (Trenberth and Olson,

1988b) have tended to be dominated by the NWP model and the associated convection

parameterization schemes. Observations of RH apparently have minimal impact. Conse-

quently there is little comfort in the recent reductions in mean differences analyzed at the

two centers. We note that even though the mean differences dropped to 10%% in RH, the

RMS differences still exceed 20% which is regarded as an unacceptably large amount for

a quantity whose total range is 0 to 100%.

Further time series of differences are presented in Section 3.4 for the poleward eddy

heat and momentum fluxes, and in Section 3.5 for the surface fields over the oceans.

3.2 Meridional cross sections

In order to better appreciate the nature of the zonal mean differences, we present a

number of monthly mean differences as latitude-height cross sections. Figure 11 presents

the meridional cross sections of the zonal mean and RMS z differences for January, April,

July and October of 1980. The mean z differences for 1983 are given in Fig. 12 and the

mean and RMS differences for 1986 are given in Fig. 13. The mean fields mainly illustrate

the huge differences over Antarctica which extend well into the SH. The differences are

greatest in the winter half year. By 1983 the systematic differences are as big as at any

time (over 400 m) but confined to south of 60°S. These enormous differences continue until

May 1986, as seen in the April panel of Fig. 13, but are much less in July and October of

1986 following the improvements at NMC discussed earlier.

The RMS z differences in Figs. 11 and 13 include the mean differences but reveal

other aspects of the analysis uncertainties. In 1980, there were big differences at all levels

south of 40°S quite aside from Antarctica problems, but also with differences becoming

larger with height and exceeding 25 m at 300 mb even in the NH in January and April

20

.0E0L.

0.J-CL

I.

CU -(lCMWF (M)

LatitudeJAN 80

.0EwCl,0

L.Q.

LatitudeAPR 80

.0

E0co5..

0Cl,Cl,wL

0.

.0Ew

IIV5..

0.

LatitudeJUL 80

LatitudeOCT 80

Fig. 11a. Meridional cross sections of [Z]NMC - Z]ECMWF for January, April, July

and October 1980. Negative values are stippled. The contour interval is 25

m.

21

SE

9,Iwm4,wa.

LatitudeJAN 80

S1..E

InL

01

LatitudeAPR 80

S

:3

toC)

a.

LatitudeJUL 80

EIL.C)

a)I

LatitudeOCT 80

Fig. lib. Meridional cross sections of zonal mean RMS z differences for January, April,July and October 1980. Values greater than 100 m are stippled and thecontour interval is 25 m.

22

i

0

.0

E

..

0.L

LatitudeJAN 83

., : :.: : l ........ .. . ........ ... ..,,,,,.... .... .... .;.;. .. ...... ,,, .{\ ................

K .. .... ........................

......., ...

..,,,.......... :

w.:,,,,.............. :' s ' '''

... . .. . .. . ... . ...:~~~~~~~~~~~~~~~~~~~~~~~~ .., ... ........

\ \ .: ..... .. ... :::M ::::........... ............... .....\\

::t l ::,:::.:.,,:: ::::::: :: :m:::: ::::: \\\R

W ,., .... ,. , ....... ~~~~~. ... .. .... ....:

:: :,: . :. ,::. ... :. . :. .... :. . :: ..: : : : :: :::: ...: . .

s , , ~~~~~~~~~~~~~~~~~~~~~....... ..... .,..,,. ̂s

" I....A....................................

............................·''·'·'....... . ;;I).~~~~~~~.......

...·~~:~ ··. ~ ~ ~ ....r.................... ................ ... . ... :~. ·· ·· _.············ · · ··

S30s os 90SN fON 30ON 0

LatitudeAPR 83

LatitudeJUL 83

LatitudeOCT 83

Fig. 12. Meridional cross sections of [z]NMC - [Z]ECMWF for January, April, Julyand October 1983. Negative values are stippled. The contour interval is25 m.

23

100

o-

E

5-3) 0 0

-

to

C70

700-

1$Oa 1I ............. - -7--- ---------------

1

.0E

S.

w,

0.

.0E

S.

I,0.S.

0.

~ I ... .. ... .. .. I , l

*

.0EvL.

A.

I,

LatitudeJAN B6

.0Ev3)

L.

0.

.0E

L.

U,6)

I.

LatitudeAPR 86

LatitudeJUL 86

.0E

U,

U,InA.0.

LatitudeOCT 86

Fig. 13a. Meridional cross sections of [z]NMC - [Z]ECMWF for January, April, Julyand October 1986. Negative values. are stippled. The contour interval is25 m.

24

.0E

I-

0.on

S.

LatitudeJAN 86

.0E6)

026v&IC.

LatitudeAPR 86

.0E

S.

0,

I..

LatitudeJUL 86

E

0.

U,IVa.l.

4ON ON 30oN O OS «S

LatitudeOCT 86

Fig. 13b. Meridional cross sections of zonal mean RMS z differences for January, April,July and October 1986. Values greater than 100 m are stippled and thecontour interval is 25 m.

25

1980. These may be associated with start-up problems at the ECMWF. By 1986 such

large RMS differences are confined to 100 mb and/or south of .40°S and with further

reductions after May 1986. But the large discrepancies remain south of -40°S.

Mean cross sections of [u] differences are not shown since they merely geostrophically

reflect the gradients in Figs. 11-13. However, sample RMS u differences for 1982 and the

most recent year, 1986, are shown in Fig. 14. Aside from Antarctica problems, RMS

differences over 5 m s - 1 occur mainly south of 20°N and above 500 mb, although they also

occur near 30°N and 200 mb in the subtropical jet stream in the NH winter. This figure

depicts, better than Figs. 11-13, the uncertainties in the analyses in the tropics. It also

shows that only minor reductions in differences have occurred with time.

We discussed the problems with [v] earlier but the differences actually have a rich

vertical structure which has varied with time, as seen for January 1981, 1983, 1985 and 1986

in Fig. 15 and for July of the same years in Fig. 16. Much of this complex structure is due

to problems at ECMWF as noted in Trenberth and Olson (1988b) who presented the total

[v] fields for July for the same months (see their Fig. 13). In 1981 [v] was generally weaker

in the ECMWF fields (see also Fig. 7), but became stronger after September 1982 following

the introduction of diabatic NNMI at ECMWF. But then [v] at NMC became stronger

after May 1986 following changes at NMC. Meanwhile the strange vertical structure in the

ECMWF [v] fields, which was associated with the number of vertical modes initialized,

has been gradually eliminated over the years.

In Figs. 17 and 18 we present the mean and RMS differences in RH for January

and July of 1985 and 1986. The pattern in January 1985 was fairly typical of that from

about 1981 to 1 May of 1985. The main change in the differences between January 1985

and January 1986 in Figs. 17 and 18 is due to the ECMWF model changes on 1 May 1985

which dried out the tropics at and above 700 mb. The changes in the difference patterns

in July 1985, compared with July 1986, reflect the impact of subsequent model changes at

NMC in May 1986. It is clear that the RMS RH differences exceed 15% and often 20%

over most of the domain.

26

.0E

S.

to0)

D.

V..

.0E

I)v

1.

a.

.0E

10

g)

to

L. -CL

Cl)

C)S.

0.

RMS DIFF. (Uwuc/rcur M/S) ZONAL MEAN

1000- oa.^. , - *< . -o - .

300ON 30N 0 305

7 00 L ..........

2.05 40 5

1000a---i ii. i-' , -.--

90N 60N 30N 0 30$ bOS 90S

:! : ...:.... .::::::i:::i:::::::i!i.i::!::: : :..:~., .:

i ·· · : * -.- w _;.: :-- :.-:-:-: .......

100 2.0S 0 :'ffN 6ON 3ON 0 30$ *0S -0S

LatitudeJUL 82

LatitudeJAN 86

LatitudeJUL 86

Fig. 14. Meridional cross sections of zonal mean RMS u differences for January andJuly of 1982 and 1986. Values greater than 5 m s - 1 are stippled and thecontour interval is 2.5 m s-.

27

.0EC)

01wC.-U)(6

5

.0E

-

go

P2

LatitudeJAN 81

.0E

S..

5-I..

LatitudeJAN 83

.0

I

so

S.

I-

LatitudeJAN 85

.0

P2

E

S..soma.

LatitudeJAN 86

Fig. 15. Meridional cross sections of [V]NMC - [V]ECMWF differences for January of1981, 1983, 1985 and 1986. Negative values are stippled and the contourinterval is 0.2 m s- 1.

28

i

i

LatitudeJUL 81

90N 6m 30W 0

LatitudeJUL 83

LatitudeJUL 85

LatitudeJUL 86

Fig. 16. Meridional cross sections of [V]NMC - [VECMWF differences for July of 1981,1983, 1985 and 1986. Negative values are stippled and the contour-intervalis 0.2 m s- 1.

29

.0E

S..

U..I

too-

.0 -

E6) 300-

w-

a . c-

a.

........ 1.. 1

I'""'' ." ti;. : t ; -: ;l.. .......: .,,-.:'':,1 ::-.::,, j j -,1,;A, .... .:.:.,:.

r. .... \ [. .......7W .;.., ........ ,...,.

, ~~~~~~~.....; ... .. ,F_,......

ZEL'l '' C'

DsS . oS

.0Ew5-

5..

I-

0t

L..SU

, lI_dl,_ _ [_ _ _ _ _- .. l_- I-__. ...___ _F

I

I.I UL"[' J''' i--- " I. --.--- .. -EN1W

Ew

0.

0.

LatitudeJAN 85

.0E

3,

L.0

a.

LatitudeJUL 85

.0

E0)

CE,3-

0)5.

Q.

LatitudeJAN 86

.0E0)

a.C.

LatitudeJUL 86

Fig. 17. Meridional cross sections of [RH]NMC-[RH]ECMWF differences for Januaryand July of 1985 and 1986. Negative values are stippled and the contourinterval is 5%.

30

i

E

L.

0.Q~

LatitudeJAN 85

O9N 6ON 30N 0

LatitudeJUL 85

LatitudeJAN 86

LatitudeJUL 86

Fig. 18. Meridional cross sections of RMS RH differences for January and July of1985 and 1986. Values greater than 20% are stippled and the contour intervalis 5%.

31

.C

EC)

a.

100-

300-

700-

1000-

·········~·\··~~·~ ··~····A *···I??J/ANK\.4····· ·······K~~~~~~~~~~~~~~~~~~~~····;·~·

· · ··. ~ ~ ~ ·SO

E

ad

tocnw

5.

5.

E.w9.00ww0.

t30S 605

3.3 Geographical distributions

A fairly small sample of the individual monthly mean maps of the NMC-ECMWF

differences is presented in this section as a guide to how representative the zonal means

are. Figure 19 shows the 1000 mb mean and RMS differences for z for January 1983.

The corresponding fields for July 1983, and January and July 1986 are shown in Figs. 20,

21 and 22. The most striking aspect of these four figures is that the RMS differences

are all quite similar to first order, with the major RMS differences occurring where there

is high orography. In this case, as noted earlier, the zonal mean differences tend to be

strongly influenced by the differences over land. One might hope that a large component

of the RMS differences, which occur because of different procedures at the two centers

for extrapolating below ground, might be systematic. It can be seen from Figs. 19-22

that, indeed, there are often large mean differences in the same places. Unfortunately,

such differences are not so consistent from month to month, since they vary in magnitude

and even in sign, so that there is a significant additional random component in all areas.

An example of contrasts in a similar environment is shown in Fig. 21 for January 1986

over Antarctica, where the mean difference of -80 m near 30°E contrasts with a +86 m

difference near 100°W.

Also evident in Figs. 19-22 are the large RMS differences over the oceans south of

-30°S; regions where limited numbers of observations are available and the daily variance

is large. As can be deduced from Fig. 1, differences are much larger prior to 1983 . Further

discussion of these aspects is contained in Section 3.5.

At 200 mb there tends to be more zonal symmetry so that the previously presented

zonal mean sections are more meaningful. Nevertheless, there are some features that

emerge in the geographic distribution that are of interest. Figures 23 and 24 present the

200 mb differences for z and u for January 1983. In the NH, mean z differences >20 m and

RMS differences up to 50 m occur over the central North Pacific Ocean, a region where the

daily standard deviation is ~150 m. They are reflected in Fig. 24 in u by RMS differences

over 6 m s - l. In the tropics u is a more sensitive indicator than z of the meaningful

differences, and in January 1983 the mean differences exceed 6 m s- 1 in the central and

32

nnnI ilnMUV U D ZNMC - Z ECMWF

150W 120o 90W 60W 30o 0

(M)30E 60E 90E 120E 150E 180

90N

60N

30N

0

305

60S

90S150W 120W 90W 60W 30W 0 30E 60E 90E 120E 150E 180

00 MB oFRMS DIFF. (ZNurrCruWF M)

150W 120W 90W 60W 30W0 , 30E

0 30E 60E 90E 120E 150E 18090N

60N

30N

0

30S

60S

90530

JAN 83

Fig. 19. Monthly mean 1000 mb z difference fields for January 1983. Top:ZNMC - ZECMWF. Negative values are stippled and the contour intervalis 20 m. Bottom: RMS z differences. The contour interval is 20 m andvalues greater than 20 m are stippled.

33

IU

18090N

60N

30N

0

30S

60S

90S180

10

18090N

60N

30N

0

30S

60S

90S1

I

1000 MBZNMC - ' CWF

150W 120W 90W 60W

(M)30W 0 30E 60E 90E 120E 150E 180

150W 120W 90W 60W 30W 0 30E 60E 90E 120E 150E 180

00 MB RMS DIFF. (ZkurCrur. M)

90N

60N

30N

0

30S

60S

90S

150W 120W 90W 60W 30W

150W 120W 90W 60W 30W

0 30E 60E 90E 120E 150E 180

0 30E 60E 90E 120E 150E 180

90N

60N

30N

0

30S

60S

90S

JUL 83Fig. 20. Monthly mean 1000 mb z difference fields for July 1983. Top:

NMC - ZECMWF. Negative values are stippled and the contour intervalis 20 m. Bottom: RMS z differences. The contour interval is 20 m andvalues greater than 20 m are stippled.

34

90N

6ON60N

30N

0

30S

60S

90S

180

180

10

90N

60N

30N

0

30S

60S

90S

180

180

1000 MB Z NMC -

150W 120W 90u 60b 30u

Z ECMWF

0 30E

(M)60E 90E 120E 150E 180

1000 MB

150W 120W

RMS DIFF. (ZNMC/ECMWF

90W 60W 30o 0 30E bOE

M)90E 120E 150E 180

90N

bON

30N

0

30S

60S

90S0

JAN 86

Fig. 21. Monthly mean 1000 mb z difference fields for January 1986. Top: ZNMC -

ZECMWF. Negative values are stippled and the contour interval is 20 m.Bottom: RMS z differences. The contour interval is 20 m and values greaterthan 20 m are stippled.

35

1800ON

>ON

30N

'Os

bOS

90S

18090N

60N

30N

0

30S

60S

I

1000 MB ZNMC - ZECMWF

150W 120W 90W 60W 30W 0 30E

150W 120W 90W 60W 30W

(M)60E 90E 120E 150E 180

0 30E 60E 90E 120E 150E 180

RMS DIFF.

150W 120W 90W 60W 30W

(ZNMC/ECMWF

0 30E 60E

M)90E 120E 150E 180

90N

60N

30N

0

30S

60S

90S0

JUL 86Fig. 22. Monthly mean 1000 mb z difference fields for July 1986. Top:

ZNMC - ZECMWF. Negative values are stippled and the contour intervalis 20 m. Bottom: RMS z differences. The contour interval is 20 m andvalues greater than 20 m are stippled.

36

9ON

60N

30N

0

30S

60S

90S

180

180

90N

60N

30N

0

30S

60S

90S

1000 MB

18090N

60N

30N

0

30S

60S

90S1

I

200 MB

150W 120W 90W

ZNMC -Z ECMWF

60W 30W 0

(M)30E 60E 90E 120E 150E 180

200 MB

9ON

60N

30N

0

30S

60S

90S180

RMS DIFF. (ZNMC/ECMWF M)

150W 120W 90W 60W 30W 0 30E 60E 90E 120E 150E 180

JAN 83

Fig. 23. Monthly mean 200 mb z difference fields for January 1983. Top:ZNMC - ZECMWF. Negative values are stippled and the contour intervalis 20 m. Bottom: RMS z differences. The contour interval is 20 m andvalues greater than 20 m are stippled.

37

180901

601

301

30!

60

90!

200 MBU NMC - U ECMWF

180 1SOW 120W 90U 60W 30W 0% P L. ,.

(M/S)30E 60E . 90E 120E 150E 180

Io -0 . W 120U1o -5 '*W 1 20W 90. 60, 30

90W 60W 30W 0 30E 60E 90E

LivV .,. U RMS DIFF.

150W 120W 90W 60W 30W

(UNMC/E CWF

0 30E

M/S)60E 90E 120E 150E 180

90N

60N

30N

0

30S

90S0s

JAN 83

Fig. 24. Monthly mean 200 mb u difference fields for January 1983. Top:UNMC - UECMWF. The contour interval is 4 m s - 1 and all values greaterthan ±2 m s - 1 are stippled. Bottom: RMS u differences. The contourinterval is 4 m s - 1 and values greater than 6 m s- 1 are stippled.

38

60N

30N

0

30S

60S

90S18

·............ ...

;;;Lk';'C,. ..,,,,,,~ ~ ~ ~~.........." ""' ··' ··· ~ ~ ~ ~ ~ ~ ~ ~ ~ . .....

. . . . . .. .. . .. . .. . .. .. .. . .. .

60N

30N

O0

30S

60S

90Sit

dry%

180

120E 150E -4S1

90N

60N

30N

0

305

60S

90S1

.-

. ......... -- .4. .i . K--. I - . . . . . . . . I .;-, �-� � . . . � . I I

urn .J L~~~~~~~~~~~~~~~~~~~~~~~ .....a..............-WV"- - l % Li

tUNr

Pnn upR

eastern Pacific, across South America and into the Indian Ocean. These are regions where

the daily standard deviation is also -6 m s-1 (Trenberth and Olson, 1988b). The RMS

differences are over 10 m s - 1 in several places.

Figures 25 and 26 show the z and u differences for 200 mb in July 1983. The July

z differences are shown mainly to emphasize the nature of the huge differences over and

near Antarctica which result in u differences of up to 26 m s-1. In the weaker gradients of

the NH summer, differences over the NH are small in July. Differences in u in the tropics

and over most of the SH are significant, the main exception being the Australasian sector

which has the most reliable data base.

Figures 27, 28, 29 and 30 show the corresponding z and u differences at 200 mb

for January and July of 1986. In January problems are still evident in the vicinity of the

NH subtropical jet over the Pacific with RMS u differences up to 11 m s- and exceeding

6 m s - 1 over a broad region. Even mean differences locally exceed 4 m s-1. RMS height

differences are bigger than in January 1983 over the north Atlantic and continue to be

large over Africa and south of 20°S while RMS differences in u over 6 m s-l are common

in the tropics and subtropics.

In July 1986 large differences >30 m in 200 mb z are present over the tropical

Atlantic and Indonesia and these are the rule through to December 1986. In December

1986 (not shown) the -28 m differences over India in Fig. 29 were reversed to be +43 m.

Problems with u differences >6 m s-l continue south of -20°N in Fig. 30.

Of particular note in Figs. 24, 26, 28 and 30 are the substantial differences that

exist throughout the tropical Pacific, an area of considerable interest for studies of the

El Nfino and Southern Oscillation phenomena. In the tropical east Pacific monthly mean

u differences commonly exceed 5 m s - ', which is comparable to the expected real inter-

annual variability, and RMS differences exceed 10 m s-1. Such differences could seriously

undermine results of diagnostic studies focussing on that area.

There are huge variations in the RH differences with time and we therefore show

only samples from the last year, 1986, in Figs. 31 and 32 at 850, 700 and 300 mb. In January

39

120W 90W

Z NC - Z ECMWF

60W 30W 0

180 150W 120W 90W 60W 30W "''

(M)30E 60E 90E 120E 150E 180

)N

)N

30E 60E 90E 120E 150E 18L

u u MtD RMS DIFF. (ZNMC/ECMWF M)

180 150W 120W 90W 60W 30W 0 30E 60E 90E 120E 150E 18090N

6ON

30N

0

30S

60S

90S

180 150W 120W 90W 60W 4S 0 30E 60E 90E 120E 150E 180

JUL 83

Fig. 25. Monthly mean 200 mb z difference fields for July 1983. Top: ZNMC -

ZECMWF. Negative values are stippled and the contour interval is 20 m.Bottom: RMS z differences. The contour interval is 20 m and values greaterthan 20 m are stippled.

40

200 MB

180 150W90N

60N

30N

0

30S

60S

90S

90N

60N

30N

0

30S

60S

90o

)N

)S

OS

3SA l ~11

d^lf^ tr,

zi

200 MB

1190N

60N

30N

0

30S

60S

90S1

UNMC U ECMWF

'I\ Ju n D RMS DIFF. (1

150W 120W 90O 60 30W

UNMC/ECMWF

0 30E

M/S)60E 90E 120E 150E 180

90N

60N

30N

0

30S

60S

905130 150W 120W 90 60W 30V 0 30E 60E 90E 120E 150E 180

90S

JUL 83Fig. 26. Monthly mean 200 mb u difference fields for July 1983. Top: UNMC -

UECMWF. The contour interval is 4 m s - 1 and all values greater than ±2m s- 1 are stippled. Bottom: RMS u differences. The contour interval is 4m s - 1 and values greater than 6 m s - 1 are stippled.

41

(M/S)

ornn tID

18090N

60N

30N

0

30S

60S

90N

6ON

3ON

30S

60S

90S

90N

30N

0

30S

60S

90S

180

180

180

200 MB Z NC -

150W 120W 90W 60W 30W

Z ECMWF

0 30E

200 MB RMS DIFF. (ZNMC/ECMWF

150W 120W 90W 60W 30W 0 30E 60E

(M)60E 90E 120E 150E 180

M)90E 120E 150E 180

9UN

60N

30N

0

30S

60S

90S150W 120W 90W 60W 30W 0 30E 60E 90E 120E 150E 180

JAN 86Fig. 27. Monthly mean 200 mb z difference fields for January 1986. Top:

ZNMC - ZECMWF. Negative values are stippled and the contour intervalis 20 m. Bottom: RMS z differences. The contour interval is 20 m andvalues greater than 20 m are stippled.

42

II

200 MBiAn 1 snu 12nu Ow

U NMC - U ECMWF

60W 30W 0

(M/S)30E 60E 90E 120E 150E 180

200 MB RMS DIFF. (UNMC/ECMWF

150W 120W 90W 60W 30W 0 30E

150W 120W 90W 60W 30W

M/S)60E 90E 120E 150E 180

0 30E 60E 90E 120E 150E 180

9ON

60NbON

30N

0

30S

60S

90S

JAN 86

Fig. 28. Monthly mean 200 mb u difference fields for January 1986. Top:UNMC - UECMWF. The contour interval is 4 m s - 1 and all values greaterthan ±2 m s - 1 are stippled. Bottom: RMS u differences. The contourinterval is 4 m s-1 and values greater than 6 m s - 1 are stippled.

43

90N

60N

30N

0

30S

60S

90S

180

180

200 MB ZNMC -

150W 120W 90W 60W 30W

Z ECMWF

0 30E

(M)60E 90E 120E 150E 180

200 MB RMS DIFF. (ZNMC/ECMWF M)

90N

60N

30N

30S

60S

90S905180 150W 120W 90W 60W 30W 0 30E 60E 90E 120E 150E 180

JUL 86Fig. 29. Monthly mean 200 mb z difference fields for July 1986. Top: ZNMC -

ZECMWF. Negative values are stippled and the contour interval is 20 m.Bottom: RMS z differences. The contour interval is 20 m and values greaterthan 20 m are stippled.

44

18090N

60N

30N

C

30S

60S

90S

)ON

SON

)ON

30S

bOS

90S

U NMC - U ECMWF

60W 30W 0

(M/S)

30E 60E 90E 120E 150E 180

200 MB150W 120W

RMS DIFF. (UNMC/ECMWF

90W 60W 30W 0 30E

M/S)60E 90E 120E 150E 180

150W 120W 90W 60W 30W 0 30E 60E 90E 120E 150E 180 -

90N

60N

30N

0

305

60S

90S

JUL 86Fig. 30. Monthly mean 200 mb u difference fields for July 1986. Top: UNMC -

UECMWF. The contour interval is 4 m s- 1 and all values greater than ±2m S- 1 are stippled. Bottom: RMS u differences. The contour interval is 4m s - 1 and values greater than 6 m s - 1 are stippled.

45

200 MB15ow 1180 20W 90W

90N

60N

30N

0

30S

60S

90S

tON

>ON

3ON

3OS

9OS~OS

90N

60N

30N

0

30S

60S

9OS

180

180

4i

. .

RH Mc - Hi £CMWF (') 850 MB

ury

?ON

W0N

3ON

~0

30S

bOs

90s

RH NMC - RH ECMF ) 700 MB

180 150W 120W 90W 60o 30o 0 30E 60E 90E 120E 150E 180

RHNuc - RH ECMWF (70) 300 MB180 150W 120W 90o 60W 30W 0 30E 60E 90E 120E 150E 180

9ON

6ON

3ON

30S

90S

9ON

6ON

3ON~0

30S

b0s

JAN 86Fig. 31. Monthly mean RH difference fields for January 1986 RHNMC -RHECMWF

at 850, 700 and 300 mb. Negative values are stippled and the contour intervalis 20%.

46

9o.

6ON

30

30S

60S

90N

60N

30N

0

30S

60S

90S

90N

60N

30N

0

30S

6OS

9051

r A L

I

RH NUC - RH ECMWF

180 150W 120W 90o 60o 30W 0 30E

(7.) 850 MB60E 90E 120E 1SOE 180

RH NMC - RH ECMWF (7') 700 MB

90N

60N

30N

0

30S

60S

90S1

RH NMC - RH ECMWF (7) 300 MB180 150s 120W 90W 60W 30o 0 30E 60E 90E 120E 150E 180

JUL 86Fig. 32. Monthly mean RH difference fields for July 1986 RHNMC - RHECMWF at

850, 700 and 300 mb. Negative values are stippled and the contour intervalis 20%. 47

90N

60N

30N

0

30S

60S

90N

60N

30N

0

30S

60S

90SI go

._O

1986, biggest differences occur over the oceans but the magnitude of these differences-over

60% in the eastern tropical Pacific at 850 mb-is astounding. At 700 mb in January the

tropical differences reverse in sign but still exceed 50% over the tropical Pacific and Indian

Oceans, and over Antarctica. Locally differences of N40% continue in July 1986 over the

tropical eastern Pacific at all three levels and over the Indian Ocean at 300 mb. The zonal

mean differences in RH shown earlier (Figs. 8, 9, 10 and 17) are representative of the

overall differences but, if anything, underestimate the huge differences that exist locally.

The RMS differences for RH are not shown as geographical distributions since they are

dominated by the mean differences.

3.4 Poleward eddy heat and momentum fluxes

The poleward eddy heat flux [v*T*] and the poleward eddy momentum flux [u*v*j

are fundamental quantities in maintaining the general circulation of the atmosphere. In

addition to their importance to the heat and momentum budgets themselves, they are

also the main quantities that go into the Eliassen-Palm (E-P) flux of wave activity (e.g.,

Trenberth, 1987b). The vertical wave activity component is proportional to [v*T*] and

the meridional component is proportional to [u*v*j. Note that we are dealing with the

total wave flux here including contributions from both the stationary and transient waves.

As background for interpreting the differences in the fluxes from NMC and ECMWF,

we first present estimates of these quantities from ECMWF for January and July 1986 in

Figs. 33 and 34. The strongest poleward eddy heat flux occurs in winter with maxima be-

tween 850 and 700 mb and a secondary maximum in the lower stratosphere near 200 mb.

The latter is associated with the vertical propagation of waves into the stratosphere which

is greatest in winter in the NH. The maximum poleward transport of westerly momentum,

Fig. 34, occurs near the tropopause and again the values are largest in the NH winter. The

annual cycles for both fluxes have much larger amplitudes in the NH.

Time series of the smoothed 15-day averaged [v*T*] at 700 mb and [u*v*] at 200 mb

are shown in Figs. 35 and 36, while time series of the differences are given in Fig. 37. The

latter are also given as meridional cross sections for January and July of 1982 and 1986 in

48

.0

E

Q)S.. I,

[" T ] ECWF (M-KS- )

LatitudeJAN 86

.0E

Q)

V)

a.

90N 60N 30N 0 305 60S 90S

LatitudeJUL 86

Fig. 33. Meridional cross sections of [v*T*] from ECMWF in K m s- 1 for Januaryand July 1986. The contour interval is 5 K m s - 1 and negative values arestippled.

49

.0Ea)S..

a)a.

LatitudeJAN 86

.0

E

(U0)

EnQ)

Q.I,

90N 60N 30N

Fig. 34. Meridional cross sections ofand July 1986. The contourstippled.

0 30S 60S 90S

LatitudeJUL 86

[u*v*] from ECMWF in m 2 s - 2 for Januaryinterval is 10 m 2 s - 2 and negative values are

50

[v T ]NMc (M-KS- 1 )

SMOOTHED

90N

60N

30N

Q5

-d

(t

0

305

60S

905

700 MB

15-DAY AVERAGES

[Iv T ]ECMWF (M-KS- )

SMOOTHED

90N,

60N

30N

0

305

605

905

700 MB

15-DAY AVERAGES

Fig. 35. Smoothed 15-day averages of [v'T*] at 700 mb as a function of latitude and

time. Values greater than ±10 K m s- 1 are stippled and the contour interval

is 5 K m s- 1. Top: from NMC; and bottom: from ECMWF.

51

0

._.-D

[u' V]NMC (M2 S- 2 )

SMOOTHED 200 MB1 I * ' I . .I I 1 r I I r I ' I , I I i I I t I I I t I

1979 1980 1981 1982 1983 1984

15-DAY AVERAGES

1985 1986

[UI V ]ECMWF

SMOOTHED

60Nt

30N4

C!

305

605

905

(M2 S- 2 )

200 MB

15-DAY AVERAGES

Fig. 36. Smoothed 15-day averages of [u*v*] at 200 mb as a function of latitude andtime. Values greater than ±40 m 2 s- 2 are stippled and the contour intervalis 20 m 2 s- 2. Top: from NMC; and bottom: from ECMWF.

52

9Er, -

30N -

0I -

30M -

A r I-

90S

v'J ~ ,.-v<.. .

)>i@6 / z~

, i I I 1 I II I I - I I I I I , I I i - 1 ) l , I I t

Iv T* ]NMC-ECMWF

SMOOTHED

90N

60N

09

305

60S

90S

(M-KS-' )

700 MB

15-DAY AVERAGES

U[ 'V ]NuC-_ECMW

SMOOTHED

60N

30t.

C

Q)

3

cl6-

._-

305

605

905

(M2 S- 2 )

200 MB

15-DAY AVERAGES

Fig. 37. Differences (NMC - ECMWF) in smoothed 15-day averages of Iv'T*] at

700 mb (top) and [u*v*] at 200 mb (bottom) as a function of latitude and

time. Values greater than ±5 K m s-1 (top) and ±10 m 2 s-2 (bottom) are

stippled.53

Figs. 38 and 39.

For [v*T*], the differences in Fig. 37 are mostly relatively small, less then 2 K m s 1,

except for the region south of 40°S and especially near Antarctica. As can be seen in

Fig. 35, very large and unrealistic values occur there in the ECMWF analyses. However,

the differences from 40-65°S are likely to be due to general uncertainties in the analyses

although no doubt with the SH problems at NMC exacerbating the problem, e.g. see

Section 4.

It turns out that the biggest discrepancies in [v*T*] are not near its maximum at

700 mb but instead occur at the surface and in the upper troposphere and lower strato-

sphere (see Fig. 38). At the surface, pronounced differences are seen in January 1986 and

are due to stronger poleward fluxes there in the NMC data. Such differences are also

found in most other months and evidently arise from the boundary layer wind properties.

Inclusion of data extrapolated below ground may also be a factor. In the lower strato-

sphere, differences of over -5 K m s- 1 are the rule, especially south of 30°S, usually with

a stronger ECMWF poleward flux (negative values, positive NMC-ECMWF difference).

Such differences, 30-40% of the total, are disconcerting and have implications for infer-

ences drawn about lower stratospheric dynamics of the SH. Evidently the wave activity

propagates vertically much more as seen from the ECMWF analyses than from the NMC

analyses.

A major concern at present is the "ozone hole", the observed trend toward lower

ozone amounts in the lower stratosphere over Antarctica in the southern spring. In order

to help unravel the role of dynamics in contributing to this phenomenon it is essential to

know the components of the E-P flux in detail. Fig. 40 therefore shows the total ECMWF

[v*T*] and the NMC-ECMWF differences for October 1986. Similar differences are found

in the previous two years in September and/or October. Clearly caution is called for in

drawing any conclusions about the P flux and the role of dynamics in the heat and ozone

budgets with uncertainties such as those that Fig. 40 presents.

The differences in [u*v*] (Figs. 37 and 39) are somewhat variable in time. Differ-

54

.0

EvLi

eL

LatitudeJAN 82

H 4ON 3ON 0

LatitudeJUL 82

3os eOS 90S

LatitudeJAN 86

.aE

S.

An5.

0.

LatitudeJUL 86

Fig. 38. Meridional cross sections of [v'T*] differences (NMC - ECMWF) in K m s-for January and July of 1982 and 1986. The contour interval is 2.5 K m s-1and negative values are stippled.

55

100-

-.0E

L3

700-

q

E5-

C,,I.

L

0.

...... I I.... I _............. mA . ....m ..... m .-~ - .-2~ ....m _ m_ __ _. m _ _

r-r

·rar ! - -. ' -_ - -. .. - . ....._~~~~~~~~~~~~~~~ .IWVV-Ia

-r

5

·::f~~l·:·~·:.........................

.............................. ................... . .....·:·:·:·...................... ·. ··. ··.·

... ... .... ... ... .. .... ..... .. ... .. ..... .

.. .. . . .. .. . .. . .. .

B litiltlirtltttt~ i~............................. ......... ..~~~~~~~~~~~i~lii~i

.......................... .. . .. . .. . .. .

.. ............ ....... ~ ~ :·:·.... ..... ... .... ... ... . .. ... ... ... ...

... .. .. .. ....... .. .. ..... .. ..... .. ...........·.·.·.·;.......... ............ ... ............::::~.r~.'.' · ~

I)

L.

Co

2 ,A.

E

4)L.co

E

3

L I.

E

S..

U)03U

5.

LatitudeJAN 82

LatitudeJUL 82

LatitudeJAN 8b

LatitudeJUL 86

Fig. 39. Meridional cross sections of [u'v*] differences (NMC - ECMWF) in m 2 s - 2

for January and July of 1982 and 1986. The contour interval is 5 m 2 s - 2

and negative values are stippled.

56

i

I

Iv' T' ] ECWF

LatitudeOCT 86

LatitudeOCT 86

Fig. 40. Meridional cross sections for October 1986 of [v*T*] from ECMWF anddifferences (NMC - ECMWF) in K m s- 1 . The contour interval is 5 (above)and 2.5 (below) K m s - 1 and negative values are stippled.

57

,0

EQ)

S)

.0

coUr)a)

CL.

E

1.0(n

to

(M-KS-' )

;._

ences are mostly small (< 5 m 2 s- 2) in the NH but often become large (>10 m 2 s- 2 ) in

the subtropical NH jet and in the tropics. For the latter region, the differences are 100%

of the total in January 1982 and in the NH jet differences can be .20O%, although they

are usually much less. Once again, the biggest differences arise in the SH and are quite

unacceptable (over 30%) in 1982 (Fig. 39) and still problematical in 1986. Such differences

have profound implications for the momentum budget (e.g. Trenberth, 1987b).

3.5. Surface differences over the oceans

Measures of how well the surface atmospheric fields are known are of interest because

of their implications for interactions of the atmosphere with the surface. In particular, in

this section we pursue how well the NMC and ECMWF analyzed fields agree over the

oceans. This has implications for the TOGA Program and the World Ocean Circulation

Experiment (WOCE). As well as having a bearing on the atmospheric forcing by the oceans,

the surface fluxes of sensible and latent heat and the surface wind stress (or momentum

fluxes) are of vital importance for driving the oceans. We have not computed the surface

fluxes themselves, since the lowest level information available from ECMWF is 1000 mb.

Rather, we have computed sea level pressures and used the 1000 mb analyzed winds to

compute differences over each major ocean.

Snapshots for individual months of these or related quantities were presented in

Figs. 19, 20, 21 and 22 for z. Here we present time series of the monthly means and

temporal RMS differences zonally averaged across each of the three main oceans and for

the oceans as a whole. In this way we can fairly concisely summarize the characteristics

of how well the analyses agree. Some "bad" days were not screened out when these

computations were made but their impact was either minimal or their effects have been

subsequently removed from the plots.

In order to average over the oceans, we have designated each point on the globe as

either land or ocean, using a mask with 2.5° resolution. The mask is conservative with

respect to the oceans by designating marginal regions and areas dominated by islands, etc.

as 'land' in order that the results will be meaningful for the open oceans. The mask is

58

shown in Fig. 41 and the non-shaded area is open ocean for the purposes of summarizing

statistics. In order to delimit the individual oceans, we have used the divisions, also shown

in Fig. 41. The ocean region from 120°E to 800W is referred to as the Pacific, from 70°W

to 20°E as the Atlantic, and from 20°E to 1200 E the Indian Ocean. Note that the region

from 80 to 700 W is in the Atlantic in the NH but in the Pacific in the SH and therefore,

for simplicity in averaging, we have omitted it from both.

Figure 42 shows the zonally averaged mean and RMS differences in sea level pressure

P for the global oceans as latitude-time sections. The corresponding u sections are shown

in Fig. 43. As the RMS differences in v are similar to those in u and the zonal means of v

are not very meaningful they are not presented. Zonal mean P differences exceed 1 mb over

the tropical oceans, where the standard deviation of the monthly means are only about

0.6 mb, until the end of 1980. Mean differences of ~0.5 mb continued through 1984. These

uncertainties in the P field are reflected in the zonal mean u field as differences commonly

exceeding 1 m s - 1, which is to be compared with the mean trade wind values of 4 to

6 m s-~. After 1980 mean P differences in the NH are small, but in the SH the previously

discussed problems with the NMC analyses prior to 1982 are again clearly evident.

The RMS differences in Figs. 42 and 43 are perhaps more meaningful in terms of the

likely uncertainty in flux quantities. RMS differences in P are rarely less than 1 mb and

exceed 2 mb in the Arctic Ocean and south of s35°S. For u the result is RMS differences

exceeding 3 m s- 1 (corresponding to the total wind speed of ~5 m s - 1) over the NH

oceans in the winter half year, in the equatorial belt and south of 30 0S. In the equatorial

belt, the differences have been due in large part to problems at NMC which were at least

partly corrected in May 1986. As shown by Bonner et al. (1986) the equatorial winds

were unrealistic and too weak prior to that date (see also Leetmaa, 1987). Harrison et al.

(1988) have compared ocean model responses to different surface wind fields for 1982-83,

including those from NMC and ECMWF, and show that there are substantial differences

among the products, apparently partly due to an inadequate data base.

The mean differences in P over the individual oceans are quite similar to those

shown in Fig. 42 and are not presented. Fig. 44 shows the RMS differences over the three

59

(2.5 X 2.5 RESOLUTION)

180 150W 120W

180

90W 60W 30W 0 30E 60E 90E 120E 150E

150W 120W 90W 60W 30W 0 30E 60E 90E 120E 150E

Fig. 41. Map showing the unmasked area designated as open oceans. The dividinglongitudes for the different oceans are also marked in.

180- 0 Pt&

180

90N

60N

30N

0

30S

60S

90S

90N

60N

30N

00I

0

305

60S

qnS

LAND/OCEAN FLAGS

0 o

I··~~ .'

GLOBAL OCEANS

p*& OQII£ (a i

ri 6'Bs ' ':;

RrJ

c?5 :...;

hss ' ,,;.- · ·// s'^ .' *';

. ,. *'**.' 1. <^ ; :' '" '

-. 739

I:: 458 ",

;..··.;5r--i'j j 5.'~~-. *. .* 5~ 5

H.149

^ ̂/ ;: -- ((n w

* *. .................................*

,~ ..

.' ·.S ,. S ,.

CONTOUP5:-2.0-1.0-0.5

0.51.02.05.0

1980 1981 1982 1983 1984 1985 1986

Months

RMS DIFF. (PN- .'ECMWF mb) GLOBAL OCEANS

1980 1981 1982 1983 19 1985 19861980 1981 1982 19B3 1984 1985 1986

CONTOURS:1.02.05.0

10.0

MonthsJPN 80 - DEC 86

Fig. 42. Latitude-time series of the monthly mean differences in sea level pressureszonally averaged over the total oceans. The uneven contours are given atright. Top: PNMC-PECMWF, negative values are dashed and values greaterthan ±1 are stippled. Bottom: RMS P differences with values greater than2 and 5 mb stippled.

61

9OJ

9 I U J

I:

305 -

605 -

905

9C'0

60:';

30rJ

-)IC

-6 .

625 -

905

./,,y

_L

: ;: : ':';;:i : .-.'. ......... :.:::.......... ........................................

1.32S~' ' ,''' '"'. '..: ::;i ''' '" " ', " "'i. ' "2 '....:..

i - , - t,,,- __ I " 4 , -1 , I tm L ·t I , " ' I ,I , ' o · -

- i I I I I Il

i I I I I

I I~ ~~~~~ .I .t I I I I.1 I . , i I I I I ..1 . . . m m ~ i -

-' r

[Phmrc -- [PIECMWF- (mb)

(

I

I .. ·

,a ':. ;.

'...1 - -

[U]NNC - [UJECVwF

90N

6 Or

30N

30

605

605

RMS DIFF. (U,t,.C/ECMWF

3C'!

305

605

9C'5

(ms- 1 ) GLOBAL OCEANS

1000 mb

Months

ms-' )

CONTOUPS;-4.0-3.C-2.0-1.0-0.5

0.51.02.03.04.05.0

GLOBAL OCEANS

1000 mb

CONTOURS:2.C3.C4.05.06.07.08.0

MonthsJRN 80 - DEC 86

Fig. 43. Latitude-time series of the monthly mean differences in 1000 mb u zonallyaveraged over the total oceans. The uneven contour intervals are given atright. Top: UNMC - UECMWF, negative values are dashed. Values greaterthan ±1 m s-1 are stippled. Bottom: RMS u differences with values greaterthan 4 and 6 m s - stippled. 62

oceans and it further illustrates the similarity of the patterns over each ocean. The biggest

uncertainties of over 2 mb RMS difference are south of 30°S over all oceans, from 40 to

50°N in the Pacific in winter, and north of 50°N in the Atlantic in winter.

Figures 45, 46 and 47 present the mean and RMS u differences over the three oceans.

Here there are more notable distinctions between them. Problems are evident near the

equator in all three oceans. The tropical differences have more seasonal character in the

Indian Ocean and are largest in the NH summer monsoon from 1980 to 1983. Stronger

equatorial easterlies have been the rule in ECMWF data in the Atlantic, but the same

applies in the Pacific only after late 1982. In the extratropics, largest RMS u differences

of over 3 m s- 1 are found in winter months ~40 to 50°N in the Pacific and Atlantic, and

south of 30°S in all three oceans with even larger differences in winter.

In closing this section, we note that sensible and latent heat fluxes at the surface,

through bulk formulae, depend on the wind speed and the covariations of wind with air-sea

temperature differences and air humidity - saturation humidity differences, respectively.

In the tropics, the latent heat flux dominates and climatological values are -200 W m -2 .

A mean wind error of 1 m s - 1 is apt to result in flux errors of ~40 W m - 2 . RMS errors in

u of 4m s - are likely to lead to 15 to 25% errors in surface wind stress in mid latitudes, or

over 40% errors near the equator. Such uncertainties are not acceptable for driving ocean

models (e.g. see Harrison et al., 1988). There has been a recent increase in attention

focussed on surface fields as a consequence of the TOGA program and fortunately there

are signs that the surface products can be and are being improved.

63

RMS DIFF. (Pwc/[cwr rnb) PACIFIC OCEAN

CONTOURS:1.02.0S.0

10.0

1980 1981 1982 1983 1984 1965 1986IMS DIFF. (PNUC/ECMuF mb) ATLANTIC OCEAN

* COT"OURS:1.02.05.0

10.0

1980 1981 1982 1983 1964 1985 1986RMS DIFF. (PWC/Cuwr mb) INDIAN OCEAN

1980 1981 1982 1983 1964 1985 1988

Months

CONTOR5:1.02.05.0

10.0

Fig. 44. Latitude-time series of the monthly mean RMS differences in P zonally av-eraged over the three oceans in mb.

64

00

Lj. 8 lO·s1.22

....... ....i·. · ..··.. ··... ..................... ....., .... ........... .... . .... .... .... ..... .... ..... .... ....:40 :~z r~s ·~:·:··~r · · ··............ . ........ ......;·:··:··:··: ·... ....... . " "

90

60SN

30SN

C)

3 o-

.3

305

605

905 -

90N -

60N

305

605

905

60N

30N

O)

.. 0

305

60S .

Io

Oii.::.

N b.J:2 1 ... .b . . .

918 ~ ~ ~ ~ ~ .42'

..A. ..... ..... ... ....ri~-~ ·~t ·: :::~'~.....

"-w .~ -- . "Z Y · ·

I Wo - -^ ^08 1 - *(

..- . , . ....!.: i: . :,:' :'-': - ; :.--.......... .. ..-.. :.-.-..-. -. : ....... , ...... , .-, ; - ... ,...F: :..-:.-.*.... .-s*.1 - **..* -

I - 1 - - - - I . I - 1 - - . - - I - - i

I . . . . . . . . . . . . . . . . . . . I . . . I � . � I4-

I- -. -. I - - 1 I - - I - -~ - 1 I . · . i-.

. . - I - - . . . . . I . . . I . .i I -- -- -- --- A -- - - --- -

[U]NMC - [U]ECMWF

60tJ

Q)

-I

I-I

M,

i C

3C S

605

9C5

RMIS DIFF. (Ul4t,/ECMwF

3CJ

'Ccl)

605

90S

(ms- 1 ) PACIFIC OCEAN

1000 mb

Months

ms-' )

CONTOURS:-5.0-4.0-3.0-2.0-1.0-0.5

0.51.02.03.04.05.06.0

PACIFIC OCEAN

1000 mb

CONTOUPS:2.03.04.05.06.07.08.0

MonthsJRN 80 - DEC 86

Fig. 45. Latitude-time series of the monthly mean differences in 1000 mb u zonallyaveraged over the Pacific. Top: UNMC - UECMWF, negative values aredashed. Bottom: RMS u differences in m s-1.

65

�t'.-··�l': 1C� �(`·\r· ijCII

G?' iOii r i%7I I

/f c..r .·r L·\

.rr. ·· C'' ' ' . ·· :i t ·''\· ··.' �·r 'r;" .1\'·' ' ''L�··? �� '' .�· r ·?�QO .. it·( ;·�·.CL I�·)

· ·I·K�� ··.-, ···II-·rI c.;Q-·,.Y�% ri....5··�r j�' I·\ *

r C.' �- 3·�

[U]NMC - [U]ECMWF (ms-' ) ATLANTIC OCEAN

1000 mb

CONTOURS:-4.0-3.0-2.0-1.0-0.50.51.02.03.04.05.06.0

Months

ms- 1 )

0

305

605

90S

ATLANTIC OCEAN

1000 mb

CONTOUPS:2.03.04.05.06.07.08.0

1980 1981 1982 1983 1984 1985 1986

MonthsJRN 80 - DEC 86

Fig. 46. Latitude-time series of the monthly mean differences in 1000 mb u zonallyaveraged over the Atlantic. Top: UNMC - UECMWF, negative values aredashed. Bottom: RMS u differences in m s-1.

66

IR t-S DIFF. (ut,,ui/E Cm W

[U]NMC - [U]ECMWF

6ON

3CKS

KS$

(ms-' )

.Months

RMS DIFF. (UIJC/ECMWF ms-' )

QE'-'

3: .

305

605

90S

INDIAN OCEAN

1000 mb

CONTOURS:-4.0-3.0-2.0-1.0-0.50.51.02.03.04.05.06.0

INDIAN OCEAN

1000 mb

CONTOURS:2.03.04.05.06.07.0e.0

MonthsJRN 80 - DEC 86

Fig. 47. Latitude-time series of the monthly mean differences in 1000 mb u zonallyaveraged over the Indian Ocean. Top: UNMC - UECMWF, negative valuesare dashed. Bottom: RMS u differences in m s-1.

67

4. A Case Study in June 1985

In this section we present the results of one case which was chosen because of impres-

sions gained through the examination of synoptic charts received over the routine facsimile

transmissions from NMC. On the day in question it was noted that several observations

were plotted on the charts over the South Pacific, but they were not compatible with the

analysis.

Fig. 48 shows the 1000 mb analyses from NMC and ECMWF for 20 June 1985

at 0000 GMT and Fig. 49 presents the differences at both 1000 and 300 mb. In the NH,

differences at 1000 mb are -50 m in several places. But by far the biggest differences of

310 m at 1000 mb and 218 m at 300 mb are found near 45°S 1000 W. Large and significant

differences are also found near 40°S 10°W (-191 m) and over the Antarctic Peninsula

(-271 m). Huge differences appear to be standard fare over Antarctica. We have therefore

examined the information available to NMC at this and adjacent synoptic times as archived

on the NMC data tapes.

4.1 The South Pacific Low

Although TOGA buoys are available and can influence the analyses, they were not a

major factor in the South Pacific. Instead, there was apparently a fleet of ships in the area,

moving very slowly, centered near 45°S 110°W. For the region 35°-50°S 900 -115°W from

0000 GMT 19-21 June 1985 there were 45 synoptic reports from 12 ships with different

call signs. The incidence of reports varied from nine at 0000 GMT 19 June to two at the

1200 GMT hours. The ships clearly reveal the development and movement southeastwards

of an intense storm that deepened to ~950 mb at 1200 GMT 19 June and sustained those

low pressures until at least 1800 GMT 21 June. From 0600 on the 19th near 45°S 109°W

pressures dropped from 970 mb to 952 mb (verified by two ships) and winds reversed

from 23 m s- 1 northeasterlies to 24 m s - 1 southwesterlies. Subsequently southwesterlies

of 29 m s-1 were recorded. Another ship downstream at 48°S 99°W recorded 25 m s-1

northeasterlies and 959.8 mb at 1800 on the 19th but the wind backed through northwest

68

90N

60N

30N

0

305

60S

905

90N

bON

30N

0

30S

b60

90S

1000 MB ZNMC (M) 20 JUN 85 OZ

180 150W 120W 90W 60b 30W 0 30E bOE 90E 120E 150E 180

180 150W 120W 90W 60W 30W 0 30E 60E 90E 120E 150E 180

1000 MB ZECMWF (M) 20 JUN 85 OZ

180 150W 120W 90W 60W 30W 0 30E 60E 90E 120E 150E 180

)UN

)ON60N

30N

30S

bOS

90S

90N

bON

30N

0

30S

60S

90S180 150W 120u 90W b6W 30W 0 30E 60E 90E 120E 150E 180

Fig. 48. 1000 mb analyses at 0000 GMT 20 June 1985 from NMC (top), and ECMWF(bottom). The contour interval is 60 m and negative values are dashed.

69

1000 MB ZNMC-ECMWF (M) 20 JUN 85 OZ

180 150W 120W qoW 6bO 30W 0 30E 60E 90E 120E 150E 180

90N

6ON

30N

0

305

60S

90S1

300 MB ZNMC-ECMWF (M) 20 JUN 85 OZ

180 150W 120W 90W W 30W 3 30E 60E 90E 120E 150E 180............... ,,., ,...

Fig. 49. Differences in analyses ZNMC - ECMWF at 0000 GMT 20 June 1985 at 1000mb (top), and 300 mb (bottom). The contour interval is 60 m at 1000 mband 100 m at 300 mb and negative values are dashed.

70

I

I

I

I

Table 1. Values of 1000 mb geopotential height 1) from NMC analyses; 2) from

ECMWF analyses; 3) from observations for the period 19-21 June 1985. Given are

values estimated for 45°S 1100W, the depth of the low center, and the maximum

differences between the NMC and ECMWF analyses in the area.

19 June 20 June 21 June

0000 1200 0000 1200 0000

45°S 110°WNMC 30 20 -10 00 15

EC -85 -175 -173 -122 -120

'observed' -100 -370 -200 -152 -120

Low CenterNMC x x -135 -223 -253

EC -125 -241 -361 -303 -257

'observed' -200 -400 -400 -380 -360

Max. Difference

NMC-EC 224 248 310 171 158

71

and then to southwest a day later with lowest pressures recorded of 954.3 mb. Pressure

tendencies generally exceeded 6 mb/3 hours for up to 12 hours both prior to and following

the disturbance.

A summary of the deduced 1000 mb heights at 450S 1100 W and in the low center is

given in Table 1 along with the values interpolated from the NMC and ECMWF analyses.

In spite of the abundance of observations, NMC failed to capture the system at all on the

19th and 20th. ECMWF generally placed the low very realistically but underestimated its

strength. Neither analysis was very good at 1200 GMT on the 19th when errors of -400 m

occurred in the NMC analysis and -160 m in the ECMWF analysis.

4.2 The South Atlantic Low

In the South Atlantic the low centers in Fig. 48 are near 40°S 10°W in both

analyses, near Gough Island. Also, two buoys were in the area and should have influenced

the analyses. In this case the NMC system is more intense by 191 m. At Gough Island

(40.35°S, 9.90 W) the sea level pressure at 0000 GMT 20 June was 1005.1 mb with a

southeasterly wind of 17 m s - 1. The NMC analysis is too low at Gough by -100 m and

there are no observations that indicate a center lower than -80 m. The ECMWF analysis

fits the available observations quite well.

4.3 The Antarctic Peninsula

A similar pattern emerges over the Antarctic Peninsula. Differences between ECMWF

and NMC were large throughout, exceeding +264 m on the 19th and -289 m on the 21st.

There were abundant observations available, not only on the 20th at 0000 GMT but at all

synoptic times for the days before and after. At 0000 GMT on the 20th between 44 and

70°W, 50-70°S there were 25 synoptic observations from surface stations plus some buoy

observations. The observations fit the ECMWF analyses reasonably well throughout the

19-21 June period but the NMC analyses are seriously in error.

72

5. Discussion and conclusions

We have presented mean and RMS differences between the NMC and ECMWF

global analyses of a number of fields. Maps, zonal mean meridional cross sections, and

latitude time sections of the differences were shown, along with a case study for a few days

in June 1985.

It is pleasing to see the fairly widespread agreement between the analyses from

the two centers over the NH extratropics. Of course, there are local differences in detail

which result in discrepancies in z of -50 m at 1000 mb and -80.6 m at 300 mb in the

north Pacific in Fig. 49, for instance. However, in general it seems that the quality of

the analyses, as revealed by their differences, is much less in the tropics and SH. This is

reflected in much greater differences in wind fields south of 200N, with RMS differences

often exceeding 5 m s- 1 above -500 mb in this region. It is further revealed in the much

greater differences in geopotential height south of n^30°S. In part, this is due to the poorer

traditional observational synoptic network in the tropics and SH, but it also appears, at

least in part, to be due to the traditional emphasis and experience gained by the centers in

dealing with the more familiar NH circulation so that procedures may have been somewhat

tuned to produce this result. Other problems are revealed by both the mean and RMS

differences over and around Antarctica. Below we comment more specifically on several

aspects.

5.1 Divergent wind

In the tropics, a major problem is related to the intense diabatic heating that

occurs with latent heat release in organized tropical convection over large areas and the

associated vertical motions and divergent winds. We showed that the zonal mean analyzed

divergent wind fields have changed enormously over the seven years from 1980 to 1986,

always increasing in intensity as NNMI procedures have been developed further to include

diabatic heating effects. It is therefore important to recognize that there have been major

discontinuities with time, more so at ECMWF, and that there is still great uncertainty as

73

to the true values. We have used [v] as a measure of the Hadley cell and differences in its

strength in the two analyses have typically been 30 to 100% of the total. This will result

in major uncertainties in any diagnostic analysis for which the divergent wind or vertical

motion fields are important. Generally, these fields appear to have become more realistic

with time (Trenberth and Olson, 1988b), in spite of sometimes larger differences, and thus

the most recent analyzed values are probably best.

5.2 Relative humidity

The RH field is one that has generally received little attention. Regardless of

the values analyzed, the NWP models typically adjust very quickly to make the model

RH values compatible with the large scale dynamics of the synoptic situation and the

cumulus parameterization scheme. Thus the analyzed fields have not been very important

for large-scale long-term forecasting. In addition, the inherent small scales in precipitation

and RH fields means that observations may not be very representative of the larger scales

and thus are given less weight in the analysis. Consequently, especially in the tropics,

it seems that the RH analyses have been dominated by the information coming from

the first guess in the NWP model rather than from the observations. The evidence for

this comes from the huge changes in RH that have occurred when changes have been

made in the model but without any change in the observations available or in the analysis

procedure. The resulting discontinuities are especially pronounced in the ECMWF RH

analyses. Even after somewhat similar changes have been made in the models at both

centers, local differences in monthly mean analyzed RH exceed 40%, so it appears that the

moisture fields are very poorly known. Although there have been some recent reductions

in the zonal mean differences in RH, there is little comfort in this fact in view of the above

discussion and it is evident that the moisture field is not known to a satisfactory degree

for almost any purpose. These problems therefore impact directly on attempts to assess

the atmospheric energetics and hydrological cycle. The main hope lies in increased use

of satellite data on the atmospheric moisture content. For instance, at ECMWF, satellite

precipitable water observations have been used in the analyses since March 1986.

74

5.3 Surface fields

Over the oceans, the surface atmospheric fields are important because of their im-

plications with regard to surface fluxes and thus ocean driving. Differences in P in the

tropics have typically been about as large as the expected signal associated with interan-

nual variability, both in a mean and RMS sense. Uncertainties in the mean westerly wind

component have been -20% of the total, and this would be reflected in uncertainties in the

latent heat flux of as much as 40 W m-2. Over the extratropical oceans, RMS differences

in wind speed exceed -5 m s-1 over the NH oceans from 40 to 50°N in winter and south of

30°S over all the southern oceans, thereby resulting in uncertainties of -25% in the surface

wind stress. A goal of TOGA and WOCE is to markedly reduce these uncertainties and

it is pleasing to see that analysis differences have been reduced during 1986.

Sea level pressures and 1000 mb heights must often be determined using artificial

information to extrapolate below the surface of the earth. It is clear that the procedures for

doing this are quite different at both centers with the result that there are major differences

in the fields where there is orography. This emphasizes the need for an accurate surface

pressure field so that the region below ground can be recognized and the information

screened. Generally such a surface pressure field is not available and the surface pressures

that are archived are of no use. This is because the NWP model surface does not correspond

to the real surface. Quite aside from resolution problems is the fact that the model surface

is usually represented nowadays in spectral space, so that there are spurious ripples in

the surface field. In addition, both centers use an enhanced orography ("envelope" or

"silhouette") as a means of compensating for the fact that the free atmospheric circulation

generally is not influenced by the air that exists in valleys within rough topography. The

envelope orography produces improved dynamical effects in the model but at the expense

of degraded physical effects. We suggest that all models, or post processed representations

of the models, should include a true representation (not spectral, not envelope) of the

orography of the surface of the earth and that this should be a standard archival level for

all data sets.

75

5.4 The Southern Hemisphere

Results have clearly shown, at least prior to May 1986, that the analyses at NMC

have been seriously in error over and near Antarctica. The quality of the analyses south

of ~30°S has also left much to be desired. Sea level pressures and 1000 mb heights were

analyzed to be too high in the vicinity of the circumpolar trough at NMC prior to 1982

relative to both Australian and ECMWF analyses. Moreover, the case study for June

1985 shows that the analyses took little notice of the available observations in this region.

There has always been a serious problem over the SH with the sparsity of observations,

but the implication from this research is that the first order problem has been the inability

to properly assimilate the observations that have been available. Changes at NMC in May

1986 have at least partly corrected the situation over Antarctica, but it remains to be seen

whether the available data are being fully utilized.

RMS differences between NMC and ECMWF continue to be large over the southern

oceans even though, since 1985 as part of the TOGA Program, a drifting buoy program

has been in place to help fill the huge data gaps. Together with the satellite soundings,

these should enable a reasonable analysis to be produced. While the ECMWF analyses

are to be preferred in the SH, the case study also shows that they too are not perfect and

they tend to be quite conservative. The evidence suggests that there is too much weight

placed upon the first guess field from the NWP model at both centers.

The reason for this, of course, is the problems in dealing with isolated station data

from ships and buoys. One problem is that the data are single level data and therefore

hard to properly assimilate. But a major problem is with quality control and the need to

catch the not uncommon errors and biases present in the data. Because of the isolation

of many ship and buoy platforms, it is often not possible to check the data by comparing

values with nearby observations since there is insufficient redundancy. Instead, alternative

kinds of checks must be made. Foremost among these is the need for time series checks on

the position of the platform and the observation sequence. It is also necessary to recognize

that NWP model forecast accuracy is not as good over the SH as in the NH and rapid

deepening rates and very intense low pressure systems are common over the southern

76

oceans (e.g. see Guymer and Le Marshall, 1981; Trenberth and van Loon, 1981).

Isolated observations, by their very nature, are more difficult to handle in any data

assimilation system, yet if correct their value is much greater. The challenge is to ensure

that these data are received, properly checked, and assimilated.

5.5 Heat and momentum fluxes

It was pleasing to see that the differences in the poleward heat fluxes near 700 mb

from the two centers were relatively small. However, larger differences were apparent at

the surface and in the upper troposphere and lower stratosphere, especially south of 30°S.

Stronger poleward heat fluxes in the SH lower stratosphere in the ECMWF analyses, even

in recent months such as October 1986, imply that the dynamics entering into the problem

associated with the depletion of ozone in the SH spring, the "ozone hole", are not well

known. Similarly, uncertainties in the poleward momentum fluxes are also largest in the

SH upper troposphere and are too large to allow definitive studies of wave dynamics or

the momentum budget.

5.6 Concluding remarks

All of the NMC and ECMWF fields are available from the Data Support Section

of NCAR. In addition, all of the ECMWF fields and summary statistics are available on a

Gaussian grid on-line at NCAR via the Community Climate Model (CCM) processor, as

described by Trenberth and Olson (1988b).

There is fairly good agreement between the analyses over the extratropics of the

NH so that some variables, including the rotational wind components, geopotential height

and temperature, can be considered to be reasonably well known there. Other variables,

including the divergent wind, vertical motion, and humidity, are not as well known any-

where. In the tropics we found substantial disagreements in the total wind and all variables

are most uncertain in the SH.

Interannual variability and climate change can only be explored to a limited extent

with these data sets and only if proper account is taken of the impacts of changes in analysis

77

procedures. Consequently a strong case can be made for a reanalysis of the original data,

preferably enhanced with further observations (either delayed or non-real time) that did

not arrive before the operationally imposed cut-off, using the same state of the art analysis

system.

In any case, there is a need to clearly document all changes in the analysis system

that have an impact on the analyses, and thus on the implied climate record. This is now

mostly done at both centers. There is also a need to document the impact of these changes

on the analyses, and this aspect could be improved.

The main approach taken in this note can only show how different the analyses are

and it does not reveal their overall fidelity. Nevertheless, the global analyses are very useful

for many purposes, especially if account is taken of the information in this report and in

Trenberth and Olson (1988a and 1988b) which provide a measure of their uncertainty.

78

References

Bonner, W. D., G. H. White, M. S. Tracton, and V. E. Kousky, 1986: Global analysis

and prediction at NMC Washington. Proc. of Second International Conference on

Southern Hemisphere Meteorology, Wellington, New Zealand 1-5 Dec. 1986. Amer.

Meteor. Soc., Boston, MA, pp. 1-9.

Guymer, L. B., and J. F. Le Marshall, 1981: Impact of FGGE buoy data on Southern

Hemisphere analyses. Bull. Amer. Meteor. Soc., 62, 38-47.

Harrison, D. E., W. S. Kessler and B. S. Giese, 1988: Ocean circulation model hindcasts

of the 1982-83 El Ninio: Thermal variability along the ship of opportunity tracks.

J. Phys. Oceanogr., submitted.

Kidson, J. W., and K. E. Trenberth, 1988: Effects of missing data on estimates of monthly

mean general circulation statistics. J. Climate, submitted.

Lau, N-C., 1984: A comparison of circulation statistics based on FGGE Level III-b analyses

produced by GFDL and ECMWF for the special observing periods. NOAA Data

Rep. ERL GFDL-6, 237 pp.

1985: Publication of circulation statistics based on FGGE Level III-b analyses pro-

duced by GFDL and ECMWF. Bull. Amer. Meteor. Soc., 66, 1293-1301.

Leetmaa, A., 1987: Progress towards an operational ocean model of the tropical Pacific at

NMC/CAC. In "Further progress in Equatorial Oceanography", E. J. Katz and J.

M. Witte Editors, Nova University Press. 439-450.

Trenberth, K. E., and J. R. Christy, 1985: Global fluctuations in the distribution of atmo-

spheric mass. J. Geophys. Res., 90, 8042-8052.

1987a: The zonal mean westerlies over the Southern Hemisphere. Mon. Wea. Rev.,

115, 1528-1533.

_ 1987b: The role of eddies in maintaining the westerlies in the Southern Hemisphere

winter. J. Atmos. Sci., 44, 1498-1508.

_ , J. R. Christy and J. G. Olson, 1987: Global atmospheric mass, surface pressure and

water vapor variations. J. Geophys. Res., 92, 14815-14826.

79

and J. G. Olson, 1988a: Evaluation of NMC global analyses. NCAR Tech. Note

NCAR/TN-299+STR. 82 pp.

and , 1988b: ECMWF Global Analyses 1979-1986: Circulation statistics and

data evaluation. NCAR Tech. Note NCAR/TN-300+STR. 94pp plus 12 fiche.

,and H. van Loon, 1981: Comment on "Impact of FGGE Buoy data on Southern

Hemisphere analyses". Bull. Amer. Meteor. Soc., 62, 1486-1489.

80

Appendix IAcronyms

CCM Community Climate Model

ECMWF European Centre for Medium Range Weather Forecasts

E-P Eliassen-PalmFGGE First GARP Global Experiment

GARP Global Atmospheric Research Program

GDAS Global Data Assimilation System

GFDL Geophysical Fluid Dynamics Laboratory

NCAR National Center for Atmospheric Research

NH Northern HemisphereNMC National Meteorological CenterNNMI Nonlinear Normal Mode Initialization

NOAA National Oceanic and Atmospheric Administration

NWP Numerical Weather PredictionPAOBs Pseudo Australian OBservations of SLP

RH Relative HumidityRMS Root Mean SquareSLP Sea level pressure

SH Southern HemisphereTOGA Tropical Oceans Global Atmosphere

WOCE World Ocean Circulation Experiment

81