NERC Centre for Global Atmospheric ModellingDepartment of Meteorology, University of Reading

Parametrization and resolution issues raised by tropical variability

on diurnal to seasonal timescales

Julia Slingo, Peter Inness, Richard Neale and Gui-Ying Yang

OR

‘THE TALE OF TWO ERRORS!’

– The Maritime Continent– The Madden-Julian Oscillation

TOOLS

• Integrations of the Met Office Unified Model

-HadAM3 AMIP II (observed SST, 1979-95)-Aquaplanet version of HadAM3 -HadCM3 Control

• CMAP Precipitation data

• High resolution (0.50, 3 hourly) window brightness temperature data from the EU Cloud Archive User Service (CLAUS)

Typical window brightness (K) image from the CLAUS dataset12z 1 January 1992

Annual Mean Precipitation (CMAP) and AMIP II Mean Model Errors

JMA

ECMWF

Climatology NCAR

NCEP

Met Office

Topography of the Maritime Continent

Resolving the Maritime Continent in GCMs

Annual Mean Precipitation Errors in HadAM3:Sensitivity to Horizontal Resolution

HadAM3 Sensitivity Experiments: Impact of removing the islands of the Maritime Continent

(Neale and Slingo, 2001: Submitted to J. Clim.)

•Land grid-points removed and replaced by ocean grid-points.•Increased moisture availability from the sea surface leads to enhanced convection and partial correction of the model dry bias.•Note also corrections to model’s wet bias in adjacent areas.

Global Impacts of Improved Maritime Continent Heat Source

DJF: 500hPa height (m) and Surface Temperature (K)

•Potential improvements in the Maritime Continent heat source can have significant remote effects.

•Related to the generation of Rossby waves by the enhanced divergent outflow from the Maritime Continent heat source.

•Substantially reduces model systematic error over the extra-tropics of the winter hemisphere.

•Emphasizes the importance of considering the global context of model systematic error in which biases in the tropics may be a key factor.

The Diurnal Cycle in the Tropics(Yang and Slingo, 2001: MWR, 129, 784-801)

Amplitude (K) of the diurnal harmonic

DJF

JJA

Phase of diurnal harmonic: Local time of maximum brightness temperature

DJF

JJA

Phase of diurnal cycle showing systematic propagation of convective signal away from the coast

Bay of Bengal, JJA:Implied propagation speed ~15-20 ms-1

?Deep gravity wave

Mexico, JJA:Implied propagation speed ~ 10 ms-1

?Shallower gravity wave associated with land/sea breeze

Rapidly propagating squall lines down Bay of Bengal as observed in JASMINE (Webster et al. 2001)

Maritime Continent, DJF: Evidence of complex land/sea breezes which organize convection for several 100 km

Are sub-gridscale land/sea breezes a crucial component of the energy and hydrological budgets of the Maritime Continent?

Schematic of a sea breeze

Sea breeze has two major impacts:•Convergence along sea breeze front provides additional convective mass flux•Winds associated with land and sea breezes enhance surface fluxes leading to increased moisture supply

CONCLUSION I

• Maritime Continent heat source is a key component of the global climate. Improvements in its simulation may have significant impacts on remote systematic errors.

• Specifically, land/sea breezes may be a crucial part of the energy and hydrological budgets of coastal regions and especially around large island complexes.

• In general, horizontally propagating gravity waves, generated by convection, may be important for organizing convection on larger scales.

Why the MJO is important• Intimately related to active/break cycles of the Australian

and Asian Monsoons• Offers potential to provide extended predictability up to 15-

20 days in tropics • Affects weather over the western US and possibly

elsewhere• Associated westerly wind events generate ocean Kelvin

waves which may significantly modify the evolution and amplitude of El Nino (e.g. 1997)

• Large interannual variability in the activity of the MJO has implications for the predictability of the coupled ocean-atmosphere system

Seoul National University

(200hPa velocity potential, 15-70day filtered, 10S-10N, 1997,1 realization)

Time-longitude cross sections of the simulated 200hPa Velocity potential by AGCMs. The contour interval is 3.0E6 m2s-1 and negative values are shaded. This is only one realization

from the 10 ensemble members.

Reanalysis

Example of MJO simulations from current AGCMs: CLIVAR Intercomparison (Kang et al. 2001)

Sensitivity of the MJO to AGCM vertical resolution(Inness et al., 2001: Clim. Dyn., 17, 777-793.)

Model Levels: L19 vs. L30

Note additional levels in free troposphere

Exploring sensitivity of convective organization to vertical resolution

• A water-covered or “aqua-planet” version of the UM is used to investigate the behaviour of tropical convection when the vertical resolution is doubled.

• Aqua-planet version of the UM chosen because:- Homogeneity of the model allows us to obtain a large sample of convective events over warm SSTs- Removal of the land areas excludes circulations forced by land-sea contrasts. Convective events in

different geographical locations are subject to the same large scale forcings, giving a cleaner comparison.

- Aqua-planet provides a more realistic test of the convection scheme than using a single-column model with idealized boundary and large scale forcing functions.

• Aqua-planet setup:- Zonally symmetric SST distribution, typical of equatorial Indian Ocean/West Pacific warm pool

values.- Incoming solar radiation fixed at zonally symmetric, equinoctial (March) values.- Aqua-planet model integrated for 15 months with both 19 and 30 levels in the vertical. First 3

months of each integration were discarded.

Time-height evolution of convective cloud over 3x3 model grid boxes (7.50 lat. x11.250 long.) centred on the equator

L30

L19

Time-height sections of specific humidity increment (g/kg/day) by the convection scheme

Note periods of moistening in the L30 case – convection is generally a moisture sink.

L30

L19

TOGA-COARE IFA apparent heat source (Q1) and moisture sink (Q2) for suppressed (A) and active (B) periods

(Lin and Johnson, 1996: J. Atmos. Sci., 53, 3367-3383)

A: Suppressed B: Active

Q1

Q2

Note periods of moistening (negative Q2) during suppressed period

Time-height sections of potential temperature lapse rate

Note presence of stronger stable layer between 600 and 400hPa in L30,and similarity with observations.

L30

L19

Observations from TOGA-COARE(Johnson et al. 1999, J. Clim.)

Inferences from aqua-planet model results• When the vertical resolution is increased, the spectrum of tropical cloud types

changes from a bimodal to a tri-modal distribution with a third peak in mid-troposphere near the melting level. Associated with periods when these mid-level congestus clouds are dominant, the detrainment from these clouds significantly moistens the mid-troposphere.

• The appearance of these congestus clouds is shown to be partly due to improved resolution of the freezing level and the convective processes occurring at this level.

• The resulting cloud distribution more closely resembles observations, particularly during the suppressed phase of the MJO when cumulus congestus is the dominant cloud type.

• The moistening of the free troposphere by cumulus congestus clouds acts to precondition the atmosphere for deep convection. This preconditioning may set the timescale for the next active phase of the MJO and thus influence the intraseasonal organization of convection.

Trimodal distribution of convection and cumulus congestus

Many conceptual models of tropical convection are based on a BIMODALcloud distribution, emphasizing shallow “trade-wind” or boundary layer cumuli and deep cumulonimbi.

TOGA COARE results emphasize the dominance of cumulus congestus and point to a TRIMODAL cloud distribution in which the freezing level inversion is the key

From Johnson et al. 1999, J. Clim.

CONCLUSION II

• Vertical resolution in the free troposphere must be adequate to resolve the formation of the freezing level inversion and the cooling associated with melting precipitation

• Convective parametrizations need to represent a TRIMODAL rather than bimodal cloud distribution.

Coupling with the upper ocean: Bringing together the diurnal cycle and the MJO

TOGA-COARE buoy data showed pronounced diurnal variations in skin temperature in excess of 1K are evident, as well as slower variations related to the MJO. Note that the diurnal variations occur only during break (B) periods. Active (A) periods are preceded by a warming on sub-seasonal timescales. (From Anderson et al. 1996, J. Clim. )

Nov. March

B A B

TOGA-COARE observations also suggest that cumulus congestus clouds are most prevalent during light wind conditions in the presence of a strong diurnal cycle in SST. Further, these cloud occur most frequently in the late afternoon suggesting that they are triggered by the diurnal cycle in SST.

Coupling with the upper ocean is

important on diurnal timescales

Cumulus congestus and the diurnal cycle

The MJO and coupling with the ocean: Observations(Woolnough et al., 2000: J. Clim., 13, 2086-2104)

Observations show a coherent relationship between convection and SST. Warm SSTs precede convection by 5-10 days and are the result of weaker winds, reduced LH flux and increased SW flux during suppressed phases of the MJO.

The MJO and coupling with the ocean: Modelling(Inness, personal communication)

CGCM has a propagating convective signal compared with standing oscillation in AGCM. Coherent variations in SST in CGCM

Coupling with the upper ocean is

important for the MJO

BUT intraseasonal SST variations in CGCMs are too small and the MJO signal is still weak:

Is the representation of the upper ocean adequate?

Large freshwater flux sets up a salt stratified barrier layer so that a shallow mixed layer forms which can respond rapidly to flux variations, such as the diurnal cycle in solar radiation. The presence of this barrier layer can potentially provide much stronger local coupling in the warm pool region than is currently found in coupled models which do not resolve the detailed structure of the warm pool upper ocean.

Schematic showing formation of salt barrier layer

(From Anderson et al., 1996: J. Clim)

CONCLUSION III

• Good evidence that MJO and diurnal cycle of cumulus congestus involve coupling with the upper ocean

• To simulate diurnal and intraseasonal variations in SST requires detailed representation of salinity and temperature gradients in the mixed layer

• Need to consider an upper ocean/atmosphere system in which the structure of the upper ocean is adequately resolved

Summary of Key Issues (1)

• Maritime Continent heat source is a major driver of the global circulation yet is poorly represented in GCMs. This complex system of islands gives rise to extensive sea/land breeze circulations which may critically influence the energy budget and hydrological cycle of the region.

• Moistening of the free troposphere by cumulus congestus clouds, which form during the suppressed phase of the MJO, may be crucial for convective preconditioning. This dominant cloud type is not represented in models which generally fail to capture the observed tri-modal distribution of convection.

Summary of Key Issues (2)

• Diurnal cycle in SST is large during suppressed or light wind conditions in the tropics and may be a trigger for cumulus congestus. It may therefore be a crucial part of the energy budget and hydrological cycle of the Warm Pool

• SSTs vary coherently with the MJO in such a manner as to suggest that they are an important component of the eastward propagation and timescale of the MJO.

• Both the diurnal and intraseasonal variations in SST involve detailed changes in the salinity and temperature structure of the mixed layer which cannot be adequately represented in current coupled models.