Lagrangian Analysis of Tropical Cirrus and Upper-Tropospheric Humidity

Z. JOHNNY LUOCity College of New York, CUNY

Acknowledgments

Dr. William RossowDr. Graeme StephensDr. Thomas Vonder Haar Dr. Richard Johnson Dr. Dieter Kley

And many others…

Tropical Cirrus

Tropical cirrus are very different from cirrus we see in midlatitude

It started from my Ph.D. research

Bill’s original plan for my thesis - “Covariability of tropical cirrus and upper-tropospheric water vapor”

What this meant was not immediately clear to me back then.

Motivation

Climatology of tropical deep convection, cirrus and UTWV shows that they are highly correlated with each other (e.g. Soden and Fu 1995).

But from these monthly mean maps, we can’t infer any cause-effect relationship or processes responsible for the good correlation. Eulerian Lagrangian

Satellite retrieval algorithm development

The split-window (11 & 12 m) to retrieve cirrus cloud-top height and emissivity (optical depth)

SSM/T2 (183 GHz) to retrieve upper-tropospheric humidity (tuned up to 6.7-m radiances in clear sky) 3-hourly deep convection data (ISCCP IR)

Starting from where convection dies out, continue to follow the large-scale UT air trajectory (as determined from NCEP/NCAR analysis) for 5 days.

The rationale is to sample the transition from deep convection to cirrus anvil to thin cirrus, as well as the associated variation of UTH along the trajectory.

Luo and Rossow (2004)

Lagrangian Trajectories

ISCCP Cloud Classification

Day 0 Day 1

Day 2 Day 3

Day 4 Day 5

The decay of deep convection is immediately followed by the growth of cirrostratus and cirrus, and then the decay of cirrostratus is followed by the continued growth of cirrus.

Cirrus CirrostratusC

loud

Am

ount

Clo

ud t

op (

mb)

Tau

Time from convection (days)

Separating detrained cirrus from in situ cirrus

For each individual trajectory (which starts where convection dies), we track the detrained cirrus until they go to zero. All other cirrus that have no direct connection to convection are called in situ cirrus.

Averaged over the whole tropics, about half of the cirrus are formed in situ well away from convection.

So, Lindzen’s iris hypothesis, even if 100% true, only applies to half of the tropical cirrus

Histogram of tropical cirrus lifetime

Lifetime (hr) 48

Detrained Cirrus

In Situ Cirrus

Cirrus Cases

Clear Cases

Upp

er t

ropo

sphe

ric h

umid

ity (

%)

Time from convection (days)

Relationship between cirrus and UTWV

Cirrus Cases

Clear Cases

Upp

er t

ropo

sphe

ric h

umid

ity (

%)

Time from convection (days)

This difference is 1-2 orders of magnitude greater than what cirrus can provide.

The most likely mechanism for the moistening is dynamic transport.

PDFs of UT vertical velocity for cirrus (solid) and clear (dashed) cases

Clear

Cirrus

All histograms are for clear-sky UTH, but 3 types of clear sky:

1) one that has upstream cirrus (solid),

2) one that has downstream cirrus (dashed)

3) one that stays clear for the past and future (dotted).

CSU

GISS

Continue with UTH topics but use in situ measurements this time

MOZAIC (Measurements of Ozone and

Water Vapour by Airbus In-Service Aircraft) sponsored by the European Union

1994 ~ present

Luo et al. (2007)

Flying between 300-200 hPa

Bimodal distribution of UTH: What does this tell us about tropical upper troposphere?

UTH (upper-tropospheric

humidity) histogram

For each aircraft measurement, we track backward in time to see how long the air parcel has traveled since its last exposure to deep convection.

Close to convection

Far from convection

Now things start to make sense: the moist mode is due to convetive moistening whereas the dry mode is due to subsidence drying.

UTH histograms

0 10050 1000 50

Close to convection

Far from convection

Now things start to make sense: the moist mode is due to convetive moistening whereas the dry mode is due to subsidence drying.

UTH histograms

0 10050 1000 50

Close to convection

Far from convection

UTH histograms

Close to convection

Far from convection

Now things start to make sense: the moist mode is due to convetive moistening whereas the dry mode is due to subsidence drying.

UTH histograms

Nawrath 2002

Conceptual model of water vapor evolution (Nawrath 2002)

UTH decreases slower when there is cirrus

Fli

gh

t le

vel

(hP

a)

RHi

Vertical structure of UTH

Luo et al. (2007)

Luo et al. 2008

Ice super-saturation

“Mysterious” moistening

Most prevalent flight level: 238 & 263 hPa

Temperature

Specific humidity (normalized)

Relative humidity

Summary

Lagrangian analysis of satellite data adds new insights into our understanding of tropical cirrus and UTH:

1. Decay of deep convection is followed by growth of thick & thin cirrus and then the decay of thick cirrus is followed by the continued growth of thin cirrus

2. About half of the tropical cirrus are formed in situ having no direct connection with convection

3. Cirrus does not moisten the UT. Rather, it is dynamic transport that does the job and makes cirrus.

Analysis of long-term aircraft in situ measurements of UTH:

1. Consistent with satellite analysis concerning the relationship b/w cirrus and UTH

2. Comparison with ECMWF analysis reveals possible model deficiency in representation of cloud and convection.

Summary

Lagrangian analysis of satellite data adds new insights into our understanding of tropical cirrus and UTH:

1. Decay of deep convection is followed by growth of thick & thincirrus and then the decay of thick cirrus is followed by the continued growth of thin cirrus

2. About half of the tropical cirrus are formed in situ having no direct connection with convection

3. Cirrus does not moisten the UT. Rather, it is dynamic transport that does the job and makes cirrus.

Analysis of long-term aircraft in situ measurements of UTH:

1. Consistent with satellite analysis concerning the relationship b/w cirrus and UTH

2. Comparison with ECMWF analysis reveals possible model deficiencies in the representations of cloud and convection.

Thank you!

Contact: Johnny LuoDept. Earth & Atmospheric SciencesCity College of New York, CUNY

luo@sci.ccny.cuny.edu