Lagrangian Analysis of Tropical Cirrus and Upper-Tropospheric Humidity Z. JOHNNY LUO City College of...
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Transcript of Lagrangian Analysis of Tropical Cirrus and Upper-Tropospheric Humidity Z. JOHNNY LUO City College of...
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