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Tropical Mid-Troposphere Clouds
A new look at
P. Zuidema, B. Mapes, J. Lin, C. Fairall
RSMAS/MPO
Miami, FL
CIRES/CDC NOAA/ETL
Boulder, CO
Why….”a new look” ???
2 recent tropical convective oceanic field campaigns included
co-located cloud and Doppler precipitation radar
New observations and data analysis provide a fresh look
into deep convective processes and its associated cloudiness
TOGA COARE:
3rd cloud population w/
cloud top heights
between 5-9 km.
Johnson et al, 1999:
Trimodal characteristics of convection
Linked to:
- melting-level stable layers
- dry air intrusions from extratropics
Cloud vertical structure inferred indirectly
(precip radar & sondes)
1 m/s bins
Eastern Pacific Investigation of Climate (EPIC)
Sept. 2001, 10N&95W
(Raymond et al., 2004, BAMS)
Joint Air-Sea Monsoon Interaction Experiment
(JASMINE) May 1999 (monsoon onset),
Bay of Bengal (Webster et al., 2002, BAMS)
40N
Upper wind
Low-level wind
equator
Height (km)
Cloud radar:
• attenuates with water vapor, liquid and precip
• point measurement but continuous
• ~ 20 dBZ max
JASMINE-convective (10 days)
14
10
6
2
0
EPIC (21 day)
Time series of coincident sonde RH at 95W, 10N
Hurricane Juliette
RHB leaves 10N, arrives 5N
Most mid-tropospheric dry layers related to the southerly phase of the easterly waves
EPIC meridional wind
Episodic southerly
mid-tropospheric advection
of dry air apparent in mean
NCEP 1990-2000 Sept. mean
RH & V along 110 W
Strong southerly
Or
Weak northerly
component
zonal
RH < 30%, 4-8km
Sept. 21:
pronounced 6-7 km dry air layer, south-southwesterly winds with easterly winds above
large-scale (~50 km range) divergence calculated from precipitation radar
shown in blue (solid line=divergence, dotted line=convergence)
• 8-14 km anvil cloud resides above dry layer (suggests sublimation)
• Surface-based convection mitigated
by both 3-5 km and 6-7 km dryness
Surface
Rainfall
rates
• Sublimation supported by
precipitation radar divergence
profile !
- Large-scale process (divergences over a 100 km^2 area) apparently
has a microphysical origin (melting of ice).
convincingly documented thanks to combined analysis of the two radars
-horizontal divergence will act to simultaneously spread and thin
The (top of the) dry layer, which is now cooler and moister
Frequency of
Layers with
Lapse rates
> -5, -4, -3 K/km
Two independent estimates of latent cooling “Q”
from sublimation !
1. Cloud radar IWC flux: (IWC)/z * fall speed*latent heat = Q
2. Horizontal wind divergence = ∂(vertical velocity)/∂p
~ ∂Q/∂p*1/static stability
Relative humidity time series
On to….
JASMINE…
convective
nonconvective
14
Height (km)
10
6
2
0
Mean diurnal cycle of 210K
~15 m/s
Meteosat-5 Infrared Imagery
Dave Lawrence
Zuidema, 2003
JASMINE
squall
(wave?)
Zuidema, 2003
SUMMARY & CONCLUSIONS:
Remarkable consistency between surface rainfall rates, vertically-pointing
cloud radar, and large-scale calculated precipitation radar divergences;
(full time series posted through http://www.rsmas.miami.edu/users/pzuidema)
New mechanism noted by which dry air intrusions influence tropical cloud
vertical structure: anvils snowing into dry layers, moistening and cooling
Dry air intrusions can be an integral dynamical component
(not just a random external forcing)
Implications and ?? for TWP-ICE:
EPIC and JASMINE took place “far” from land - TWP-ICE likely to capture
even more complexity.
What role do dry air intrusions play, if any ? If in evidence, how can models
reproduce their influence on convection and mean climatology ?
Might divergence structures other than convective/stratiform also be seen ?
What cloud overlap constructions best describe the various regions ?
Background JASMINE cloud images courtesy of U. of Washington JASMINE webpage