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A new look atA new look at – – Tropical Mid-Troposphere Tropical Mid-Troposphere
CloudsClouds P. Zuidema, B. Mapes, J. Lin, C. P. Zuidema, B. Mapes, J. Lin, C.
FairallFairall CIRES/CDC NOAA/ETL Boulder, CO
RSMAS/MPO Miami, FL
Why….”a new look” ???
2 recent tropical convective oceanic field campaigns includedco-located cloud and Doppler precipitation radar
New observations and data analysis provide a fresh lookinto deep convective processes and its associated cloudiness
TOGA COARE: TOGA COARE: 3rd cloud population 3rd cloud population
w/ cloud top w/ cloud top heights between 5-heights between 5-9 km.9 km.
Johnson et al, 1999: Trimodal characteristics of convection
Cloud vertical structure inferred indirectly (precip radar & sondes)
Linked to: - melting-level stable layers- dry air intrusions from extratropics
m/s bins
Eastern Pacific Investigation of Climate (EPIC) Sept. 2001, 10N&95W (Raymond et al., 2004, BAMS)
equator
40N
Low-level wind
Upper wind
Joint Air-Sea Monsoon Interaction Experiment (JASMINE) May 1999 (monsoon onset), Bay of Bengal (Webster et al., 2002, BAMS)
Cloud radar: • attenuates with water vapor, liquid and precip• point measurement but continuous• ~ 20 dBZ max
EPIC (21 day)JASMINE-convective (10 days)
Heig
ht (k
m)
14
6
10
2
0
Melting-level outflow
Hurricane JulietteRHB leaves 10N, arrives 5N
Time series of coincident sonde RH at 95W, 10N
EPIC meridional wind
Most mid-tropospheric dry layers related to the southerly phase of the easterly waves
meridional
zona
l
RH < 30%, 4-8km
Strong southerlyOrWeak northerlycomponent
Episodic southerly mid-tropospheric advection of dry air apparent in mean
NCEP 1990-2000 Sept. meanRH & V along 110 W
Sept. 21: pronounced 6-7 km dry air layer, south-south-westerly winds with easterly winds above
• 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
SurfaceRainfallrates
• Sublimation supported byprecipitation radar divergenceprofile !
large-scale (~50 km range) divergence calculated from precipitation radarshown in blue (solid line=divergence, dotted line=convergence)
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
- Large-scale process (divergences over a 100 km^2 area) apparentlyhas a microphysical origin (melting of ice). convincingly documented thanks to combined analysis of the two radars
-horizontal divergence will act to simultaneously spread and thinThe (top of the) dry layer, which is now cooler and moister
Frequency of Layers withLapse rates> -5, -4, -3 K/km
convective
Heig
ht (k
m)
14
6
10
2
0
nonconvective
Relative humidity time seriesOn to….JASMINE…
Meteosat-5 Infrared Imagery
Mean diurnal cycle of 210K
Zuidema, 2003
~15 m/s
Dave Lawrence
JASMINE JASMINE squallsquall(wave?)(wave?)
prevailing wind
Zuidema, 2003
SUMMARY & CONCLUSIONS:Remarkable consistency between surface rainfall rates, vertically-pointingcloud 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 cloudvertical structure: anvils snowing into dry layers, moistening and cooling
Background JASMINE cloud images courtesy of U. of Washington JASMINE webpage
Implications and ?? for TWP-ICE:EPIC and JASMINE took place “far” from land - TWP-ICE likely to captureeven more complexity.
What role do dry air intrusions play, if any ? If in evidence, how can modelsreproduce 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 ?
Dry air intrusions can be an integral dynamical component (not just a random external forcing)