epic 2001 se pacific stratocumulus cruise 9-24 october 2001 rob wood, chris bretherton and sandra...
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EPIC 2001 SE Pacific Stratocumulus Cruise
9-24 October 2001Rob Wood, Chris Bretherton and Sandra Yuter
(University of Washington) Chris Fairall, Taneil Uttal (NOAA/ETL)
Bob Weller (Woods Hole OI)
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Clouds types, SST and
wind stress in the
East Pacific
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Goals
• Document cloud and boundary layer structure in the SE Pacific
• Assess the importance of drizzle processes to cloud thickness and extent
• Compare results with other Sc regimes and with large-scale models
• Maintain WHOI IMET buoy at 20S 85W
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NOAA/ETL (surface met., fluxes, mm radar, lidar)
Participants
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Participants
UNAM (aerosol concentration and characterization)
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Participants
WHOI (CTDs, IMET buoy)
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Participants
University of Washington (sondes, 5 cm scanning radar,
meth blue, cloud photos)
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EPIC Sc cruise9-24 Oct. 2001
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Radiosonde observations
qv
Cloud baseSharp PBL inversion
At Buoy
Figure by Kim Comstock
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cld top
cld base
LCL
Cloud top and dBZ (MMCR), base (ceilometer), LCL (surf. met.)
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[m s-1] ECMWF VERTICAL VELOCITY
[dBZ]10
-10
0
DiurnalCycle
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Large scale observations of the diurnal cycle of low cloud (cont.)
• TRMM TMI (microwave imager) observations of LWP
Wood, Bretherton, and Hartmann (2002)
SE PACIFIC [85W, 20S], 1999-2000
LOCAL TIME [hr]
LW
P [
g m
-2]
0 6 12 18 24
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Geographical variationsMEAN LWP
AMPLITUDE [fraction]
TIME OF MAXIMUM LWP
• Strongest diurnal cycle of LWP found in the
regions of low cloud to the west of continents
• Larger amplitudes in southern hemisphere
• Peak LWP at 02-06 hr showing diurnal cycle of
insolation is key modulator
• Summer max. in amplitude
12601812
0.40.30.20.10.0
150
100
50
0
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Diurnal cycle of subsidence ws, entrainment we, and zi/t
NIGHT DAY NIGHT DAY
ws
we
dzi/dt
we=0.24 cm s-1; ws
=0.26 cm s-1; zi/t=0.44 cm s-1
zi/t + u•zi = we - ws
0.05 cm s-1
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Mixed layer budgets
LatentEvap
zi
Free trop.
Mixed layerSensibleHeating
qT SL
Precip
EntrainmentLW SW
Net radiative flux divergence
qT SL
Ocean
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Estimating budget terms
LE
zi
SH Rain
EntrainmentLW SW
Ocean
bulk flux algorithm usingRon Brown met measurements
3hr radiosonde profiles, observed LWP
and Fu-Liou radiation scheme
cloud radar, ECMWF subsidence,
+ MODIS BL depth climatology
cloud radar +microphysical model
• Advectionterms estimated using NCEP and ECMWF reanalyses
• Storage terms estimated using sonde profiles
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Solar absorption and drizzle evaporative stabilization
zi
Ocean
Solar absorption
SW
Drizzle evaporaton
warmingcooling
Evaporative cooling
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Drizzle + solar decoupling?
Strong TKE source in cloud
Evaporating drizzlesuppresses TKEbelow cloud
Drizzle
Solar
2 5 8 11 14 17 20 23 Local time Buoyancy flux
heig
ht [
m]
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Diurnal cycle of
convective velocity
scale
Drizzle and solar radiation reduce
mean w* by comparable amounts
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Sample of C-band
scanning radar and coincident
MMCR
dBZ
0530 0730
20 km
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Meth BlueRain Rate
Num
ber/
seco
ndZ=58R1.1
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Remotely-sensed cloud microphysics
(Rob Wood, UW)
…UNAM also found a strong diurnal signal in submicron aerosol conc.
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MODIS 10/16/2001; 10:00 Local (16:00 UTC)
1 3 10 30 100 300 1000 cm-3
0 50 100 150 200 250 300 g m -2
0 5 10 15 20 25 30 mm
Liquid water path Droplet concentrationEffective radius
SHIP
250 km
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Comparison of 6-day mean 20S 85W profiles with models
• All models (esp CAM) have too shallow a PBL.• CAM2 LWC all in lowest 3 levels (70-630 m).• Observed LWC mainly at 800-1300 m.
(Peter Caldwell, UW)
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Conclusions• A remarkable dataset was gathered documenting both
spatial and diurnal variability of the SE Pacific Sc regime
• Pronounced diurnal cycle, amplified by subsidence wave from S America.
• Mesoscale drizzle cells ubiquitous, especially at night; rain mostly evaporates above surface, sensitive to cloud drop conc.
• Boundary layer deeper than current global models; well-mixed in early evening but near total shut-down of entrainment in early morning due to combination of solar absorption and drizzle evaporation