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)
Clouds types, SST and
wind stress in the
East Pacific
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
NOAA/ETL (surface met., fluxes, mm radar, lidar)
Participants
Participants
UNAM (aerosol concentration and characterization)
Participants
WHOI (CTDs, IMET buoy)
Participants
University of Washington (sondes, 5 cm scanning radar,
meth blue, cloud photos)
EPIC Sc cruise9-24 Oct. 2001
Radiosonde observations
qv
Cloud baseSharp PBL inversion
At Buoy
Figure by Kim Comstock
cld top
cld base
LCL
Cloud top and dBZ (MMCR), base (ceilometer), LCL (surf. met.)
[m s-1] ECMWF VERTICAL VELOCITY
[dBZ]10
-10
0
DiurnalCycle
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
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
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
Mixed layer budgets
LatentEvap
zi
Free trop.
Mixed layerSensibleHeating
qT SL
Precip
EntrainmentLW SW
Net radiative flux divergence
qT SL
Ocean
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
Solar absorption and drizzle evaporative stabilization
zi
Ocean
Solar absorption
SW
Drizzle evaporaton
warmingcooling
Evaporative cooling
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]
Diurnal cycle of
convective velocity
scale
Drizzle and solar radiation reduce
mean w* by comparable amounts
Sample of C-band
scanning radar and coincident
MMCR
dBZ
0530 0730
20 km
Meth BlueRain Rate
Num
ber/
seco
ndZ=58R1.1
Remotely-sensed cloud microphysics
(Rob Wood, UW)
…UNAM also found a strong diurnal signal in submicron aerosol conc.
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
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)
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