1. EPIC 2001 SE Pacific Stratocumulus Cruise 9-24 October 2001 Rob Wood, Chris Bretherton and Sandra Yuter (University of Washington) Chris Fairall, Taneil Uttal (NOAA/ETL) Bob Weller (Woods Hole OI)

2. Clouds types, SST and wind stress in the East Pacific

3. 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

4. NOAA/ETL (surface met., fluxes, mm radar, lidar)
Participants
NOAA/ETL (surface met., fluxes, mm radar, lidar)

5. UNAM (aerosol concentration and characterization)
Participants
UNAM (aerosol concentration and characterization)

6. Participants
WHOI (CTDs, IMET buoy)

7. Participants
University of Washington (sondes, 5 cm scanning radar, meth blue, cloud photos)

8. EPIC Sc cruise
9-24 Oct. 2001

10. Radiosonde observationsq qv
Cloud base
Sharp PBL inversion
At Buoy
Figure by Kim Comstock

11. Cloud top and dBZ (MMCR), base (ceilometer), LCL (surf. met.)
cld top
cld base
LCL

12. [m s-1]
ECMWF VERTICAL VELOCITY
Diurnal
Cycle 10
[dBZ]
-10

13. Large scale observations of the diurnal cycle of low cloud (cont.)
SE PACIFIC [85W, 20S],
LOCAL TIME [hr]
LWP [g m-2]
TRMM TMI (microwave imager) observations of LWP Wood, Bretherton, and Hartmann (2002)

14. Geographical variations
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 hr showing diurnal cycle of insolation is key modulator
Summer max. in amplitude
MEAN LWP
AMPLITUDE [fraction]
TIME OF MAXIMUM LWP
150
100 50
0.4
0.3
0.2
0.1
0.0 12 6 18

15. Diurnal cycle of subsidence ws, entrainment we, and zi/t
 0.05 cm s-1
zi/t + u•zi = we - ws
NIGHT DAY NIGHT DAY we
dzi/dt ws
swe=0.24 cm s-1; sws=0.26 cm s-1; szi/t=0.44 cm s-1

16. Mixed layer budgets Net radiative flux divergence Free trop. LW SW
Entrainment
DqT
DSL zi qT SL
Mixed layer
Latent
Evap
Sensible
Heating
Precip
Ocean

17. Estimating budget terms
cloud radar, ECMWF subsidence,
+ MODIS BL depth climatology
3hr radiosonde profiles, observed LWP
and Fu-Liou radiation scheme
Advection
terms estimated using NCEP and ECMWF reanalyses
Storage terms estimated using sonde profiles
Entrainment LW SW zi
bulk flux algorithm using
Ron Brown met measurements
cloud radar +
microphysical model LE SH
Rain
Ocean

18. Solar absorption and drizzle evaporative stabilization
Drizzle evaporaton SW zi
Evaporative cooling
cooling
warming
Ocean

19. Drizzle + solar decoupling?
Strong TKE source in cloud
Solar
Evaporating drizzle
suppresses TKE
below cloud
Drizzle
height [m]
Local time
Buoyancy flux

20. Diurnal cycle of convective velocity scale
Drizzle and solar radiation reduce mean w* by comparable amounts

21. Sample of C-band scanning radar and coincident MMCR
20 km
dBZ
0530
0730

22. Number/second
Meth Blue
Rain Rate
Z=58R1.1

23. Remotely-sensed cloud microphysics
(Rob Wood, UW)
…UNAM also found a strong diurnal signal in submicron aerosol conc.

24. MODIS 10/16/2001; 10:00 Local (16:00 UTC)
cm-3
g m-2 mm
SHIP
Liquid water path
Effective radius
Droplet concentration
250 km

25. Comparison of 6-day mean 20S 85W profiles with models
(Peter Caldwell, UW)
All models (esp CAM) have too shallow a PBL.
CAM2 LWC all in lowest 3 levels ( m).
Observed LWC mainly at m.

26. 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