1. The importance of precipitation in marine boundary layer cloud
Robert Wood, Atmospheric Sciences,
University of Washington

2. Motivation
Marine boundary layer (MBL) clouds cover about 1/3 of the world’s oceans and have an enormous impact on
top-of-atmosphere (TOA) and surface radiation budgets
the general circulation
How clouds change remains one of the major uncertainties in future climate prediction
Until recently, precipitation in MBL clouds was assumed to be of secondary importance – this view is changing

3. SST anomaly from zonal mean
ERBE net cloud forcing

4. ISCCP inferred St/Sc amount

5. Tropical-subtropical general circulation
from
Randall et al.,
J. Atmos. Sci.,
37, , 1980
warm SST
cold SST

6. SST and wind stress coupled ocean-atmosphere GCM
Prescribed ISCCP clouds
SST and wind stress coupled ocean-atmosphere GCM
Climatology
Model clouds
from Gordon et al. (2000)

7. Clouds in climate models - change in low cloud amount for 2CO2
GFDL
Clouds in climate models - change in low cloud amount for 2CO2
CCM
model number
from Stephens (2005)

8. Precipitation in MBL clouds?
Pioneering study by Albrecht (1989)
importance of drizzle in cloud thermodynamics
suggestion of microphysical controls upon cloud coverage/lifetime
Early 1990s saw the development of sensitive radars that can detect even light drizzle (few tenths of a mm/day)
Petty (1995) highlighted prevalence of drizzle in volunteer ship observer reports

9. Fraction of precipitation reports indicating “drizzle”
Drizzle is prevalent form of
precip. in MBL cloud regions
0% % % % 40% 50% >50%

10. Field campaigns with focus on low clouds
ISCCP stratus/stratocumulus cloud amount

11. Low cloud amount (MODIS, Sep/Oct 2000)
The southeast Pacific
Low cloud amount (MODIS, Sep/Oct 2000)
 Mean cloud fraction
 Mean MBL depth

12. The EPIC Stratocumulus study
Part of the East Pacific Investigation of Climate (EPIC) field program
Ship cruise (NOAA R/V Ronald H Brown,10-25 October 2001) under the stratocumulus sheet
Surface meteorological measurements, 3 hourly radiosondes, aerosols
Suite of remote sensors: scanning C-band radar, 35 GHz profiling radar (MMCR), lidar, ceilometer, microwave radiometer
Bretherton et al. (2004), BAMS

13. Drizzle challenges
What is the frequency and strength of drizzle over the subtropical oceans?
What are the structural properties of precipitating MBL cloud systems?
Can drizzle affect cloud dynamics, structure and coverage - how does it do so?
What controls drizzle production in MBL clouds?

14. EPIC Sc. visible reflectance (MODIS) SST (TMI) & winds (Quikscat)
Wood et al. (2004)

15. Diurnal cycle and drizzle
Surface-derived LCL
Ceilometer cloud base

16. Quantification of drizzle

17. Quantifying drizzle Marshall-Palmer
Z-R relationships derived using MMCR are then applied to the scanning C-band radar

18. Quantifying drizzle

19. Structural properties of precipitating stratocumulus

20. u
20 km
10 km

21. Mesoscale dynamics dBZ 0 10 20 30 [km] 23:09 UTC VRAD [m s-1] 1.5 km
dBZ
23:09 UTC
VRAD [m s-1]
1.5 km
23:18 UTC
[km]

22. Animation of scanning C-band radar
30 km
mean wind

23. Echo Tracking
Comstock et al. (2004)

24. Structure and evolution of drizzle cells
Time to reflectivity peak (hours)
Average cell reflectivity (dBZ) 15 10 5
Drizzle cell lifetime 2+ hours
Time to rain out < ~ 30 minutes
Implies replenishing cloud water
This plot shows average reflectivity with time for 8 example cells, plotted with respect to the time at which each cell reached its peak average reflectivity. We didn’t capture the entire lifecycle of any single drizzle cell, but we have parts of the life cycle from different cells From this plot, we can infer that the lifetime of the drizzle cell is a little over 2 hours.
Shallow marine cumulus clouds form quickly, reach a peak reflectivity, and then rain themselves out within minutes. If this were to occur in these clouds, the rain-out time would take about 30 minutes, due to their lower rain rates. In fact, these example cells are actually drizzling during the entire lifecycle captured here (if we can see them on the C-band radar, then they are drizzling). So they are continuing to grow even while they are losing water due to drizzle.
This implies that the clouds are being replenished even as they drizzle.
(time to rain out = LWP/rain rate in kg/m^2 s, LWP ~ 300 g/m^2, R = mm/hr = kg/m^2 s, time = min)
Comstock et al. (2004)

25. Can drizzle affect MBL dynamics?

26. What controls drizzle production?

27. Pollution plumes in the SE Pacific
Chile is world’s largest copper producer
Copper smelting SO2 emissions from Chile (3 Tg yr-1) comparable to total SO2 emissions in Germany
90% of Chilean SO2 emissions from seven smelters!
Andes mountains prevents eastward transport
Put Seasonal
Mean SON

28. How rapidly are CCN lost through coalescence?

30. Summary of drizzle observations from previous field programs

31. Open Cells
Closed Cells
Satellite
Ship Radar

32. Drizzle and cloud macrostructure
MODIS brightness temperature difference ( mm), GOES thermal IR, scanning C-band radar

33. Summary Precipitation is common in MBL clouds
The mean precipitation rates 1 mm day-1 are observed and can have significant thermodynamic impact upon the MBL
Precipitating MBL clouds display interesting mesoscale dynamics that may influence their macroscopic properties
Results suggest that drizzle is modulated by cloud LWP and by cloud droplet number

34. Future directions
Broaden the scope of EPIC using a combination of satellite remote sensing, reanalysis, and buoy data (NSF funded, )
Plan and participate in a more extensive field program in the SE Pacific (VOCALS 2007)
Use Cloudsat (launch summer 2005) to begin to develop climatologies of precipitation in low cloud

35. Fraction of areal mean precipitation observed
How long do we need to average?