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Robert Wood, Atmospheric Sciences,
University of Washington
The importance of precipitation in marine boundary layer cloud
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
ERBE net cloud forcing
SST anomaly
from zonal mean
ISCCP inferred St/Sc amount
Tropical-subtropical
general circulation
from Randall et al., J. Atmos. Sci., 37, 125-130,
1980
cold SST
warm SST
SST and wind stress
coupled ocean-atmosphere GCM
Prescribed ISCCP clouds
Model clouds
Climatology
from Gordon et al. (2000)
Clouds in climate models
- change in low cloud
amount for 2CO2
from Stephens (2005)
GFDL
CCM
model number
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
Fraction of precipitation reports indicating “drizzle”
0% 10% 20% 30% 40% 50% >50%
Drizzle is prevalent form of precip. in MBL cloud regions
Field campaigns with focus on low clouds
ISCCP stratus/stratocumulus cloud amount
The southeast
Pacific
Low cloud amount (MODIS,
Sep/Oct 2000)
Mean MBL depth
Mean cloud fraction
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
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?
EPIC Sc.
Wood et al. (2004)
SST (TMI)& winds
(Quikscat)
visible reflectance(MODIS)
Diurnal cycle and drizzle
Ceilometer cloud baseSurface-derived LCL
Quantification of drizzle
Quantifying drizzle
Z-R relationships derived using MMCR
are then applied to the scanning C-band
radar
Marshall-Palmer
Quantifying drizzle
Structural properties of precipitating
stratocumulus
20 km
u
10 km
Mesoscale dynamics
-10 -5 0 5 10 15
-3 -2 -1 0 1 2 3
dBZ
VRAD [m s-1]
1.5 km
0 10 20 30 [km]
23:09 UTC
23:18 UTC
Animation of scanning C-band radar
30 km
mean wind
Echo Tracking
Comstock et al. (2004)
Structure and evolution of drizzle cells
• Drizzle cell lifetime 2+ hours
• Time to rain out < ~ 30 minutes
• Implies replenishing cloud water
Time to reflectivity peak (hours)
Average cell reflectivity (dBZ)15
10
5
-1.5 -1 -0.5 0 0.5 1 1.5
Comstock et al. (2004)
Can drizzle affect MBL dynamics?
What controls drizzle production?
Summary of drizzle observations from previous field programs
Open Cells Closed CellsS
atel
lite
Shi
p R
adar
Drizzle and cloud macrostructure
MODIS brightness temperatu
re difference
(3.7-11 m), GOES thermal IR, scanning C-band radar
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
Future directions
• Broaden the scope of EPIC using a combination of satellite remote sensing, reanalysis, and buoy data (NSF funded, 2004-2007)
• 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
Fraction of areal mean precipitation
observed
How long do we need to average?