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, , 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 2  CO 2 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 dB Z V RAD [m s -1 ] 1.5 km [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) Comstock et al. (2004)

Can drizzle affect MBL dynamics?

What controls drizzle production?

Summary of drizzle observations from previous field programs

Open Cells Closed Cells Satellite Ship Radar

Drizzle and cloud macrostructure MODIS brightness temperature difference (  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, ) 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?