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?