EPIC 2001 SE Pacific Stratocumulus Cruise

9-24 October 2001Rob Wood, Chris Bretherton and Sandra Yuter

(University of Washington) Chris Fairall, Taneil Uttal (NOAA/ETL)

Bob Weller (Woods Hole OI)

Clouds types, SST and

wind stress in the

East Pacific

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

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

Participants

Participants

UNAM (aerosol concentration and characterization)

Participants

WHOI (CTDs, IMET buoy)

Participants

University of Washington (sondes, 5 cm scanning radar,

meth blue, cloud photos)

EPIC Sc cruise9-24 Oct. 2001

Radiosonde observations

qv

Cloud baseSharp PBL inversion

At Buoy

Figure by Kim Comstock

cld top

cld base

LCL

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

[m s-1] ECMWF VERTICAL VELOCITY

[dBZ]10

-10

0

DiurnalCycle

Large scale observations of the diurnal cycle of low cloud (cont.)

• TRMM TMI (microwave imager) observations of LWP

Wood, Bretherton, and Hartmann (2002)

SE PACIFIC [85W, 20S], 1999-2000

LOCAL TIME [hr]

LW

P [

g m

-2]

0 6 12 18 24

Geographical variationsMEAN LWP

AMPLITUDE [fraction]

TIME OF MAXIMUM LWP

• 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 02-06 hr showing diurnal cycle of

insolation is key modulator

• Summer max. in amplitude

12601812

0.40.30.20.10.0

150

100

50

0

Diurnal cycle of subsidence ws, entrainment we, and zi/t

NIGHT DAY NIGHT DAY

ws

we

dzi/dt

we=0.24 cm s-1; ws

=0.26 cm s-1; zi/t=0.44 cm s-1

zi/t + u•zi = we - ws

0.05 cm s-1

Mixed layer budgets

LatentEvap

zi

Free trop.

Mixed layerSensibleHeating

qT SL

Precip

EntrainmentLW SW

Net radiative flux divergence

qT SL

Ocean

Estimating budget terms

LE

zi

SH Rain

EntrainmentLW SW

Ocean

bulk flux algorithm usingRon Brown met measurements

3hr radiosonde profiles, observed LWP

and Fu-Liou radiation scheme

cloud radar, ECMWF subsidence,

+ MODIS BL depth climatology

cloud radar +microphysical model

• Advectionterms estimated using NCEP and ECMWF reanalyses

• Storage terms estimated using sonde profiles

Solar absorption and drizzle evaporative stabilization

zi

Ocean

Solar absorption

SW

Drizzle evaporaton

warmingcooling

Evaporative cooling

Drizzle + solar decoupling?

Strong TKE source in cloud

Evaporating drizzlesuppresses TKEbelow cloud

Drizzle

Solar

2 5 8 11 14 17 20 23 Local time Buoyancy flux

heig

ht [

m]

Diurnal cycle of

convective velocity

scale

Drizzle and solar radiation reduce mean

w* by comparable amounts

Sample of C-band scanning

radar and coincident

MMCR

dBZ

0530 0730

20 km

Meth BlueRain Rate

Num

ber/

seco

ndZ=58R1.1

Remotely-sensed cloud microphysics

(Rob Wood, UW)

…UNAM also found a strong diurnal signal in submicron aerosol conc.

MODIS 10/16/2001; 10:00 Local (16:00 UTC)

1 3 10 30 100 300 1000 cm-3

0 50 100 150 200 250 300 g m -2

0 5 10 15 20 25 30 mm

Liquid water path Droplet concentrationEffective radius

SHIP

250 km

Comparison of 6-day mean 20S 85W profiles with models

• All models (esp CAM) have too shallow a PBL.• CAM2 LWC all in lowest 3 levels (70-630 m).• Observed LWC mainly at 800-1300 m.

(Peter Caldwell, UW)

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