SE Pacific Sc Research at UW

Chris Bretherton, Rob Wood, Peter Caldwell, Kim Comstock

April 23, 2004

Recent work at UW

• Patches of open cells (POCs)• Inferring MBL structure by combining

knowledge from satellites and reanalysis• Diurnal cycle of subsidence,

entrainment.

The first satellite remote sensor - Tiros 1

TV Camera in space

Mesoscale cellular convection

C-band reflectivity and radial velocity Mesoscale

wind fluctuations related to

drizzle cells

Radar reflectivity

Radial velocity

fluctuations

The “POCS” mystery

Pockets Of Open Cells (POCS) are frequently

observed in otherwise unbroken

Sc.

Their cause is unknown

POCS

MODIS 250m visible imagery

100 km

POCS regions drizzle more

Figure by Kim Comstock/Rob Wood

POCS and cloud droplet size

MODIS brightne

ss temperat

ure difference, GOES thermal

IR, scanning

C-band radar

Figure by Sandra Yuter/Rob Wood

Climatology of open

and closed cellular

regions…role of

deepening-induced

Cu-coupling

vs. drizzle?

POCS and drizzle – summary

• POCS last for days, and are associated with large cloud droplet effective radius and enhanced drizzle.

• Patchy drizzle evaporation in POCs drives spreading mesoscale cool pools, reinforcing cellular structure

• Drizzle scavenging promotes aerosol depletion in POCs.

MBL depth, entrainment, and decoupling

Methodology (Wood, Park, Bretherton)

• Independent observables: LWP, Ttop, SST

• Unknowns: zi, q (= )

• Use COADS climatological surface RH and air-sea temperature difference

• Use NCEP reanalysis free-tropospheric temperature and moisture

• Iterative solution employed to resulting non-linear equation for zi

Mean MBL depth (Sep/Oct 2000)

NE Pacific SE Pacific

Mean decoupling parameter q

Decoupling scales well with MBL depth

q vs zi-zLCL

Deriving mean entrainment rates

• Use equation: we=uzi+ws

• Estimate ws from NCEP reanalysis

• Estimate uzi from NCEP winds and two

month mean zi

Mean entrainment ratesEntrainme

nt rate [mm/s]

◄ NE Pacific

SE Pacific ►

Subsidence rate [mm/s]

Cross section

through SE Pacific

stratocumulus sheet

Diurnal subsidence

wave - ECMWF• Daytime dry heating leads to ascent over S American continent

• Diurnal wave of large-scale ascent

propagates westwards over the SE Pacific at

30-50 m s-1

• Amplitude 0.3-0.5 cm s-1

• Reaches over 1000 km from the coast,

reaching 90W around 15 hr after leaving

coast

EPIC 2001 [85W, 20S]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

Conclusion: Subsidence and entrainment contribute equally to

diurnal cycle of MBL depth