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 Radial velocity fluctuations Radar reflectivity

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 brightness temperature 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=uzi+ws Estimate ws from NCEP reanalysis Estimate uzi from NCEP winds and two month mean zi

Mean entrainment rates Entrainment 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 zi/t + u•zi = we - ws  0.05 cm s-1 NIGHT DAY NIGHT DAY ws we dzi/dt swe=0.24 cm s-1 sws=0.26 cm s-1 szi/t=0.44 cm s-1 Conclusion: Subsidence and entrainment contribute equally to diurnal cycle of MBL depth