Feedbacks between drizzle and open cell cloud structure in the SE Pacific (Bretherton)
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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 Radar reflectivity Mesoscale wind fluctuations related to drizzle cells Radial velocity fluctuations The “POCS” mystery Pockets Of Open Cells (POCS) are frequently observed in otherwise unbroken Sc. POCS Their cause is unknown 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 deepeninginduced Cucoupling 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 airsea temperature difference • Use NCEP reanalysis free-tropospheric temperature and moisture • Iterative solution employed to resulting nonlinear 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 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] 0.05 cm s-1 zi/t + u•zi = we - ws Diurnal cycle of subsidence ws, entrainment we, and zi/t NIGHT DAY NIGHT DAY we ws swe=0.24 cm s-1 sws=0.26 cm s-1 szi/t=0.44 cm s-1 dzi/dt Conclusion: Subsidence and entrainment contribute equally to diurnal cycle of MBL depth