1. SE Pacific Sc Research at UW
Chris Bretherton, Rob Wood, Peter Caldwell, Kim Comstock
April 23, 2004

2. Recent work at UW Patches of open cells (POCs)
Inferring MBL structure by combining knowledge from satellites and reanalysis
Diurnal cycle of subsidence, entrainment.

3. The first satellite remote sensor - Tiros 1
TV Camera
in space
Mesoscale cellular
convection

4. C-band reflectivity and radial velocity
Mesoscale wind fluctuations related to drizzle cells
Radial velocity fluctuations
Radar reflectivity

5. The “POCS” mystery
Pockets Of Open Cells (POCS) are frequently observed in otherwise unbroken Sc. Their cause is unknown
POCS

6. MODIS 250m visible imagery
100 km

7. POCS regions drizzle more
Figure by Kim Comstock/Rob Wood

8. POCS and cloud droplet size
MODIS brightness temperature difference, GOES thermal IR, scanning C-band radar
Figure by Sandra Yuter/Rob Wood

9. Climatology of open and closed cellular regions… role of deepening-induced Cu-coupling vs. drizzle?

10. 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.

11. MBL depth, entrainment, and decoupling

12. 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

13. Mean MBL depth (Sep/Oct 2000)
NE Pacific SE Pacific

14. Mean decoupling parameter q
Decoupling scales well with MBL depth

15. q vs zi-zLCL

16. 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

17. Mean entrainment rates
Entrainment rate [mm/s] ◄ NE Pacific SE Pacific ► Subsidence rate [mm/s]

18. Cross section through SE Pacific stratocumulus sheet

19. 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 m s-1 • Amplitude cm s-1 • Reaches over 1000 km from the coast, reaching 90W around 15 hr after leaving coast

20. 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