Regional and Interannual Comparisons of Marine Stratocumulus Precipitation Detected Using an AMSR-E 89-GHz Passive Microwave Based Method

This study examines the diurnal, seasonal, and interannual variability of drizzle occurrence within the three main marine stratocumulus geographic regions: the Southeast Pacific (SEP), the Northeast Pacific (NEP), and the Southeast Atlantic (SEA). Heavily drizzling cells within marine stratocumulus are detected using 89 GHz Advanced Microwave Scanning Radiometer - Earth Observing System (AMSR-E) passive microwave brightness temperature data.

Radiant emission at 89 GHz by liquid-water cloud and precipitation particles from drizzling cells in marine stratocumulus regions yields local maxima in brightness temperature against an otherwise cloud-free background brightness temperature. The background brightness temperature is primarily constrained by column-integrated water vapor and sea surface temperature. Clouds containing ice, which yield a scattering signal, are screened out. Once heavily drizzling pixels are identified, connected pixels are grouped into discrete drizzle cell features. The identified drizzle cells are used in turn to determine several spatial statistics for each satellite scene, including drizzle cell number and size distribution. The 1445 km wide swath width of AMSR-E provides information on mesoscale spatial organization of drizzle that is not available from 1.4 km curtain width of CloudSat.

The entire period of AMSR-E availability (2002-2011) was exploited to analyze precipitation frequency over 3-month time scales using the daytime (≈ 1:30pm LT) and nighttime (≈ 1:30am LT) overpasses separately. The SEP has the most frequent drizzle, up to 30% per 55 km x 52 km area at night for Sept-Oct-Nov, the season where lower-tropospheric static stability is highest. Diurnal and seasonal variability are present in all regions. Additionally, all three regions exhibit interannual variability in drizzle area frequency for a given season. The interannual variability of total precipitation area is larger for SEA and NEP than SEP. The 3-month period with maximum drizzle area during the night coincides with the maximum drizzle area during the day for SEA (Sept-Oct-Nov 2006) and NEP (June-July-Aug 2010) but not for SEP (Sept-Oct-Nov 2010 for night and Sept-Oct-Nov 2005 for day). The environmental controls on marine stratocumulus drizzle are complex and numerous. High cloud droplet number concentrations (> 130 cm^-3) near the western coasts of continents reduce the probability of heavy drizzle except where local coastal winds are convergent. Further from the coast, probability of heavy drizzle increases associated with the joint environmental characteristics of lower cloud droplet number concentration, higher boundary layer depth and higher sea surface temperatures.