Understanding the Importance of Microphysics and Macrophysics for Warm Rain in Marine Low Clouds. Part I: Satellite Observations

The importance of macrophysical variables [cloud thickness, liquid water path (LWP)] and microphysical variables (effective radius r(e), effective droplet concentration N-eff) on warm drizzle intensity and frequency across the tropics and subtropics is studied. In this first part of a two-part study, Moderate Resolution Imaging Spectroradiometer (MODIS) optical and CloudSat cloud radar data are used to understand warm rain in marine clouds. Part 11 uses simple heuristic models. Cloud-top height and LWP substantially increase as drizzle intensity increases. Droplet radius estimated from MODIS also increases with cloud radar reflectivity (dBZ) but levels off as dBZ > 0, except where the influence of continental pollution is present, in which case a monotonic increase of r(e) with drizzle intensity occurs. Off the Asian coast and over the Gulf of Mexico, r(e) values are smaller (by several mu m) and N-eff values are larger compared to more remote marine regions. For heavy drizzle intensity, both r(e) and N-eff values off the Asian coast and over the Gulf of Mexico approach r(e) and N-eff values in more remote marine regions. Drizzle frequency, defined as profiles in which maximum dBZ > -15, increases dramatically and nearly uniformly when cloud tops grow from 1 to 2 km. Drizzle frequencies exceed 90% in all regions when LWPs exceed 250 g m(-2) and N-eff values are below 50 cm(-3), even in regions where drizzle occurs infrequently on the whole. The fact that the relationship among drizzle frequency, LWP, and N-eff is essentially the same for all regions suggests a near universality among tropical and subtropical regions.