Precipitation in Stratocumulus Clouds: Observational and Modeling Results

Abstract

The spatial and temporal variability of precipitating stratocumulus layers is examined using aircraft observations, satellite retrievals of cloud optical depth, and one-dimensional models that include coalescence and a simple representation of layer turbulence. The aircraft observations show large horizontal variations in cloud thickness and precipitation, with local rain rates 4–5 times larger than the replacement moisture flux, and evidence for precipitation scavenging of small cloud droplets. The satellite observations show that, despite this local water loss, the distribution of cloud optical thickness remains nearly constant over the course of a day, indicating that on larger scales precipitation removal and cloud-top entrainment are in approximate balance with the vapor flux. The authors apply analytic and numerical models of steady-state precipitation to the observed microphysical conditions, and find that the models can match the drop size distributions observed during both heavy and light stratocumulus rainfall, but are especially sensitive to the processes governing the growth rate of the smallest drizzle drops.