PRECIPITATION IN STRATOCUMULUS CLOUDS - OBSERVATIONAL AND MODELING RESULTS

The spatial and temporal variability of precipitating stratocumulus la yers is examined using aircraft observations, satellite retrievals of cloud optical depth, and one-dimensional models that include coalescen ce and a simple representation of layer turbulence. The aircraft obser vations show large horizontal variations in cloud thickness and precip itation, with local rain rates 4-5 times larger than the replacement m oisture flux, and evidence for precipitation scavenging of small cloud droplets. The satellite observations show that, despite this local wa ter loss, the distribution of cloud optical thickness remains nearly c onstant over the course of a day, indicating that on larger scales pre cipitation removal and cloud-top entrainment are in approximate balanc e with the vapor flux. The authors apply analytic and numerical models of steady-state precipitation to the observed microphysical condition s, and find that the models can match the drop size distributions obse rved during both heavy and light stratocumulus rainfall, but are espec ially sensitive to the processes governing the growth rate of the smal lest drizzle drops.