Drizzle in the SE Pacific stratocumulus region

Kimberly Comstock, Robert Wood, Sandra Yuter, and Christopher Bretherton
Department of Atmospheric Sciences
University of Washington

The quantity of precipitation falling from stratocumulus (Sc) clouds is small but important to climate feedbacks. Ship-based radar measurements obtained during EPIC 2001 Sc in the SE Pacific are used to derive several characteristics of the rainfall from drizzling Sc. In previous work, reflectivity to rain rate (Z-R) relationships were derived by combining reflectivity profiles from the shipborne NOAA ETL millimeter-wavelength cloud radar and results from an evaporation-sedimentation model. The model is used to derive information on the mean raindrop radius and drizzle drop concentrations at cloud base and to show how Z-R relationships change with height below the cloud base. This leads to the development of a bi-level Z-R relationship for relating cloud-base reflectivity and surface rain rate. The derived Z-R relationships are used in conjunction with shipborne C-band radar reflectivity data to estimate arealy-averaged precipitation with uncertainties at cloud base and at the surface. Substantial evaporation occurs within the boundary layer. Areally-averaged rain rates are found to be highly intermittent, with most of the accumulation originating from a small fraction of the drizzling area. The observations suggest that rain rate in marine Sc is strongly dependent on cloud liquid water path and inversely correlated with cloud droplet concentration.