Observations of the Structure of Heavily Precipitating Marine Stratocumulus

Abstract

The second research flight of the Second Dynamics and Chemistry of Marine Stratocumulus (DYCOMS-II) field study is analyzed. This case attracted attention because it combined the presence of high drizzle rates with the occurrence of clearings in the cloud cover, which previous work has suggested could be due to a drizzle-induced change in cloud structure. Recent work has named the configuration of these open-cell-like features pocket of open cells (POC). A division of the data, based on the difference of brightness temperature of the 11- and 4-?m channels of the Geostationary Operational Environmental Satellite-10 (GOES-10), is used to condition averages over POC and non-POC sections of the data. Based on this division, significant precipitation is observed almost entirely within the POC. Overall, the observed PBL was markedly energetic and well mixed, commensurate with observations of similarly forced nonprecipitating boundary layers. Regions of elevated equivalent potential temperature, Θe, are encountered in association with the POC and are particularly pronounced below cloud in regions of active precipitation. In the same areas, evaporative cooling and moistening of the subcloud layer air and a marked reduction of the vertical velocity variance below cloud is noted. The POC, and in particular the drizzling areas in the POC, have a mean upward velocity, while the non-POC is associated with descending air. In addition to exhibiting more mesoscale variability the POC differs microphysically from the non-POC: in the POC liquid water amount is higher, cloud droplet number concentration lower, and the effective radii larger. Factors maintaining regions of elevated Θe are discussed, and targets for future modeling or observational studies are identified.