Pockets of Open Cells and Drizzle in Marine Stratocumulus

In order to better understand the role of precipitation in cloud dynamics we will discuss a flight with heavy drizzle as observed during the dynamics and chemistry of marine stratocumulus (DYCOMS-II) field study. This experiment took place in July 2001 off the coast of California and mainly consisted of flights in nocturnal marine stratocumulus. In a previous study we showed that in general during DYCOMS-II higher drizzle rates could be associated with smaller spatial scales, that is the flight averaged precipitation rate was mainly determined by heavily drizzling cores. Those observations are consistent with the idea that heavy drizzle can induce a transition in cloud structure.

Serendipitously a change in cloud structure was probed during the second flight of DYCOMS-II. Visual inspection of the reflectivity data as measured by the Wyoming cloud radar data reveal that the areas of depleted cloud regions are surrounded by 'walls' of heavy precipitation. We named those areas, resembling open cellular convection, pockets of open cells or POCs for short.

Compositing the data into POCs versus 'nonPOCs' (that is, more stratiform cloud regions) based on a satellite derived threshold clearly establishes the relation between drizzle and the POCs. Precipitation is moderate to heavy (more than 1 mm per day) in the POCs while it is insignificant in the nonPOCs at the same time. The analysis further shows the POCs in the sub cloud layer to be colder and higher in moisture due to the evaporation of drizzle. Consistent with more cumulus like convection the POCs can be associated with ascending air in general and more specifically with ascending air in the drizzling parts of the POCs.