Unexpectedly strong convection under an inversion-topped marine boundary layer

Low, warm stratocumulus clouds top the marine boundary layers in eastern subtropical oceans and exert a net radiative cooling effect on the climate. Current models simulate climate over these regions with critical errors. Improving understanding of the aerosol-cloud-precipitation interactions within marine stratocumulus will help improve numerical modeling of cloud radiative properties at a range of model spatial and time scales. The recent VOCALS Regional Experiment (VOCALS REx), held 6 October to 2 December 2008, examined the joint aerosol-cloud-precipitation characteristics of the marine stratocumulus region off the coast of Peru and Chile using a variety of ship and aircraft observations. Instrumentation on the NOAA Ship Ronald H. Brown included a scanning C-band radar, a vertically-pointing W-band cloud radar and a scanning Doppler LIDAR, as well as upper air soundings and a suite of surface meteorological measurements.

Previous cruises to the region have documented drizzle cells organized into mesoscale open cellular structures and their relation to cloud properties and surface fluxes. In addition to drizzle cells, unexpectedly strong convection was also observed by the C-band radar during the VOCALS cruise. These precipitating structures had radar reflectivity values up to 42 dBZ , well defined convergence at low levels and divergent outflow near the inversion altitude. Strong precipitation was observed during 7 independent events during the 34 day cruise. Preliminary analysis of upper air sounding data indicates that the strong convective events were associated with boundary layers that were both moist and deep (1.4-2 km in height). In contrast, the more ‘typical' weaker drizzle events tended to be either drier or shallower, relative to the strong convective cases. We will present observational analysis and high-resolution numerical modeling results to illuminate our progress on unraveling the dynamical and microphysical structures responsible for the strong convective events.