Airborne Doppler radar observations of the dynamics of marine stratocumulus

David Leon, University of Wyoming

The past two decades have seen a surge in interest in marine stratus/stratocumulus driven in large part by the forcing that these cloud decks exert on global climate. Research into these cloud systems has been conducted using numerical simulations as well as numerous field campaigns. The DYCOMS-II project was intended to provide a dataset for testing Large-Eddy Simulations (LES) as well as extending previous observations by including a suite of new fast-response instruments. The field campaign took place off the coast of southern California in July 2001 using the NCAR/NSF C-130 aircraft and consisted of seven nighttime and two daytime flights. The cloud deck was essentially unbroken during all flights. Precipitation varied widely from non-drizzling to cases with strong drizzle through the boundary layer.

Included among the suite of instruments on the C130 was the Wyoming Cloud Radar in a dual-beam downward-looking configuration. This configuration allows the retrieval of two-dimensional particle velocities in a vertical plane below the aircraft. Results of the dual-Doppler analysis show evidence of cellular circulations with a horizontal scale of 4-5 km. These circulations are clearly evident in the heavily strongly drizzling cases, and may also exist in the weakly and non-precipitating cases. Such circulations have been hypothesized to play a role in organizing and maintaining the entrainment of air from above the boundary layer. In these models entrainment occurs preferentially in the downward branch of the circulations, negative buoyancy resulting from the evaporation of cloud water then strengthens the circulation. However, these circulations have not previously been observed directly.

The cases presented will include both strongly drizzling and non-drizzling cases. The presentation will be skewed towards the stronger-drizzle cases for the simple reason that extensive drizzle allows the WCR to retrieve the velocity field below cloud base. However, the heavy, uniform drizzle that makes cases such as RF07 ideal for investigations using the WCR is not representative of the DYCOMS-II dataset as a whole. Thus, comparisons between the weakly- and strongly-drizzling cases will also be presented.