Large-eddy simulation of the VOCALS RF06 pocket of open cells: cloud macrostructure, entrainment and microphysical timescale

The VAMOS Ocean-Land-Atmosphere Study (VOCALS) REx field campaign sampled several excellent cases of pockets of open cells (POCs) embedded in a fully cloud-covered stratocumulus layer, most notably NSF C-130 flight RF06, which sampled across the boundary of a well defined POC between 0500 and 1000 local time on October 26th, 2008. We present the results of a series of large-eddy simulation (LES) studies of RF06, examining the role of microphysical gradients in droplet concentration Nc on cloud macrostructure and entrainment.

The SAM LES of Marat Khairoutdinov was run using CAM radiation and Morrison (2005) microphysics, with cloud droplet concentration Nc treated either as a fixed distribution or as an advected scalar without microphysical sources and sinks. The simulations were initialized with soundings constructed from aircraft data and large-scale subsidence from NCEP reanalysis. The simulations were initialized based on the aircraft data with Nc = 60 cm^-3 within the overcast region and 10 within the POC region; the advected case sets Nc equal to 40 in the free troposphere. A doubly periodic ‘bowling alley' domain with horizontal dimensions of 192km x 24km is used to simulate a transect across the POC. The horizontal resolution is 125m and vertical resolution varies from 30m near the surface to 5m in a layer across the inversion, then stretching above. The 18 hr runs start at 2200 local time and include diurnally-varying radiation.

Mesoscale circulations rapidly develop within the domain, with low level outflow from the POC to overcast regions and inflow near the top of the boundary layer from the overcast region into the POC. Drizzle cells develop within the POC and along its boundaries, consistent with observations. Actual precipitation amounts are somewhat less than observed and too much thin cloud remains inside the POC. The difference in cloud properties and precipitation across the POC edge results in entrainment in the overcast region twice as large as inside the POC, yet throughout the simulation, the inversion height inside the POC remains within 25m of that in the overcast region because of the development of buoyancy-driven mesoscale circulations that reduce subsidence into the POC compared to the overcast region. A pair of separate identically-initialized and forced horizontally-homogeneous simulations with different Nc representing either the POC or the overcast evolve different inversion heights and have different entrainment rates, confirming the dynamical coupling across the POC boundary. Sensitivity tests treating Nc as an advected scalar show that the mesoscale circulations take 12 hours to significantly mix out the initially prescribed Nc gradient, before which the overall evolution is remarkably similar to the fixed-Nc cases. We are working towards implementing a simplified interactive aerosol scheme within SAM to simulate the mechanisms that form and maintain the POC-edge microphysical boundary.