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Department of Atmospheric Science

Tues., Feb. 7, 3:10 pm, EN6085

Collision-Coalescence and Precipitation Formation in Marine Stratocumulus

Dr. Mikael Witte

National Center for Atmospheric Research


Precipitation in ice-free clouds forms by two processes: condensation and collision-coalescence. A long-standing problem in cloud physics relates to the theoretical minimum in droplet growth rate where neither process is particularly effective (from roughly 30<d<60 µm). This process rate bottleneck, termed the “warm rain problem” in the context of shallow cumulus, has been the subject of study for over 80 years. Numerous solutions have been proposed to open the bottleneck, ranging from ultragiant cloud condensation nuclei to entrainment broadening to direct acceleration of collision-coalescence by turbulence, though none of these has yet proven to definitively “solve” the problem. This talk explores how collision-coalescence is initiated in marine stratocumulus using a combination of in situ aircraft measurements of the cloud drop size distribution (DSD) and large eddy simulations (LES) with bin microphysics. 
Observations come from the Physics of Stratocumulus Top (POST) field campaign, flown by the CIRPAS Twin Otter off the coast of Monterey, CA during July and August 2008. This campaign primarily sampled the top 100 m of the cloud deck, where drizzle is initiated. First, we will demonstrate the limits of the observations to test existing formulations of collision-coalescence rates, even when turbulence effects are included. These limits are primarily imposed by instrumentation and aircraft sampling strategies, but have broad implications for the parameterization of DSD shape in bulk microphysics schemes. The remainder of the presentation is focused on the sensitivity of modeled DSDs to bin microphysics numerics using case studies derived from POST flights that span a range of cloud base precipitation rate, time of day, and inversion strength. A high resolution bin microphysics scheme was implemented in the LES model in order to meaningfully compare model output with DSD observations. We find that spectral resolution has a significant impact on the evolution of the DSD because of numerical diffusion at low (standard) resolution. This diffusion results in greater precipitation rates at low resolution, which in turn alters the dynamics of the boundary layer compared to the high resolution simulations. Finally, simulations with a turbulent collision-coalescence kernel are presented to illustrate the differing response of the DSD to kernel formulation across meteorological contexts.

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