UC San Diego

Stratocumulus is the most common cloud type and has a strong impact on global climate. Over coastal lands, which are frequently well populated, these clouds have a strong impact on surface temperature and radiation through reflection of incoming solar radiation. The cloud behavior is determined both by small-scale processes such as turbulent mixing between two-phase, two-component, fluids at the cloud top and large-

scale circulation such as anticyclones and their climatological occurrence. Due to the large range of spatial scales associated with stratocumulus clouds, global climate models (GCM) and numerical weather prediction models (NWP) parameterize the physical processes occurring in the stratocumulus-topped boundary layer (STBL). However, these models are unable to simulate the clouds accurately. For instance, in the North American Model stratocumulus clouds over the California coast in the summer dissipate earlier than observed via satellite.

In this thesis, we first employ high-resolution Large Eddy Simulations (LES) and Mixed Layer Models (MLM) to study mechanisms regulating the timing of the break up. We find that over coastal lands, as the cloud thins during day, turbulence generated by surface fluxes becomes larger than turbulence generated by longwave cooling across the cloud layer. To capture this shift in turbulence generation in the MLM, we extend an existing entrainment parameterization to account for both sources. We find that cloud lifetime depends on a combination of surface moisture content, cloud-top entrainment flux, and large-scale horizontal advection by sea breeze.

Next, we evaluate three different planetary boundary layer (PBL) parameterization schemes in the Weather Research and Forecasting (WRF) model in simulating the STBL by benchmarking them against high-resolution LES. We find that the schemes do not take into account the turbulence generated by longwave cooling across the cloud layer and therefore underestimate the mixing of warm-dry tropospheric into the STBL at the cloud top. Thus, we propose a correction to the eddy diffusivity coefficient by adding a term that accounts for turbulence generated throughout the cloud layer as well as at the surface due to buoyancy flux. The modified scheme is then able to simulate the cloud physics similar to that of the LES. The modeling tools developed in this thesis have improved the understanding of and the ability to forecast stratocumulus-topped boundary layers.