Employment of Mixed Layer Models and Large Eddy Simulations to Determine the Factors Controlling Stratocumulus Cloud Lifetime over the Coast

Abstract:

Summertime marine boundary layer stratocumulus (Sc) clouds have a strong impact on ecology and infrastructure over the coast of California. Modeling the lifetime of such clouds in global climate models (GCM) or numerical weather prediction models (NWP) is difficult and significant errors are typically observed. For instance, stratocumulus clouds over the coast of southern California in the Weather Research and Forecasting (WRF) model were found to dissipate, on average, 1.9 hours earlier than observed via satellite. In order to determine the factors controlling the Sc lifetime, we have employed large eddy simulations (LES) and a mixed layer model (MLM). Enhancements to previous MLMs include a temperature dependent radiation scheme, a land surface model, and a novel entrainment parameterization scheme for stratocumulus clouds over land in which the entrainment velocity is derived as a function of the surface buoyancy flux and the buoyancy flux integrated over the cloud layer. The advantage of using the MLM is that different mechanisms and feedbacks controlling stratocumulus cloud thickness can be examined rapidly through sensitivity studies.

We find that during the night cloud lifetime is modulated by longwave cooling of the boundary layer and entrainment flux warming and drying. During the day, surface shortwave radiative heating drives surface flux therefore increasing the turbulence within the boundary layer and increasing entrainment flux. For wet surface conditions, the increase in latent heat flux moistens the boundary layer and offsets the increase in entrainment flux warming and drying of the boundary layer and clouds persist throughout the day. For dry surface conditions, the combination of increased surface sensible heat flux warming the boundary layer and increased entrainment flux act to dissipate the cloud within a couple of hours after sunrise. For both cases, the sea breeze advects cool ocean air that acts to thicken and prolong the cloud lifetime. Next, we utilize the MLM, validated through the LES, to study the sensitivity of the Sc cloud lifetime to a comprehensive set of parameters such as initial cloud thickness and inversion strength.