Stratocumulus Dissipation Dependence on Initial and Boundary Conditions in a Mixed Layer Model

For cities in midlatitude west coasts stratocumulus clouds affect solar energy variability and pose a challenge to the grid integration of solar power. For Southern California these clouds are common in summertime, covering large parts of the land during nighttime, and usually dissipating during the morning.

A Mixed Layer Model (MLM) is applied to model basic stratocumulus clouds properties. Simple parameterizations model the complex processes in the atmospheric boundary layer. In this work we study how the different parameters used by the MLM for the initial and boundary conditions affect the predicted cloud lifetime over land. The parameters values reflect real conditions over land in Southern California. Radiosonde observations and Numerical Weather Prediction model output for 179 cloudy days in the summer months between 2014 and 2017 serve as the evaluation dataset. The parameters included are moisture and temperature profiles, inversion height and cloud thickness, large scale divergence, wind speed, Bowen ratio, sea surface temperature, effective sky temperature, shortwave irradiance above the cloud, and sea level pressure.

Our simulations use a two column approach to account for ocean advection and are initialized with a state derived from the observations that is brought closer to equilibrium in terms of cloud thickness evolution, considering that the Mixed Layer idealization may alter the stability of the cloud. We study the effect of each parameter by modeling all 179 cloudy days.

The most important parameters determining cloud lifetime are inversion base height and cloud thickness, followed by temperature and moisture in the cloud at the initial time, effective sky temperature, Bowen ratio, and large scale divergence. On the other hand, shortwave irradiance above the cloud, sea level pressure, and average wind speed show little or no clear effect on cloud lifetime.