Dissipation time and spatial organization of coastal Stratocumulus clouds
- Author(s): Zamora Zapata, Mónica Natalia
- Advisor(s): Kleissl, Jan
- et al.
Stratocumulus clouds (Sc) cover 23% of the Earth's ocean surface and adjacent coastal areas, greatly affecting the available solar energy resource. Sc clouds are hard to predict in Numerical Weather Prediction (NWP) models due to their complex turbulent structure and challenging parameterizations of physical processes. In the Southern California summer, Sc clouds over coastal land typically dissipate during the morning hours. The cloud layer gradually thins, then breaks, and eventually dissipates completely. This dissertation investigates two aspects of importance for coastal Sc cloud dissipation: how different meteorological variables affect dissipation time, and the effects of wind shear on their spatial organization.
In the first part, we analyze the impact of initial states and meteorological forcing parameters on coastal Sc dissipation time using a Mixed-Layer Model. We use a dataset with 15 variables derived from observations and NWP model outputs for 195 days in San Diego, California. Since some variables co-vary in nature, a simple sensitivity analysis can be misleading, hence we follow two approaches. First, we simulate the 195 real cloudy days, allowing for co-variability amongst the initial states and meteorological forcing parameters. Second, we vary a single variable around a reference case. While both analyses agree on initial conditions influencing dissipation time more than forcing parameters, some results with co-variability differ greatly from the traditional sensitivity analysis and previous studies.
In the second part, we analyze the effects of wind shear on the organization of Sc clouds. Updraft and downdraft motions are a key component of the Sc topped boundary layer and strong surface shear can yield updraft rolls, while strong top shear can diminish cloud fraction. We investigate the effect of combined variations of surface and top wind shear using Large Eddy Simulations. We focus on the spatial features of updrafts and downdrafts and how their turbulent flux contributions change with varying conditions of shear. Stronger surface shear generates updraft rolls, less well-mixed thermodynamic profiles, and decreases cloud fraction. Stronger top shear also decreases cloud fraction and can shape the top of the cloud field. Larger objects dominate turbulent flux contributions, which are sensitive to wind shear conditions.