Modelling Coastal Stratocumulus Dissipation Under Idealized Variations of Sea Breeze

Stratocumulus (Sc) clouds that develop over coastal land are strongly influenced by sea-breeze advection. This study investigates the dissipation and fragmentation processes for a coastal cloud based on the DYCOMS II RF01 case, using Large-Eddy Simulations (LES) and a Mixed-Layer Model (MLM) to analyze its day evolution under idealized sea-breeze profile variations. These variations consist of weak versus strong speeds and different starting times. Our results show that the MLM closely follows the LES main cloud evolution with a similar liquid water path, but it overestimates cloud base height near dissipation due to its inability to describe cloud breaking. Of the studied sea-breeze variations, the influence of the breeze starting time on cloud dissipation is negligible compared to the maximum breeze speed. A close examination of the cloud thinning budget reveals that an earlier, stronger sea breeze delays fragmentation by enhancing moisture advection, whereas a weaker, later breeze accelerates it. Since there is no standard definition for cloud fragmentation, we compare four metrics and find that only those based on averaged cloud fraction or liquid water path thresholds yield consistent results. Finally, a critical cloud thickness is found to trigger cloud thinning for all studied cases, while afternoon thickening occurs for a different critical value, exhibiting hysteresis behaviour.