The Impact of Offshore-Propagating Squall Lines on Coastal-Mountain Flows

Dynamical physical processes associated with an onshore moving marine atmospheric boundary layer (MABL, i.e., sea breeze) over sloping terrain, sensitivity of these processes to MABL characteristics, and flow modifications induced by an offshore-moving squall line are investigated using idealized simulations. The moving MABL gradually advances inland, exhibiting farther advancement and greater upslope wind speed for deeper and cooler MABLs. The local acceleration is primarily driven by a MABL-generated perturbation pressure gradient force (PPGF). As the moving MABL air accumulates onshore over time, an opposing force associated with the increasing negative buoyancy eventually balances the PPGF and results in a quasi-steady upslope flow. The approaching squall line disrupts this flow in two distinct ways; Initially the storm's cold pool enhances the ambient downslope winds which diminishes the upslope wind speeds, and subsequently the storm-generated high-frequency waves and the associated surface pressure low enhances the upslope-directed PPGF which reintensifies the upslope flows.