The Behavior of Squall Lines in Horizontally Heterogeneous Coastal Environments

Inland squall lines respond to the stable marine atmospheric boundary layer (MABL) as they move toward a coastline and offshore. As a storm’s cold pool collides with the marine layer, characteristics of both determine the resulting convective forcing mechanism over the stable layer and storm characteristics. Idealized numerical experiments exploring a parameter space of MABL characteristics show that the postcollision forcing mechanism is determined by the buoyancy of the cold pool relative to the MABL. When the outflow is less buoyant, storms are forced by a cold pool within the marine environment. When the buoyancies are equivalent, a hybrid cold pool–internal gravity wave develops after the collision. The collision between a cold pool and less buoyant MABL initiates internal waves along the stable layer, regardless of MABL depth. These waves are inefficient at lifting air into the storm, and ascent from the trailing cold pool is needed to support deep convection. Storm intensity decreases with deeper and less buoyant MABLs, in part due to the reduction in elevated instability. Precipitation is enhanced just prior to the collision between a storm and the deepest marine layers. Storms modify their environment downstream, leading to the development of a moist adiabatic unstable layer and a lowering of the level of free convection (LFC) to below the top of the deepest marine layer. An MABL moving as a sea breeze into the storm-modified air successfully lifts parcels to the new LFC, generating convective towers ahead of the squall line. This mechanism may contribute to increased coastal flash flooding risks during observed events.