The Rain-Induced Transition of the Boundary Layer in Quasi-Linear Convective Systems

The rain-induced transition (RIT) is a planetary boundary layer (PBL) phenomenon which can occur in light precipitation ahead of a quasi-linear convective system (QLCS). Evaporation of rain statically stabilizes the PBL and can lead to frictional erosion of near-surface winds. Like the afternoon-to-evening transition, this can ultimately increase vertical wind shear, storm-relative helicity, and the potential for tornadogenesis. This static stabilization may also occur without significant convective stabilization such that maintenance of the QLCS is not severely impacted. This study outlines the mechanisms associated with the RIT and analyzes cases where the RIT may have played an important role in QLCS tornadogenesis - in particular, the 2019 March 3 QLCS event which produced the Smiths Station-Beauregard EF4 tornado in eastern Alabama. The focus is placed on the southeastern United States, where precipitation well ahead of a QLCS is common.