Convective Cloud Bands in the Lee of Mesoscale Mountain Ranges

Recent observations have shown that elongated cloud bands, some capable of producing heavy precipitation, may develop past prominent midlatitude mountain ranges (e.g., the Alps and Rockies). The location (relative to the mountain) and morphology of these bands contrast with other types of terrain-locked convective bands previously studied in detail, including windward bands past small-scale topographic features and lee-side bands at the downstream edge of a converging mountain wake. The few relevant studies on these bands have attributed their formation to one or more mesoscale instabilities (static, symmetric, and inertial), but no definitive conclusions on the underlying mechanism(s) have been reached. Herein, idealized, explicit-convection WRF simulations of moist, baroclinic flows crossing mesoscale ridges (oriented in the south-north direction) are conducted to gain further insight into this phenomenon. The simulations reveal that lee-side bands are favored in environments with a three-layer tropospheric static-stability profile: a weakly stable midlevel layer, based near crest level, in-between two more stably stratified layers. Elevated convection within the conditionally unstable midlevel layer is initiated by a lee-side hydraulic jump in conjunction with additional low-level convergence in the wake. In nonrotating flow, this additional convergence stems from the collision between a lee vortex circulation and deflected flow streaming around the ridge edge. In rotating flow the initiation mechanism is similar, but the northern band is reinforced (and the southern band suppressed) by ageostrophic southerly acceleration within the wake. The sensitivities of the simulated bands to various parameters, including static stability, winds, and terrain geometry (height, width, and ruggedness) are also evaluated.