The evolution of convective storms initiated by an isolated mountain ridge

The evolution of convective storms over the Black Hills, an isolated mountain ridge in the northern US and a regional convection hotspot, is investigated using a 10-year observational climatology and quasi-idealized numerical simulations. Radar-observed diurnally forced mountain-convection events are classified according to their maximum cell track length and duration, which are quantified using an automated cell-tracking algorithm. Environmental conditions during these events are obtained from operational radiosonde and model-analysis data. These data suggest that mountain-forced convective cells generally struggle to survive in the convectively inhibited flow downwind of the Black Hills. Those cells that do survive downwind favor environments with strong bulk vertical shear over the 0-6-km layer, which favors organized multicellular or supercellular convection. Under slightly weaker shear, the cells tend to dissipate rapidly as they propagate downwind. Relatively weak winds aloft, when coupled with low-level winds aligned with the long terrain axis, support longer-lived, quasi-stationary cells with flash-flooding potential. The weak winds favor slow cell propagation while the along-ridge flow limits the negative feedbacks of storm outflow on the elevated convergence over the ridge, allowing convection to repeatedly initiate in the same location. The storm evolution is relatively insensitive to the background thermodynamic profile, provided that sufficient moist instability exists to support deep convection. Convection-permitting numerical simulations reinforce that changes in the background wind profile alone can explain the observed variations in cell evolution. They also suggest that the longevity of convective cells downwind of the ridge is sensitive to terrain-induced modifications to the vertical wind shear.