Evolution of convective storms downwind of an isolated mountain ridge

Although intensive study has been devoted to convective-storm initiation over mountainous terrain, relatively little attention has been given to the factors controlling the subsequent evolution of these storms. This study conducts an observational and numerical investigation of the evolution of thunderstorms initiated over the Black Hills mountains of South Dakota, a local hot-spot for warm-season thunderstorm genesis due to its access to moist-unstable airflow and its relatively steep terrain (height of 1.2 km and horizontal half-width of 25 km). Automated convective-cell tracking of 53 locally forced summer convection events over 2010-2012 reveal three basic modes of storm evolution: (1) short-lived, short-track cells, (2) long-lived, short-track cells, and (3) long-lived, long-track cells. Although the latter two categories are relatively infrequent, they pose significant hydro-meteorological hazards and thus represent an important operational forecast problem. Analysis of the background flows from the collection of events reveals only marginal differences in thermodynamic profiles between the different modes of storm evolution along with more significant (though still modest) differences in the wind-velocity profiles. Physical hypotheses drawn from the observational analysis are tested using quasi-idealized, convection-permitting numerical simulations. These simulations agree with the observations in that the three different modes of storm evolution may be largely explained by modest differences in the background wind profiles. Although some of these findings are consistent with well-known modes of cell organization over flat terrain, others are unique to mountainous regions, notably the relationship between the low-level wind direction and development of stationary, potentially flood-producing storms.