Flow-Over or Flow-Around? Upstream Blocking over Heated Mountain Ridges

Whether an impinging fluid will ascend or detour around an obstacle in its path is a fundamental question of mountain meteorology. Although a well established theory exists for predicting this response in inviscid, single-layer, unheated stratified flows, little such guidance is available in the presence of diurnal surface heating. In heated flows, the upstream destabilization of the convective boundary layer (CBL), convectively enhanced vertical momentum transport, and elevated-heating effects may all increase the tendency for low-level flow to ascend the barrier. This study examines the impact of diurnal surface heating on upstream flow blocking using systematic large-eddy simulations of airflow over an idealized mountain ridge. To isolate each of the three above-mentioned diurnal-heating impacts, reference simulations without diurnal heating and with a synthetic CBL but no elevated heating are also conducted. The key diagnostic used in the analysis is the "flow-over" ratio (FO), defined as the fraction of impinging momentum flux that breaches crest level. The simulations indicate that, for the range of conditions sampled in the simulations, the impacts of upstream destablization and CBL mixing on FO are small compared to that of elevated heating. The magnitudes of all three effects are highly sensitive to the upstream wind speed and the ridge height, with increasing FO more than ten-fold in weak flows past tall ridges. A simplified hydrostatic scaling of mechanically and thermally forced pressure perturbations is shown to reasonably match corresponding simulated values. The scaling also provides a basis for characterizing the mountain flow regime (thermally driven, mechanically driven, or combined) and predicting FO.