Cross-barrier flow during orographic precipitation events: Results from MAP and IMPROVE

Ground-based and airborne Doppler radar data collected during passage of frontal rainstorms over the European Alps and Cascade Mountains of Oregon are found to exhibit characteristic cross-barrier flow and precipitation patterns. A stably stratified layer of blocked (or partially blocked) low-momentum flow below mountain-crest level is separated from strong cross-barrier flow aloft by a concentrated layer of vertical wind shear. This shear layer slopes upward toward the mountain crest and persists for several hours as the storm passes over the mountain range. This pattern is remarkably similar from case to case and from one mountain range to the other. The orographically enhanced precipitation in these cases exhibits stratiform structure, marked by a radar bright band, which often drops to a lower height (indicating locally cooler conditions) immediately adjacent to the windward mountain slopes. A terrain-induced gravity wave produces strong downslope flow and spillover of precipitation onto the lee side of the barrier. An elevated reflectivity maximum appears above the mountain crest and extends a short distance (generally 20-40 km) upstream, apparently as a result of gravity wave lifting. While baroclinically induced shear may contribute to the observed pattern, two-dimensional idealized simulations indicate that, in the presence of sufficient upstream static stability, orographic effects alone are sufficient to support development of the upward-sloping shear layer on the windward side of the barrier. Furthermore, idealized simulations show that if there is preexisting vertical shear in the incident upstream flow, this shear is strengthened by the orography and that surface friction may also enhance the strength of the shear layer. The repeatable sheared velocity pattern identified here is apparently an essential feature of cases in which stably stratified low-level flow associated with a baroclinic system impinges upon a sufficiently tall mountain range. Since the layer of orographically generated shear promotes turbulence, which has been shown to aid precipitation growth over the windward slopes, these stable flow dynamics represent a potentially important mechanism for orographic precipitation enhancement in association with the passage of frontal systems over mountainous terrain.