Influence of terrain resolution on banded convection in the lee of the Rocky Mountains

Abstract

On 16-17 February 2007, snowbands formed in the lee of the Rocky Mountains in Wyoming, Colorado, and Nebraska on the anticyclonic-shear side of a midlevel jet streak. Two types of bands were prevalent: a longer, wider band associated with frontogenesis along an equatorward-moving cold front (major band) and multiple shorter, narrower bands farther poleward (minor bands). To understand how the upstream terrain affected the occurrence and intensity of the bands, multiple mesoscale model simulations were performed in which the terrain was incrementally smoothed. The evolutions of the synoptic patterns were similar in all simulations that included topography, but the synoptic pattern differed and no bands developed in a simulation with a flat land surface. These results allowed a focus on the changes to the banded precipitation due to the terrain resolution. Remarkably, although the exact location of the bands differed from run to run, the bands in all simulations with topography were in roughly the same region where they occurred on 16-17 February 2007. The major band was associated with frontogenesis along an equatorward-moving cold front that became stalled against the terrain. The minor bands formed from the release of conditional, symmetric, and inertial instabilities by ascent up the large-scale topography, rather than by ascent up specific small-scale topographic features. Because the bands were not tied to specific terrain features, these results suggest that the precise location of the minor bands is unpredictable.