The response of statically unstable orographic clouds to small-scale topographic features

Recent observations have shown that, under favorable atmospheric conditions, convective orographic precipitation tends to organize into quasi-stationary longitudinal rainbands with axes parallel to the low-level wind vector. These bands may form when potentially unstable atmospheric layers are lifted to saturation by a mesoscale mountain ridge and then excited by small-scale topographic irregularities superimposed on the main mountain profile. In this study we investigate the effects of small-scale topographic variations on orographic rainbands by performing cloud-resolving numerical simulations of moist, potentially unstable flow over the Coastal Range in western Oregon, a location where these bands are known to develop. Simulations using a section of the real Coastal Range topography and a simple idealization of the upstream flow during one case in November 2002 develop similar banded precipitation patterns as that in radar observations, and suggest a novel physical mechanism leading to the formation of the bands. Additional numerical experiments, which test the sensitivity of the rainbands to the wavenumber spectrum of the underlying topography, reveal key factors controlling the band organization and spacing.