Sensitivities of Orographic Precipitation to Terrain Geometry and Upstream Conditions in Idealized Simulations

Abstract

his study examines how variations in relatively simple terrain geometries influence orographic precipitation and its spatial patterns of sensitivity to small changes in upstream conditions. An idealized three-dimensional model is used to simulate a moist flow impinging upon three alpine-scale terrain shapes: a straight ridge, a concave ridge, and a convex ridge. A variety of simulations are conducted to investigate the sensitivity of precipitation patterns to ridge length and upstream thermodynamic and wind conditions for an impinging flow with a nondimensional mountain height of approximately unity. It is found that for the straight and convex ridges, the flow response is mostly linear for the conditions examined here and passes over the obstacles with little lateral deflection. The concave ridge, however, exhibits strengthened flow deceleration, wave breaking in the lee, and flow confluence between the ridge arms. The concave ridge also generates substantially more precipitation than the other two ridge geometries via an established precipitation-enhancing funneling mechanism near the ridge vertex. However, for some concave ridge configurations the results feature dual-precipitation maxima, which is an important difference from previous findings. Finally, results from a simple ensemble of simulations elucidate the sensitivity of precipitation patterns to small variations in upstream conditions and how these vary for the different terrain geometries.