Response of orographic precipitation to subsaturated low-level layers

Abstract

In orographic precipitation events, there are times when subsaturated low-level layers are observed to be below saturated, nearly moist-neutral, upper-level layers. By performing a series of idealized two-dimensional simulations, this study investigates the response of orographic precipitation to subsaturated low-level layers. When the nondimensional parameter N2zt/U, where N2 and zt are, respectively, the dry Brunt–Väisälä frequency and depth of the subsaturated low-level layer, and U the cross-mountain wind speed, exceeds a critical value, the decelerated region on the upwind side of the mountain moves upwind, resulting in weak surface precipitation near the mountain peak. The critical value determined from the simulations is close to that derived from linear theory. When N2zt/U is less than the critical value, increasing zt has two competing effects: 1) the vapor-transport effect, meaning that increasing zt decreases the amount of vapor transported to the mountain, and hence tends to decrease surface precipitation; and 2) the updraft-width effect, meaning that increasing zt enhances flow blocking, producing a wider updraft over the upwind slope, and hence tends to increase surface precipitation. When the vapor-transport effect dominates, surface precipitation decreases with zt. When the updraft-width effect dominates, surface precipitation increases with zt. Increasing the maximum mountain height hm or U generally increases surface precipitation. However, for certain combinations of hm and U, the simulations produce lee waves, which substantially reduce surface precipitation. Finally, the response of orographic precipitation in the simulations with both liquid-phase and ice-phase microphysics is similar to that in the simulations with only liquid-phase microphysics.