Orographic Enhancement of Precipitation over Low Mountain Ranges. Part I: Model Formulation and Idealized Simulations

Abstract

A diagnostic model for simulating orographic precipitation over low mountain ranges is presented. It is based on linear theory of hydrostatic flow over mountains and calculates condensation rates from vertical lifting at the different model layers. Several other physical processes, such as hydrometeor drifting, evaporation, and moisture loss, are incorporated in the model by simple parameterizations. Idealized simulations of precipitation with different model performances provide insight into the physical processes of orographic precipitation. Evaporation, in combination with hydrometeor drifting into descent regions, is identified as one of the key aspects that primarily determine the spatial distribution of precipitation. The variability in orographic precipitation that results from changes in model parameters and ambient conditions is investigated in sensitivity studies. Simulated intensities as well as their spatial distributions are very sensitive to the temperature T0 at the lowest layer and to the variables that define the Froude number Frm: the horizontal wind speed U, static stability Nm, and mountain height H. Most of the parameters exhibit a nonlinear relation to the simulated precipitation intensities. Relative to ambient conditions, orographic precipitation is found to be less sensitive to changes in formation time tice, terminal velocity of ice particles ?ice, and melting level ?z. In each case, the sensitivities of simulation results strongly depend on the location in the model domain.