Precipitation over multiscale terrain

Stratiform precipitation over multiscale terrain is examined based on two-dimensional simulations of moist airflow past two-ridge terrain with a narrower ridge superposed on the windward slope of a wider ridge. It has been demonstrated that the narrower ridge could significantly modulate the precipitation intensity and distribution by inducing upslope ascent, leeside descent, and gravity waves. Three distinct flow regimes have been identified based on simulations, namely linear, non-linear, and blocking regimes. For low terrain, the wave response is primarily linear, and more precipitation occurs over the narrower ridge. Precipitation over the major ridge is much reduced by precipitation over the narrower ridge, which decreases the moisture flux that reaches the major ridge, and by leeside descent over the narrower ridge, which limits the horizontal dimension of the updraft zone over the major ridge. In the linear regime, the total precipitation produced by the two-ridge terrain and the corresponding wide smooth ridge is comparable. For terrain of moderate heights, the wave response is nonlinear and oscillation occurs with updraft and precipitation maxima alternating between the upwind slopes of the two ridges, likely due to non-linear wave–wave interaction. Strong lee waves are produced in the lee of the first ridge due to non-linear amplification and could significantly enhance precipitation over the major ridge through directly increasing the condensation rate over the upwind slope of the major ridge and through generating snow above the freezing level. Correspondingly, precipitation rates over the two ridges become comparable and the total precipitation over the tworidge terrain is considerably larger than over the corresponding reference ridge. For higher terrain, blocking becomes significant and precipitation over the first ridge tends to detach from the terrain and propagates upstream. Additional simulations indicate that the precipitation intensity and distribution are also sensitive to terrain geometry such as the ratio of the two ridge heights, the depth of the relative valley, and the distance between the two ridges.