NONLINEAR INFLUENCES OF TOPOGRAPHY ON OROGRAPHIC PRECIPITATION

Linear models of orographic precipitation have successfully reproduced several important aspects of the feedback between topography and precipitation in mountainous regions. These models, however, do not capture some important nonlinear interactions between topography and precipitation. The main controlling parameter for this interaction is the Froude number: Fr=Nh/U, where U is the speed of the oncoming flow, N the atmospheric static stability, and h is the maximum mountain height. For Fr < 1, the winds are able to flow over the mountainous topography and linear theory will be valid. For Fr > 1, the impinging atmospheric flow is blocked from flowing over the topography. Blocked flows will produce precipitation fields that may not be well estimated by linear theory. Here I present numerical simulations of the precipitation field associated with this nonlinear regime for an evolving two-dimensional fold-and-thrust belt. The simulations are computed using a 2-D version of the Weather Research and Forecasting model (WRF). Preliminary results show that precipitation along upper windward slopes decreases with the onset of blocking while precipitation increases along lower windward slopes. The implications of these results for climatic forcing of erosion in mountain belts will be discussed.