Rain shadow development during the growth of mountain ranges: An atmospheric dynamics perspective

An idealized atmospheric model is used to explore the links between climate and topography in the development of orographic rain shadows during orogenesis. The atmospheric dynamics theory of density stratified fluid flow over topography is used to interpret the results. The controlling nondimensional parameter is Nh/U, where N is the Brunt-Vaisala frequency, a measure of atmospheric stability, h is the terrain relief, and U is the initial horizontal wind speed. Rain shadow development is found to be a nonlinear and nonunique function of both topography and atmospheric state, indicating that geological records of orographic aridity cannot be interpreted in terms of relief alone. When upstream topography exceeds Nh/U approximate to 1 during surface uplift, downstream orographic precipitation vanishes, and downstream orographic cloud mass decreases by as much as 90%. Upstream blocking of air flow can generate a forward projecting rain shadow in which a relatively low ridge (Nh/U < 1) situated upstream of a relatively high ridge (Nh/U > 1) may be decoupled from the atmospheric flow by a zone of flow stagnation extending upstream of the high terrain. Such an effect may occur if the valley separating the two ranges is narrower than the length scale of flow stagnation. In the model configuration used here, lateral widening of a relatively low (Nh/U < 1) range changes downstream orographic cloud mass by only a few percent, while lateral growth of a relatively high (Nh/U > 1) range increases downstream cloud mass by up to a factor of 3. These results help to refine interpretations of climate-tectonic interactions in shaping the geological record of the Sierra Nevada and Andes.