Over geologic timescales, patterns of precipitation shape mountain ranges. Since precipitation is strongly influenced by topography, a potential for feedback between evolving topography and spatial patterns of precipitation exists. At the spatial scale of entire mountain ranges evidence for a feedback between topography and precipitation patterns is apparent in the case of a strong rain shadow. Are there similar feedbacks at the smaller spatial scale of individual ridges and valleys?

To investigate this question we analyzed the precipitation over the Olympic Mountains as simulated at a 4km resolution by a mesoscale weather prediction model (the MM5). Examination of the model results from individual storms as well as cumulative precipitation totals over two calendar years predicted by twice daily model runs reveals a strikingly consistent pattern of precipitation that is related to small-scale topography. On the southwest flank (typically the upwind side of the range), simulated precipitation totals on ridges are consistently 2-4 times the totals in adjacent valleys. The pattern of enhanced precipitation in MM5 on ridges relative to valleys is observed in annual totals, seasons and individual storms. Storm events, identified by large 12-hour accumulations of precipitation at any point in the domain, show remarkably little variability in the local patterns of precipitation despite variability in wind speed and direction, and the temperature and humidity of the incoming air. We present analysis of several storms in terms of their consistency with simple upslope and linear mountain wave models of orographic precipitation.

Sustained enhanced precipitation on ridges relative to valleys has several implications for landscape evolution including gently sloping ridge crests and lower overall relief (difference between highest and lowest elevation) relative to a case with uniform precipitation. The feedback between precipitation patterns and topography is an exciting new area of research at the interface between atmospheric science and geology. Increased understanding of the controls on patterns of precipitation in mountains and the stability of these patterns through changing climates will greatly advance this field. In addition, coupled landscape evolution and atmospheric modeling promises to provide an intriguing look at the rich system of evolving interactions between mountains and weather.