OROGRAPHIC PRECIPITATION OVER THE OLYMPIC MOUNTAINS OF WASHINGTON STATE

Over geologic timescales, patterns of precipitation shape mountain ranges. Since precipitation patterns are, in turn, strongly influenced by topography, a potential exists for significant feedback between evolving topography and spatial patterns of precipitation. At the spatial scale of entire mountain ranges evidence for a feedback between topography and precipitation patterns has been widely recognized in the case of a strong rain shadow. Are there similar feedbacks between patterns of precipitation and erosion at the smaller spatial scale of individual ridges and valleys? To begin to investigate this question we analyzed the precipitation over the Olympic Mountains as simulated at a 4 km resolution by a mesoscale weather prediction model (MM5). Individual storms, as well as cumulative precipitation totals over two calendar years, predicted by twice daily model runs, reveal a strikingly consistent pattern of precipitation that is related to small-scale topography. On the southwest flank simulated precipitation totals on ridges are consistently 2-4 times higher than those in adjacent valleys. The pattern of enhanced precipitation in MM5 on ridges relative to valleys is observed in annual totals, seasons and individual storms despite variability in wind speed and direction, and the temperature and humidity of the incoming air. This suggests that the topography itself, at this spatial scale, is a strong control on the distribution of precipitation as modeled by MM5.

We compare the spatial pattern of a fluvial erosion rate index computed using topographic data for the Olympic Mountains and the MM5 precipitation pattern with that computed assuming uniform precipitation to illustrate the potential impact of orographic precipitation. Sustained, spatially non-uniform erosion due to enhanced precipitation on ridges relative to valleys has several implications for landscape evolution including more gently sloping ridge crests and lower overall relief relative to a case with uniform precipitation. The robustness of the observed pattern of precipitation suggests that precipitation patterns and topography may co-evolve. Our continuing work with coupled landscape evolution and atmospheric modeling promises to yield new insights into the rich linkages between topography and climate.