87th AMS Annual Meeting

Monday, 15 January 2007
Understanding the climatology of small-scale patterns of orographic precipitation: progress from the Olympic Mountains
Exhibit Hall C (Henry B. Gonzalez Convention Center)
Justin R. Minder, University of Washington, Seattle, WA; and A. M. Anders, G. H. Roe, and D. Durran
Knowledge of patterns of orographic precipitation on small spatial scales (less than ~20km) has been limited due to the challenges of obtaining detailed observations and high quality model output over complex mountain topography. Recent observational campaigns are beginning to illuminate small-scale orographic precipitation patterns and processes for individual storms, however an understanding of the precipitation climatology on these scales remains quite elusive.

Small-scale variations in mountain precipitation can profoundly influence hydrology and landslide hazard, as well as landscape erosion and evolution. Yet, fully understanding these impacts requires knowledge of how precipitation patterns and amounts vary on timescales of years to millennia.

We have created a unique dataset characterizing precipitation patterns over the Olympic Mountains of Washington state by utilizing 8 years of mesoscale model output at 4km horizontal resolution and 3 years of observations from a densely spaced network of precipitation gauges. Modeling and observations both show a greater than 60% enhancement of precipitation on the windward ridges of the range, relative to adjacent valleys just a few kilometers away, for all major storms and in the annual total.

We use our detailed knowledge of precipitation patterns from hundreds of storms, sampling a wide range of synoptic variability, to build an understanding of how likely it is that the precipitation patterns we have observed have remained (or will remain) a pronounced feature of the Olympic Mountain climate through major past (or future) climate changes. We explore the problem by comparing general circulation model predictions of past and future climate changes in variables such as wind direction, with our record of current storm-to-storm variability. Furthermore, we utilize a linear model of orographic precipitation (already proven successful over the Olympics) to explore the sensitivity of the precipitation pattern to a range of plausible parameters suggested by our analysis of individual storms.

Our results suggest that small-scale variation in precipitation, with enhancement on major ridges, is a fundamental and robust feature of the Olympic Mountain climate. Persistence of such patterns, and their attendant affects on erosion, over millennia allows for interactions wherein orographic precipitation patterns can act to sculpt the very mountains that create them in the first place. These feedbacks between precipitation and topography are highlighted with results from a landscape evolution model.

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