Abstract : C.1
Understanding the climatology of small-scale patterns of orographic precipitation: Progress from the Olympic mountains

Dale Durran, Justin Minder, Gerard Roe, Alison Anders
University of Washington Department of Atmospheric Science

Small-scale spatial variations in mountain precipitation have profound influences upon hydrology, landslide hazard, and landscape erosion. Yet, due to observational inadequacies and modeling limitations, the climatology of such precipitation patterns is only beginning to be documented and understood.

We have created a unique dataset characterizing precipitation patterns over the Olympic Mountains of Washington state by utilizing 8 years of mesoscale model output (from the PSU-NCAR MM5) at 4km horizontal resolution and 4 years of wet-season 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 virtually all major storms and in annual means.

We have examined case studies using high-resolution (1.33km) hindecasts. Results indicate that the dominant process shaping the precipitation pattern is enhancement by collection within thick, low clouds produced during stable ascent over the ridges. Furthermore, we have identified conditions under which the “wet ridge” pattern appears to break down (during post frontal convection, and periods of low melting-level and strong cross-ridge winds). A comparison of storm-to-storm and climate variability in the Olympics suggests that the observed precipitation patterns have been a robust feature of the local climate for thousands of years, raising the possibility of intriguing climate-landscape interactions.

This past winter (2006-2007) saw an expansion of our observational network to gain a more detailed depiction of the dynamical and microphysical processes controlling precipitation in the Olympics. New instrumentation includes a 24GHz vertically pointing radar (MRR2), two precipitation distrometers (JWD and PARSIVEL), and two 10m anemometer towers. Preliminary results from the augmented network will also be presented.