Small-scale gradients in climatological precipitation on the Olympic Peninsula

Persistent, 10km-scale gradients in climatological precipitation tied to topography are documented with a fine-scale rain and snow gauge network in the Matheny Ridge area of the Olympic Mountains of Washington State. Precipitation totals are 50% higher on top of a ~800m high ridge relative to valleys on either side, 10 km distant. Operational MM5 model runs on a 4-km grid produce similar precipitation patterns over 6 water years. A change in MM5 microphysical parameterization from Simple Ice to Reisner 2 does not produce profound changes to the precipitation pattern at the annual timescale.

The performance of the MM5 is compared to the gauge data for two wet seasons and for the largest precipitation events of each season. The cumulative MM5 precipitation forecasts for both seasons and for the sum of the largest events compare well with the precipitation measured by the gauges, although some of the individual events are significantly over or under forecast. This suggests that the MM5 is correctly reproducing the precipitation climatology in the vicinity of our gauges, but that errors for individual events may arise due to predictability issues.

A computationally simple model of orographic precipitation is shown to reproduce the major features of the precipitation pattern. This simple model can be coupled to landscape evolution models to examine the impact of long-term spatial variability in precipitation on the evolution of topography over thousands to millions of years.

Current work is concentrating on understanding the physical mechanisms responsible for the observed precipitation pattern, as well as predicting the robustness of the pattern under climate changes.