Differential flow blocking along the Sierra Nevada mountains and its effect on the precipitation distribution

For this study, typical characteristics of moderate to heavy precipitation events (i. e. storm averaged rainrates greater than about 100 mm/day) over the Sierra Nevada mountains of California were examined using both observed and reanalyzed data for the fall and winter seasons from the years 2002 - 2006. This threshold of rainrate was chosen since it has been found to be the optimal precipitation rate at which flash flooding in the westernmost U. S. occurs. Precipitation observations indicate heavier accumulations are more favored toward the northern half of the western flank of the mountains. Observations also show that the flow incident to the Sierra Nevada mountains tends to experience stronger deflection along the southern half of the western face of the mountains, which, in turn, leads to enhanced convergence toward the north and is a likely cause for the observed trend in precipitation patterns. Reanalysis data shows that most of these cases are characterized by a north-to-south gradient in wind speed, U, and stability, N, upstream of the mountain complex with faster winds and lower stability to the north. This, coupled with the fact that the Sierra Nevada mountains have a slightly higher average peak height to the south, appears to be responsible for differential blocking along the western face of the mountains. The effects of this differential blocking on the formation of precipitation systems is explored through a series of numerical simulations and sensitivity tests which confirm the importance of an upstream gradient in U and N in determining the distribution of precipitation. Additional experiments address the importance of terrain resolution on adequately representing differential blocking and the precipitation distribution.