Observed Variations in the Strength of the Sierra Nevada Rain Shadow

The extent of precipitation spillover into the lee of the Sierra Nevada significantly affects the region’s hydrology. For example, during the winter (Oct.-Apr.) of 2016-2017 many areas in western Nevada, including Reno, set new records for seasonal precipitation due to high occurrence of spillover events. While some of the factors impacting variations in lee side precipitation are well known, e.g., the number of intense atmospheric rivers, the mesoscale factors affecting both intra-storm and inter-storm variability in spillover are less understood.

To address this knowledge gap, we use composite NEXRAD radar from KRGX near Reno, NV and KDAX in Sacramento, CA to document variations in lee side precipitation throughout the 2016-2017 cool season. The radar data are used to produce Hovmöller diagrams that reveal preferential cutoff locations for precipitation and episodic periods of enhanced spillover. We subsequently examine the structure of the mountain wave leading to these different cut-off locations. These radar analyses are further contextualized with observations and mesoscale model analysis fields to examine the role of both upstream forcing (e.g., Froude number) and downstream stability in modulating the structure of the mountain wave, and thus spill over precipitation.