Spatial Precipitation Patterns Associated with Atmospheric River Storms Over the Complex Terrain of Northern California

The supply of fresh water to California is dependent on infrequent, but long-lasting storms. These storms are associated with atmospheric rivers, which are narrow plumes of strong horizontal water vapor flux. The details of where and how much precipitation this region receives are important for short-term flood forecasting and long-term water resource management. The study focuses on the Central Valley of northern California and the adjacent mountain slopes where operational radar data from NWS radars at Davis, CA and Beale Air Force Base are available. Precipitation frequency based on a 13 dBZ (0.2 mm/hr) threshold is used to characterize the spatial distribution and variability of precipitation from 64 high-impact events between 2005 and 2010. Wind profiler data from Bodega Bay and Chico are used to group events by low-level wind characteristics and the presence and altitude of a barrier jet. Atmospheric river events produce a range of spatial patterns of precipitation frequency from event to event. A composite of all events shows three locations in the radar domain that typically experience locally higher precipitation frequencies: the lee slopes of the Coastal Mountains, the northern end of the Central Valley, and the windward slopes of the northern Sierra Nevada.

Consistent with previous studies, greater mean upslope wind speeds are associated with higher frequencies of precipitation. Events in this study with the highest total and upslope wind speeds are more likely to have southerly wind directions at the onset of atmospheric river conditions. We document a number of covariations among wind, storm duration, and barrier jet variables. Southerly wind events are associated with longer durations of atmospheric river conditions, higher altitude Sierra barrier jets, higher magnitudes of upslope wind (from 230 degrees) and storm total IWV flux compared to atmospheric river conditions with westerly winds. Longer storm durations, stronger upslope flow, and larger IWV flux would each independently produce greater values of precipitation frequency. The co-variation among these variables makes it difficult to attribute specific precipitation frequency patterns to a single variable.

Our radar-derived results for the Sierra Nevada slopes indicate an increase of precipitation frequency from the valley to 1 km MSL, and decreasing precipitation frequency above 1 km MSL. These changes in precipitation frequency with elevation generally agree with output from Smith and Barstad's (2004) linear model but do not agree with output from the PRISM model (Daly et al. 1994). As wind direction shifts from southerly to westerly, there is little change in the gradient of precipitation frequency with elevation. The amount of precipitation at middle elevations near 1 km is primarily governed by the change in precipitation frequency at low elevations in the northern portion of the Central Valley.