Value of a Dual-Polarized Gap-Filling Radar in Support of Southern California Post-fire Debris-Flow Warnings

A portable truck-mounted C-band Doppler weather radar was deployed to observe rainfall over the “Station fire” burn area near Los Angeles, California, during the winter of 2009-2010 to assist with debris-flow warning decisions. The deployments was a component of a joint NOAA-USGS research effort to improve definition of the rainfall conditions that trigger debris flows from steep topography within recent wildfire burn areas. A procedure was implemented to blend various dual-polarized estimators of precipitation (for radar observations taken below the freezing level) using threshold values for differential reflectivity and specific differential phase shift that improves the accuracy of the rainfall estimates over a specific burn area sited with tipping-bucket rain gauges. The portable radar outperformed local WSR-88D National Weather Service network radars in detecting rainfall capable of initiating post-fire runoff-generated debris flows. The network radars underestimated the precipitation by about 50%. Consistent with intensity-duration curves determined from past debris-flow events in burned areas in southern California, the portable radar derived rainfall rates exceeded the empirical thresholds over a wider range of storm durations with a higher spatial resolution than local Weather Service operational radars. Moreover, the truck-mounted C-band radar dual-pol-derived estimates of rainfall intensity provided a better guide to the expected severity of debris-flow events, based on criteria derived from previous events using rain gauge data, than traditional radar-derived rainfall approaches using reflectivity-rainfall relationships for either the portable or operational network WSR-88D radars