Validation of the NEXRAD DSP Product with a Dense Rain Gauge Network

The Next-Generation Weather Radar (NEXRAD) system digital storm-total precipitation product (DSP; $2\mathrm{km}\times 1°$ at 4–7-min intervals) is examined for its quality in terms of precipitation quantity and spatial rainfall distribution. Observations from a network of 50 rain gauges in the Upper Guadalupe River Basin are compared to the DSP rainfall estimates from two radars: KEWX at New Braunfels, Texas, and KDFX (14 km east of Brackettville, Texas), for the period of September 2006 to June 2008. The rainfall data comparisons are conducted at three temporal scales: 6 min, 1 h, and storm-total accumulations, and at different distances from the radars, from near ($<50\mathrm{km}$) to middle (50–100 km), far (100–160 km), and very far ranges ($>160\mathrm{km}$). A strong range dependence is found from radar estimates, i.e., radar underestimates at near and very far ranges ($<50$ and $>160\mathrm{km}$), matches well or slightly overestimates at middle ranges (50–100 km), and overestimates at far ranges (100–160 km). The correlation coefficients between paired radar and gauge precipitation estimates are moderate ($r$ from 0.62 to 0.76) for both radars at the three time scales, however, they are spatially different (at the county level, showing a decreasing trend from middle to far ranges, passing west to east through Kerr, Kendall, Comal, and Guadalupe counties) under the KDFX umbrella. Under the KEWX umbrella, there is no trend of spatial correlation coefficients from county to county. Similarly, there is an overall increase of mean relative difference between radar and gauge values from middle to far ranges from the KDFX radar, but mixed results for the KEWX radar. The probabilities of rainfall detection (POD) for both radars are greater than 90% and are always higher than the gauge PODs, which increase from $\sim 10\%$ at the 6-min temporal scale, to $\sim 30\%$ at the hourly scale, to $\sim 70\%$ for the storm-total scale. There is no range dependence for POD. Overall, the DSP product suffers from severe range dependence, and its rainfall estimates agree better with rain gauge estimates during the warm season than during the cold season. However, the DSP’s advantages in high temporal and spatial resolutions and the near real time storm-total rainfall estimations provide great potential for near real time flash flood monitoring and forecasting at local and regional scales, although it still needs more testing and corrections, especially at very close ranges, far ranges, and during the cold season.