Validation of NEXRAD products with rain gauge networks

The purpose of this study is to improve our understanding of Next Generation Weather Radar (NEXRAD) precipitation products by comparing them to high accuracy of rain gauge network, for the estimation of precipitation quantity and spatial rainfall distribution. The first part of the study was the calibration and installation of double-gauge platforms within a single radar cell (1 km by 1°), with some preliminary data analysis based on collected data. The second part was a comparison study of a network of 50 rain gauges with NEXRAD's Digital Storm Total Precipitation (DSP) product.;The first part consists of two phases: a testing phase for hardware calibration and development of data processing based on one double-gauge platform on the UTSA campus, and a second phase where a network of four double-gauge platforms within one radar cell is installed in the Government Canyon State Park, near San Antonio. The preliminary results indicate a correlation coefficient of 85% with a p-value of 0.0001 and a radar underestimation by 23% for paired gauge-radar events within a 6-month period (August 2006 to March 2007). Rain gauge rainfall data collected from the Park (August 2007 to June 2008) did not show a large spatial variation of rainfall distribution, which indicate most of the rainfall events in study area and time period are uniform events, although it is indicated that the "SE" of the radar cell has relative higher rain rates at certain storms and lower correlations to other three pairs of gauge measurements.;In the second part, a network of 50 rain gauges rainfall in the Upper Guadalupe River Basin were used to compare the DSP rainfalls from two radars, the KEWX radar at New Braunfels, Texas and the KDFX radar at Laughlin Air Force Base near Del Rio, Texas, for the period of September 2006 to May 2007. The rainfall data comparisons were examined based on different time scales: 6 minutes, 30 minutes, one hour, and storm total, different distances from radar to gauges (from near range, middle range to far range), and different gauge elevations from 200 m to 700 m. It is found that there is a strong radar range dependence as previously found: underestimate in the near range (150 km), while overestimate in the middle range (50-150 km). The results found that at the storm total time scale that the DPS product offers a good rainfall estimation and detection, and that the correlation with rain gauges is the highest (KEWX R2 = 0.59 and KDFX R2 = 0.37); and the mean relative differences between gauges observations and radars estimates are the lowest (52% for the KEWX and 59% for the KDFX). It is found there is no gauge elevation dependence in the study area.;The probability of rainfall detection (POD) is dependant on time scales. It is found that radar PODs are less than gauge PODs in the 6 minutes, 30 minutes and hour time scales, while in the storm total scale, the radar PODs are higher than the gauge PODs. It is 92% for the KEWX and 85% for the KDFX at the storm scale, which are higher than the rain gauges PODs of 84% and 79%, respectively. It is supposed that radar PODs should be always larger than the gauge PODs. The reason for this inconsistencies needs further discussion.;Overall, it is clear that the DSP product is range dependant; its rainfall estimation carries some errors due to the Z-R relationship power law. This product can be used for storm monitoring and flash flood warning. A longer study period with a different rain gauges network is needed to provide further comparison and analysis between radar DSP product and rain gauges rainfall observations.