Assessing satellite-based rainfall estimates in semi-arid watersheds using the Walnut Gulch gauge network and TRMM-PR

The gauge network of the USDA-ARS Walnut Gulch Experimental Watershed (WGEW) in southeastern Arizona provides a unique opportunity for direct comparisons of in-situ measurements and satellite based rain rate estimates. Direct comparisons between gauge measurements and satellite estimates (e.g., TRMM, GPM) are subject to large uncertainties. On the one hand, comparisons of TRMM snapshots with rain gauge network observations are associated with large uncertainties due to volume sampling discrepancies; on the other hand, comparisons of rainfall accumulations are associated with large uncertainties due to satellite temporal sampling errors. The general strategy used in evaluating the TRMM Precipitation Radar (PR) rain rate estimates, for example, is to conduct comparisons of the satellite products with ground radar estimates, in which the latter are adjusted, or are not adjusted, to the rain gauge observations. However, the configuration of the WGEW network and its high degree of temporal synchronization, which does not exist elsewhere under the TRMM PR coverage area, justifies direct comparison of the in-situ ground measurements with the satellite instantaneous rain rate estimates. The WGEW network consists of 88 weighing rain gauges within a 150 km² area. Therefore, on average, approximately 10 gauges can be found in each PR pixel. All gauges are very well synchronized (within seconds with 1 minute reporting intervals during precipitation). This allows generating very-high-temporal-resolution rainfall rate fields and obtaining accurate estimates of the area-average rain rate for the entire watershed and for a single TRMM PR pixel.

The objective of this study is to conduct such "instantaneous" rain rate comparisons, and to investigate how well both fields compare in the Semi-Arid climate of the southwest US. The comparisons are based on data from all TRMM overpasses in which the PR recorded rain within the WGEW. Special attention is given to the distance of the watershed from the TRMM subsatellite track. The closer the watershed is to the nadir-line the closer the PR observations are to the surface, and less effected by evaporation and other processes typical to such environment.