The evaluation of rainfall rate estimates from low-orbital satellites like TRMM is conventionally performed by comparisons with other remote sensing products (e.g., ground radar fields). Direct comparisons with in-situ measurements (e.g., rain gauge) have been limited to rainfall accumulations. Such comparisons are associated with large uncertainties due to satellite temporal sampling errors. Comparisons of instantaneous rainfall rate fields (snapshots) from satellite and gauge observations have been avoided, as they are associated with large uncertainties due to volume sampling discrepancies. However, the configuration of the gauge network in the USDA-ARS Walnut Gulch Experimental Watershed (WGEW) in southeastern Arizona and its high degree of temporal synchronization justify such comparisons. 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 TRMM Precipitation Radar (PR) field of view (~ 5-km diameter). Such density does not exist elsewhere under the TRMM PR coverage area. 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 field of view.

Very high temporal (1-min) and spatial resolution (100-m) rainfall rate maps were generated using the gauge multiquadric-biharmonic (MQB) spatial interpolation scheme. The high-resolution data also allows for time/space shifting of the rain rate fields with respect to each other to account for the change in position of the hydrometers from which they were observed. The comparisons are based on data from all TRMM overpasses (~30) in which the PR recorded rain within the WGEW. Both the current and the future version of TRMM PR algorithms are examined (i.e., V6 and V7). 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. The results indicate very good agreement between the fields with high correlation and low bias values, especially for the near-nadir cases; these values are typically not seen when comparing remote sensing observations (i.e., satellite vs. ground radar rainfall rate fields).