Comparisons between NEXRAD Radar and Tipping-Bucket Gage Rainfall Data: A Case Study for DuPage County, Illinois

The ability to use radar-based rainfall estimates either alone or as a supplement to rain-gage data in real-time or historical rainfall-runoff simulations or other hydrologic applications would be valuable because of the relative sparseness of available gaged rainfall data. To investigate this ability, a comparison study between rainfall estimates from the National Weather Service (NWS) Next Generation Weather Radar (NEXRAD) and the recorded measurements collected from a network of 27 tipping-bucket rain gages located in or near DuPage County in northeastern Illinois over the period from July 1997 through September 2005 was carried out at the daily time scale. The NEXRAD data used in this comparison consist of Stage III (1997–2001) and Multisensor Precipitation Estimate (MPE) (2002–2005) gridded hourly products. These NEXRAD products were corrected using rain-gage data, but data from the DuPage County gage network were not used for this purpose. Periods of missing or frozen precipitation were excluded from the comparisons. Results show that early in the study period, NEXRAD data first under-estimated and then over-estimated the gage measurements on a spatial average basis, while since 2001, the long-term spatial averages were similar. For most of the study period, the differences between gage and NEXRAD estimated daily totals decreased with distance from the nearest radar site. An analysis of the distribution of daily rainfall values shows that over most of the period, the radar data has more small values and fewer large values than the gage data, which is consistent with the radar sampling a larger area. Overall, these findings indicate that, at least in this region, while total NEXRAD precipitation estimates have become more comparable to the rainfall recorded by the gage network over the period of the study, spatially-variable biases may remain, and extreme values may have a different distribution. As a result, caution should be employed when using NEXRAD Stage III and MPE data for hydrologic modeling.