Intercomparison of Mean Areal Precipitation from Gauges and a Multisensor Procedure

This study examines differences between rain gauge precipitation estimates and those from the National Weather Service's (NWS's) Multi-sensor Precipitation Estimator (MPE) which blends hourly gauge data with estimates from NWS radars onto a 4×4 km2 grid. Comparisons are made between 1996-2003 on five watersheds within Florida that have a range of sizes and gauge densities. The comparison statistics are found to improve during the period, likely due to improvements in the gauge networks, both the number of gauges and their data quality. MPE hourly totals generally are less than gauge totals, and differences between MPE and gauge amounts (MPE-gauge) often are greater (more negative) for larger gauge values than smaller ones. Results also reveal that the larger the basin, the greater the volume differences and standard deviation of differences. Differences between MPE and gauge-derived precipitation are found to depend greatly on gauge density within the basin which generally increases during the study period. The comparison statistics also are affected by the spatial and temporal variability of rainfall within the basins. Thus, a particular gauge distribution may adequately sample one rainfall scenario and not another. Differences between MPE and gauge-derived precipitation are found to be related to the seasons because of the different types of precipitation generally associated with each. The two data types correlate better during the cold season than the warm season and the standard deviations of differences are greater during the warm season. The tendency for MPE amounts to be less than gauge amounts is more pronounced during the cold season. On the other hand, the trend for MPE amounts to be much less than gauge amounts for the greater gauge totals is more pronounced during the warm season. These seasonal differences are due to several characteristics of the rainfall and its sampling by the two products. The cold season consists of more low top rainfall events than the summer. Radars often overshoot this precipitation, leading to MPE amounts less than gauge amounts. Cold season events also are larger in area and more likely to be sampled adequately by gauges. Conversely, warm season events are more small scale in character and may not be adequately sampled by gauges. Several factors are thought to explain the tendency for smaller MPE than gauge totals with increasing gauge amounts. The standard NWS reflectivity-rainfall (Z-R) relationship produces large underestimates compared to the tropical Z-R relationship. Additionally, the NWS caps reflectivity to avoid hail contamination, and this cap might be set too low for intense precipitation events. Finally, truncation errors in the MPE algorithm prior to 2001 may be a factor in causing the smaller MPE values. Differences between MPE and gauge totals are compared for hourly, 6-hourly, and daily accumulations. The difference statistics improve with increasing periods of accumulation because when hourly observations are added, random errors in the data tend to cancel although systematic errors remain. The results indicate that there are definite differences between MPE and gauge-derived precipitation. However, it is not possible to say which product is superior because there is no "ground truth" at the scale at which Florida precipitation occurs.