FLOOD03496 - Laserfiche WebLink

Home Browse Search

rainfall estimates in the locations over which high-reflectivity thunderstorm cores have passed. In contrast, changing the l-R parameters will affect the rainfall estimates radar-wide, regardless of the n:flectivity intensity. Since there appears to be a widespread occurrence of radar overestimation over the whole spectrum of rainfall amounts based on the scatter plots in Fig. II, changing the l-R parameters to values more appropriate for this case would likely improve the overall estimates more effectively than changing the rain rate threshold, although this has not been explicitly examined in this study. It is also possible that both the rain rate threshold parameter and the l-R parameters may have been appropriate for this event but there exists improper (i,e., "hot") radar reflectivity calibration causing the widespread radar overestimation. A gauge-radar sample bias of 0.6 from Table I, for example, could be explained completely by a radar in which the reflectivitie,s were too high by 3 dBl. Calibration information for the FTG radar was not available on this day. Clearly, proper radar calibration is important for quantitative rainfall estimation algorithms like the PPS that use Z-R technology. Another possible explanation for the radar overestimation in this case is evaporation of the radar-observed rain as it falls to the ground. This is common in drier climates such as Colorado. However the ambient envirorunent was much moister than usual in Colorado on this day with surface dewpoint temperatures above 60 deg F, and below-beam evaporation would not be expected to playas large a role as it usually does in explaining radar overestimation relative to the gauges, If the Adjustment algorithm had been performing more optimally for this case, then it could have made the necessary bias corrections automatically to the radar estimates to correct for 20 /.