FLOOD05056 - Laserfiche WebLink

Home Browse Search

The HMS methodology to relating radar reflectivity to rainfall approaches the solution of this problem from another direction. In over 90 percent of the operational heavy rain days in the Urban Drainage & Flood Control District since 1985, HMS has observed thaI: the heaviest rainfall has occurred when the strongest radar reflectivity fiElld of a thunderstorm pa~.ses over the rain gauges. The HMS method uses the radar reflectivity to Ilocate the portion of the cloud where the heaviest rainfall is located rather than using its strength to calculate a rainfall rate. Given the validity of this assumption, the next step is to calculate the peak rainfall rate associated with the storm which can in turn be related to the strongest radar reflectivity values, HMS has predicted the quantitative precipitation associated with thunderstorms since 1979 in the Urban Drainage & Flood Control District. Since late 1981, it has used a combination of surface weather station data, upper air soundings plotted on a Skew T, Log P diagram and a 2-D cloud methodology to predict the peak rainfall rate associated with thunderstorms. HMS has found that the depth of a thunderstorm's updraft which is warmer than freezing is directly related to the rain-making potential of the cloud. Henz (1995) describes this process in detail and a copy of the paper is included in Appendix A. When the warm depth of the updraft exceeds 1,5 km in Colorado, for instance, the rain- making potential of the cloud doubles, Equations 2 to 4 below show simplified forms of this relationship: (2) Peak GO-minute rainfall = PWI times (Depth of updraft warm layer) times 2' 1.5km (3) Peak 30-minute rainfall = 0.7'0(Peak GO-min rain) (4) Peak 10-minute rainfall = 0.6;0(Pellk 30-min rain) . Note that the doubling occurs onll' if the depth of warm layer eXlceeds 1.5 km where the Precipitable Water Index (PWI) is a measure of IIhe amount of water in the air from the surface to about 20,000 feet. In effect, the calculated peak 60- ,30-,10- minute rainfall rates are assumed to occur in the grids covered by the 50 dBZ or greater radar reflectivity in the thunderstorm with appropriate time apportionment. Lower rain rates are logarithmically down-stepped to the lower radar reflectivity values. HMS generates a matrix of rainfall rates which are derived from surface temperature and dew point fields used to initialize the 2-D model output. For each set of surface temperature-dew point combinations, HMS creates a unique radar-rainfall relationship for prec:ipitation mapping. A weather station network provides information on observed surface temperature/dew point values in the District. For the night of July 1 in, HMS used the PROFS mesonet of automated '5