A Raingage Evaluation of the Miami Reflectivity-Rainfall Rate Relation

Abstract

To provide a foundation for other radar studies in the Miami areas, 50 comparisons were made between shower rainfall recorded by raingages and observed with radar to evaluate the reflectivity Z, rainfall rate R relation, Z = 300R1,4, referred to here as the Miami Z-R relation. Total shower rainfalls measured by recording raingages were compared with estimates derived from the Miami Z-R relation in conjunction with radar reflectivity measurements with iso-echo contouring and the analysis scheme described. Rainfall rate comparisons were not possible because of the poor time resolution of the raingage observations. The radar and raingage estimates of shower rainfall were highly correlated (+0.93, significant at the 1% level); they had an average difference of 8% and a mean absolute difference of 30%. Stratification by shower amount revealed that the radar estimate of gage-recorded rainfall was too high for small shower amounts (<0.25 inch) and too low for large shower amounts. In terms of percentage the comparison was best for the heavy showers. Stepwise regression analysis showed that consideration of the square of the range from gage to radar, in addition to range normalization already provided in the radar receiving system, made a small (3%) but statistically significant (<1% level) reduction in the variance and improved the correlation (0.93 to 0.944) between the gage and radar estimates of precipitation. It is concluded that the Miami Z-R relation, when used with the radar system described, is an effective tool in representing point and area rainfall from South Flordia convective showers.