Convective Rain Rates and their Evolution during Storms in a Semiarid Climate

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Rain rates and their evolution during summertime convective storms were analyzed for the semiarid climate of the northern High Plains. Radar data from a total of 750 radar echo clusters from the 1980 and 1981 summer cloud seeding operations of the North Dakota Cloud Modification project (NDCMP) were used. The analysis suggests that the average rain rate R¯ among storm is, in a first approximation, independent of the total rain volume if the entire storm duration is considered in the averaging process. This average rain rate depends primarily on the reflectivity threshold considered in calculating the area coverage integrated over the lifetime of the storm, the storm, the area-time integral (ATI). For the 25 dBz reflectivity threshold used in the ATI computations, R¯ was 4.0 mm h−1 with a standard deviation of 1.55 mm h−1, being ∼20% higher for wet mason conditions. The evolution of rain rates during storms was analyzed by dividing each storm lifetime into 10 min. 1, 2 and 4 h, and growing and decaying periods. A 10 min time increment was used in computing the parameters for all time intervals. A storm cluster reached its maximum growth after an average of 56% of its lifetime. The average rain rate for the growing period exceeded that for the decaying period by about 10%. As the time interval used in computations approached the storm lifetime, the scatter of the average rain rates was reduced, thus increasing the accuracy of rainfall estimates using the area time integral. The value of R¯ remained independent of the total rain volume when the growing or decaying periods of storms were considered separately. The total rain volume was also well correlated with the maximum single-scan rain volume. These findings suggest the possibility of estimating total storm rain volume at its maximum stage of development. It is hoped that improvements in rainfall estimation over area using satellite data may result from further studies since the precipitating part of a cloud picture can be more accurately defined for the growing period of a cloud's history.