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<br />AUGUST 1984 <br /> <br />DONEAUD, IONESCU-NISCOV AND MILLER,JR. <br /> <br />1611 <br /> <br />volumes using satellite data. Many of the satellite <br />schemes (Augustine et al., 1980; Griffith et al., 1981; <br />Stout et aI., 1979) developed toestimate rain amounts <br />depend on the delineation of both the rain area and <br />the lifetime of the storm. Larger errors in delineating <br />rain areas from satellite pictures might be generated <br />by the stratification of the convective components <br />and other storm-weakening processes during its de- <br />caying phase. A scheme based only on data from the <br />growing period of the convective cloud history may <br />help reduce errors now encountered. <br /> <br />.. <br /> <br />7. Conclusions <br /> <br />Radar,data from the 1980 and 1981 NDCMP were <br />used to investigate the evolution of rain rates during <br />storms in the semiarid climate of North Dakota. The <br />conclusions are: <br /> <br />"'1'," <br /> <br />1) The value of the average rain rate over the <br />storm duration depends primarily on the reflectivity <br />threshold considered in calculating the area-time <br />integr'al (the area coverage integrated over the lifetime <br />of the storm). For a selected threshold, the average <br />rain rate R could be considered in a ,first approxi- <br />mation independent of the A TI. The 25 dBz A TI <br />threshold seems to be a suitable compromise, but the <br />very small. clusters should be ignored. <br />2) When a 25 dBz A TI threshold was used, the <br />average rain rate is -4 mm h-1 for a dry season (i.e., <br />1980) and -4.8 mm h-I for a wet season (i.e., 1981) <br />. in North Dakota. This suggests that the average rain <br />rate depends on weather conditions, being - 20% <br />higher for wet conditions. In Florida, the average rain <br />rate was found to be 2.5 times larger and was <br />considered independent of the rain volume. <br />3) The division of a cluster or storm into its <br />growing and decaying periods was made by consid- <br />ering the radar scan with maximum 'echo area (for <br />A TI ~ 25 dBz), maximum reflectivity, or radar <br />maximum echo height. On average, a cluster reached <br />its maximum growth after - 56% of the total cluster <br />lifetime. <br />4) The average rain rate for the growing period <br />exceeds that for the decaying period by an average of <br />-20%; in comparison, in the subtropical climate of <br />south Florida the rain rate for the growing period <br />was found to be twice that for the decaying period. <br />5) When the single-scan, area-time product was <br />used in the rain rate computation, the scatter of the <br />data increased. The correlation coefficient dropped <br />from 0.98 to 0.94 and the standard error of estimate <br />increased from 0.14 to 0.20. As the time increment <br />used in the A TI computations comes closer to the <br />total storm duration, the scatter of the average rain <br />rates is reduced and the predictive power of the A TI <br />increases. A comparison of the rain volumes computed <br /> <br />~ <br /> <br />.. <br /> <br /> <br /> <br />from the 1981 radar reflectivity data using both the <br />1980 average rain rate and the standard Marshall- <br />Palmer relationship applied to the 1981 data showed <br />no significant differences. <br />6) A multiple linear regression analysis demon- <br />strated that the radar-estimated rain volqme is well <br />correlated with the maximum single-scan rain volume, <br />suggesting the possibility that the total rain volume <br />of a storm can be estimated following identification <br />of its maximum stage of development. This could <br />improve satellite rain volume estimates since larger <br />errors might be encountered in such calculations. due <br />to overestimation of rain volumes during a storm's <br />weakening or decaying phase. <br /> <br />Acknowledgments. Support for this research was <br />partially provided by the North Dakota Weather <br />Modification Board under Contract WMB-IAS-80-1 <br />and partially by the National Aeronautics and Space <br />Administration under Grant NAG-5-386. The paper <br />is based in part on a thesis submitted by Stefano <br />Ionescu-Niscov in fulfillment of the requirements for <br />the M.S. degree in meteorology. <br />Thanks are given to Mr. Dave Priegnitz for sub- <br />stantial efforts in programming and computer pro- <br />cessing of the radar data tapes, to Dr. P. L. Smith for <br />discussing the manuscript, and to the unknown re- <br />viewers for their helpful suggestions. Special thanks <br />are given to Sandra Palmer and Joie Robinson for <br />work on the manuscript, and to the late Mel Flan- <br />nagan for drafting the figures. <br /> <br />REFERENCES <br /> <br />Augustine, J. A., C. G. Griffith, W. L. Woodley and J. G. Meitin, <br />1980: Insights into errors of SMS-inferredGATE convective <br />rainfall. J. Appl.Meteor., 20, 509-520. <br />Austin, P. M., and R. A. Houze, Jr., 1972: Analysis of the structure <br />of precipitation patterns in New England. J. Appl. Meteor., <br />11, 926-935. <br />Byers, H. R., 1948: The use of radar in determining the amount <br />ofrain falling over a small area. Trans. Amer. Geophys. Union, <br />29, 187-196. <br />Doneaud, A. A., P. L. Smith, A. S. Dennis and S. Sengupta, 1981: <br />A simple method for estimating convective rain volume over <br />an area. Water Resour. Res., 17, 1676-1682. <br />-, S. Ionescu-Niscov, D. L. Priegnitz and P. L. 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