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<br />PRECIPITATION MEASUREMENTS FOR <br />EVALUATION OF PRECIii>ITATION ENHANCEMENT PROJECTS <br /> <br />William Lee Woodley <br /> <br />National Hurricane & Experimental <br />Meteorology Laboratory <br />Coral Gables, Florida, U.S.A. <br /> <br />1.0 Introduction <br /> <br />Natural rainfall variability and errors in the measurement of <br />precipitation are typically major problems in weather modification ex- <br />periments. The problem of natural rainfall variability is minimized by <br />using predictors and covariates in the evaluation of the experiment. The <br />problem of precipitation measurement errors, which compounds the natural <br />rainfall variability problem, is minimized by giving careful attention to <br />the nature of the precipitation to be measured and by designing an opti- <br />mum system of precipitation measurement. This paper concentrates on the <br />measurement problem because its solution is essential to any weather <br />modification experiment if one is to dl~tect an effect of seeding. <br /> <br />2.0 General Considerations <br /> <br />Three factors determine the magnitude of the precipitation mea- <br />surement problem in weather modification experiments: the nature of the <br />precipitation itself, the terrain over which the precipitation measure- <br />ments are to be made, and the design of the experiment. For example, the <br />measurement problem is much greater in an experiment in which the treat- <br />ment unit is individual convective storms and the evaluation interval is <br />the lifetime of the storm as compared to an experiment in which large <br />scale stratiform cloud systems are the treatment unit and the evaluation <br />interval is 24 hours or longer. Mountainous or inhospitable terrain only <br />serves to magnify the measurement problem. <br /> <br />Because the design of an expElriment is dictated in part by the <br />nature of the precipitation, the first consideration must be careful def- <br />inition of the precipitation characteristics in the region where the <br />experimentation is to be conductl~d. The first task is examination of the <br />historical precipitation record to establish a precipitation climatology. <br />This climatology should include: the frequency, intensity and duration <br />of precipitation, area amount, spatial distribution and maximum amount. <br />Not all of the information may b,~ available in the historical record, <br />particularly if the raingage network is coarse and the raingages are non- <br />recording. Nevertheless, it should still be possible to determine <br />whether the precipitation is from conv~~ctive or stratiform clouds or from <br />a combination of the two cloud forms. <br />