On the Effect of Natural Rainfall Variability and Measurement Errors in the Detection of Seeding Effect

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  • a Battelle-Northwest, Richland, Wash. 99352
  • b National Hurricane and Experimental Meteorology Laboratory, Coral Gables, Fla. 33124
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Abstract

Natural rain variability and measurement errors are obstacles to the determination of the seeding effect in convective cloud seeding experiments. The relative importance of these problems in Florida is evaluated in this paper. Its major thrust is embodied in a computer simulation of area cloud seeding experiments for two areas (570 km2 and 1.3 × 104 km2) using field measurements as input. The effect of natural rain variability is studied as it relates to the power functions of selected statistical tests for seeding effect. Measurement errors for gage and radar systems are introduced by modifying the underlying distribution of area mean rainfall.For the two Florida areas, natural rain variability is by far the major obstacle to the determination of a seeding effect. Errors are of lesser importance for the system of rain measurement used in Florida, which involves radar-rain estimates adjusted by gages. With a less accurate system of rain measurement, errors would assume greater relative importance. It is concluded that to detect a particular seeding effect with a minimum number of cases, the importance of natural rain variability must be decreased through either stratification of the experimental days or through meteorological predictors. The measurement system used by the Experimental Meteorology Laboratory is adequate for the evaluation of its seeding experiments and little will be gained through the expenditure of time and effort to improve it further.

Abstract

Natural rain variability and measurement errors are obstacles to the determination of the seeding effect in convective cloud seeding experiments. The relative importance of these problems in Florida is evaluated in this paper. Its major thrust is embodied in a computer simulation of area cloud seeding experiments for two areas (570 km2 and 1.3 × 104 km2) using field measurements as input. The effect of natural rain variability is studied as it relates to the power functions of selected statistical tests for seeding effect. Measurement errors for gage and radar systems are introduced by modifying the underlying distribution of area mean rainfall.For the two Florida areas, natural rain variability is by far the major obstacle to the determination of a seeding effect. Errors are of lesser importance for the system of rain measurement used in Florida, which involves radar-rain estimates adjusted by gages. With a less accurate system of rain measurement, errors would assume greater relative importance. It is concluded that to detect a particular seeding effect with a minimum number of cases, the importance of natural rain variability must be decreased through either stratification of the experimental days or through meteorological predictors. The measurement system used by the Experimental Meteorology Laboratory is adequate for the evaluation of its seeding experiments and little will be gained through the expenditure of time and effort to improve it further.

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