Radar Results of the 1986 Exploratory Field Program Relating to the Design and Evaluation of PACE

Authors

  • Nancy Westcott Climate and Meteorology Section Illinois State Water Survey Champaign, Illinois

Abstract

The initial phase of the Precipitation Augmentation for Crops Experiment (PACE), directed at enhancing rainfall was conducted in Illinois during the summer of 1986. This first experiment resulted in a limited sample, (19 clouds and experimental units), but one sufficient to provide information pertinent to the design and evaluation of future efforts. In particular, it was determined that differences in meteorological conditions may mask any seeding signature present, requiring that the experimental units be stratified or normalized. It was found that for these clouds, the height of the echo at first detection and the age of the echo at treatment (AgI or placebo) have an important bearing on the expected growth of the echo. Additionally, the area and reflectivity of the echo at 6 km at the time of treatment seem to be related to the maximum size attained by the echo cores. That is, the larger the echo at treatment, the larger the echo can be expected to grow. However, the growth of the echo core in terms of reflectivity, height and area appeared to slow as the echo cores matured. This suggests that explosive growth sometimes expected from cloud seeding may not be the rule in this area of the country and that a comparison of before and after treatment growth rates may not be a good evaluation tool. Rather, the post treatment growth, with the experimental units stratified by the age of the echo at treatment and by the height of formation may be more useful in discriminating seeding effects. Radar derived predictor variables were examined to assess echo behavior based on the ambient weather conditions, and response variables were developed which may be useful in detecting potential seeding effects. Inferences were made with regard to stratification of the data, experimental unit definition, and cloud behavior. This work emphasized, as others have found, the need for predictor variables and a large sample.

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Scientific Papers