Effects of Cloud Seeding in West Texas: Additional Results and New Insights

Abstract

Additional results and new insights have been obtained regarding the effect of randomized dynamic seeding of supercooled convective clouds in west Texas. These have resulted in a revised conceptual model that aids in understanding the new results and in designing future physically based experimentation and cloud modeling studies. Attention is focused initially on further evidence for seeding effects on convective cells, which are the treatment units for the experiments. A total of 183 cells [93 seeded (S) and 90 nonseeded (NS)] have been identified and their properties computed through analysis of three-dimensional, volume-scan, C-band radar data using cell-tracking software. The results indicate that AgI seeding increased the maximum heights by 7%, the areas by 43%, the durations by 36%, and the rain volumes by 130%. Cell merger occurred nearly twice as often in the AgI-treated cases. The rainfall and merger results are significant at better than the 5% level using rerandomization procedures. Additionally, it was found that AgI-treated cells produced more rainfall than untreated cells of the same height. Time-height reflectivity composite cross sections for the S and NS cells revealed stronger reflectivities aloft immediately after seeding for the AgI-treated cells. This region of enhanced S reflectivities expanded and descended with time, reaching the earth's surface by 40 min after initial seeding. The next step focused on the areas surrounding the treated cells. A new and improved ?focused area? approach, involving calculations for radii of 7, 10, 15, 25, 50, and 100 km around each treatment position was developed and applied. The rainfalls from the nonseeded cells exceeded the rainfalls from the nonseeded cells at radii less than 10 km early in the treatment period, and this apparent effect spread to larger radii with time. These results are consistent with a positive effect of AgI treatment on rainfall that begins on the cell scale, where the seeding takes place, and spreads outward with time. Analysis of the 34 (17 S and 17 NS) small mesoscale convective clusters (i.e., the experimental units) obtained to date produced ratios of S to NS rainfalls of 1.37, 1.27, 1.37, 1.26, and 1.27 for the time periods 0?30, 0?60, 0?90, 0?120, and 0?150 min, respectively, after initial seeding. None of the results has strong P-value support. The ratios are larger and more significant when the five experimental units that failed to meet the qualification criteria are eliminated from the sample. The revised dynamic seeding conceptual model gives much more attention to microphysical processes than before. How dynamic seeding likely induces greater cell rainfall without an appreciable increase in maximum cell height in some case is addressed by the model. The model is shown to be useful also in identifying which cloud processes should be the focus of recommended future physically based experimentation and cloud modeling.