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William L. Woodley

Abstract

In an attempt to specify the changes in precipitation produced by alteration of cloud dynamics, airborne seeding with silver iodide pyrotechnics was carried out in South Florida during May 1968. Emphasis was placed on altering cloud dynamics and on increasing precipitation as a by-product of the dynamic alteration. Nineteen clouds were studied; 14 were seeded and 5 unseeded (controls) as dictated by the randomized seeding instructions. Each of the 14 clouds received approximately 1 kg of AgI smoke. Seeding was found to be effective in promoting increased cloud growth; the average growth difference between the seeded and control clouds was 11,400 ft, significant at the 0.5% level. The induced growths took many forms and in many cases were produced in clouds containing significant amounts of natural ice.

A 10-cm radar with iso-echo contouring was used to infer changes in precipitation. Analysis indicates that seeding increased rainfall an average of 100–150 acre-ft 40 min after the seeding pass, an increase of over 100%. The result is changed little by using an alternate analysis scheme or by including five additional control clouds selected after the program. The rainfall increases would probably have been greater it calculations had been possible for entire cloud lifetimes. The significance of the rainfall results ranged between 5 and 20% based on two-sided statistical tests.

Comparison between radar and raingage rainfall demonstrates that the rainfall calculations are probably underestimates by no more than 30%. The Z-R relation used in the rainfall calculations was equally valid for the seeded and control clouds. The amount of rain from the seeded clouds was positively correlated with the maximum top growth following seeding. The seeded rainfall increases were apparently the result of larger and more lasting clouds that were the by-product of the dynamic invigoration. The natural glaciating behavior of the experimental clouds would appear to preclude the “colloidal instability” approach to rainfall augmentation from Florida cumuli.

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William L. Woodley

Abstract

No abstract available.

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William L. Woodley

Abstract

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William L. Woodley and Daniel Rosenfeld

Abstract

A method for the objective evaluation of short-term, nonrandomized operational convective cloud-seeding projects on a floating-target-area basis has been developed and tested in the context of the operational cloud-seeding projects of Texas. The computer-based method makes use of the Next-Generation Radar (NEXRAD) mosaic radar data to define fields of circular (25-km radius) floating-target analysis units with lifetimes from the first echo to the disappearance of all echoes and then superimposes the track and seeding actions of the project seeder aircraft onto the unit fields to define seeded (S) and nonseeded (NS) analysis units. Objective criteria (quantified herein) are used to identify “control” (C) matches for each of the seed units from the archive of NS units. To minimize potential contamination by seeding, no matching is allowed for any control unit if its perimeter came within 25 km of the perimeter of a seed unit during its lifetime. The methodology was used to evaluate seeding effects in the High Plains Underground Water Conservation District (HP) and Edwards Aquifer Authority (EA) programs during the 1999, 2000, and 2001 (EA only) seasons. Objective unit matches were selected from within and outside each operational target within 12, 6, 3, and 2 h of the time on a given day that seeding of a particular unit took place. These were done to determine whether selection biases and the diurnal convective cycle confounded the results. Matches were also drawn from within and outside each target using the entire archive of days on which seeding was done. Although the results of all analyses are subjected to statistical testing, the resulting probability (P) values were used solely to determine the relative strength of the various findings. In the absence of treatment, randomization P values cannot be used as proof of seeding efficacy. The apparent effect of seeding in both programs was large—even after determining the effect of selection biases and the diurnal convective cycle. The most conservative and credible estimates of seeding effects were obtained from control matches drawn from outside the operational target within 2 h of the time that each unit was seeded initially. Under these circumstances, the percentage increase exceeds 50% and the volumetric increment was greater than 3000 acre-feet (3700 kt) per unit with strong P-value support (i.e., <0.0001) in both the HP and EA programs. This is in good agreement with the apparent percentage effects of seeding for the randomized Texas and Thailand cloud-seeding programs, which were 43% and 48%–92%, respectively. The results and their P-value support after partitioning gave even stronger indications of positive seeding effects. Although the results of these and other analyses described herein make a strong case for enhanced rainfall by the operational seeding programs, such programs must not be viewed as substitutes for randomized seeding efforts that are conducted in conjunction with realistic cloud modeling and are followed by replication, preferably by independent groups for maximum credibility.

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Joanne Simpson and William L. Woodley

Abstract

After four summer periods of randomized experimentation with dynamic cumulus seeding in a 1.3 × 104km2 target area in south Florida, 14 seed and 23 control cases are available, with increased documentation of radar measurement accuracy.Seed-control rainfall comparisons are made for “floating” and total target for the 6 h period following the first seeding. On days screened as suitable for the experiments, natural rain volume varied by a factor of 62 for floating target and by a factor of 25 for total target. Area seed-control rainfall differences are not significant with six classical tests, nor is the difference between random and non-random controls.Analysis of isolated experimental clouds obtained on days of multiple cloud seeding produced significant findings. Results were stratified depending on whether the single clouds dissipated in the target area without merger or whether they merged with a neighbor. With the former stratification, the mean seeded rainfall exceeded the mean control rainfall by a factor of 2, a result (one-tailed significance of 3%) that is consistent with earlier single cloud studies. No meaningful rainfall comparison was possible with the latter stratification because, on the average, the seeded clouds merged (and were dropped) 13 min earlier than the controls. This disparity in mean lifetimes before merger (two-tailed significance level of 0.5%) suggests that seeding is promoting merger in FACE as intended.Several Bayesian approaches are used to estimate a probability distribution of a multiplicative seeding factor, based on gamma rainfall distributions, with the same shape parameter for seeded and control populations. The most general treatment assigns prior probabilities to three variables, the common shape parameters, the mean of the control distribution, and the multiplicative seeding factor. With existing data, 95% of the area under the marginal density of the seeding factor lies between about 0.7 and 1.7, with a mean just above and a mode just below 1.After extensive search for physically meaningful covariates or predictors, radar echo motions in or near the target related to two distinct rainfall populations. Category 1 comprised those cases where echoes were “marching” across the area. Category 2 comprised those cases with growth and dissipation virtually without motion. Echo motion is shown to be a statistically significant covariate, accounting for 30% of the variation in the total rainfall. For the afternoon measurement period, the mean target rainfall in Category 2 cases exceeded that in Category 1 cases by a factor of 2.5.Separate seed-control comparisons in the two categories indicate that different effects of seeding might be sought in continued experimentation. Although the existing sample is small, there is evidence that in Category 1 (marching) the seeding effect is probably not multiplicative.Attempts are in progress to estimate the number of further cases required to resolve a range of postulated seeding effects in this experimental context.

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Daniel Rosenfeld and William L. Woodley

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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.

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Daniel Rosenfeld and William L. Woodley

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The effect of randomized seeding with droppable silver iodide (AgI) flares in West Texas during the Southwest Cooperative Program is addressed. Attention is focused on individual convective cells and on the small mesoscale convective clusters that contain the cells.

Analysis of three-dimensional, volume-scan, C-band radar data using sophisticated tracking software indicates that AgI seeding increased the areas, durations and rain volumes of the cells. The radar-estimated rainfall volume at bases of the AgI-treated cells was more than double the rain volume from the cells that received simulated treatment. This result is significant at the 3% significance level using rerandomization procedures. The apparent effect of seeding and its significance increases slightly when control cells are incorporated into the analysis. The effect of treatment on maximum cell height, as measured by radar, generally averaged less than 5%.

In moving from the cell scale to the larger sca;es, it was found that cell merger occurred twice as often in the AgI-treated cases. Merging was most pronounced for cells treated early in their lifetimes with 9 or more AgI flares.

The next step focussed on the areas in which the cells received treatment. This “focused area” approach involved calculations for radii of 5, 7, 10, 15. 20, 25 and 35 km around each treatment position, providing eight separate analyses. The rainfalls from the seeded cells exceeded the rainfalls from the non-seeded cells in the focused area by over 50% by the end of the analysis period. 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 into the overall experimental unit with time.

The final step in the study involved examination of the experimental units themselves. The ratios of Seed (S) to No Seed (NS) rainfalls by half-hour interval and cumulatively generally exceed a factor of 1.20 for the two approaches employed in the analyses. The ratios are largest for mean cumulative rainfalls at 2.0 to 2.5 hours after qualification of the experimental units. None of the results have strong P-value support.

Because of the small sample and the large natural rainfall variability it is likely that chance has confounded this assignment of the results of treatment in the SWCP. In order to obtain a clearer picture of the effect of cloud seeding in West Texas, it in recommended that the sample be expanded further and that subsequent analyses include the use of predictive equations to reduce the impact of the natural rainfall variability. It is recommended further that cloud microphysical measurements be incorporated into the next stage of the studies in order to better understand how the apparent increases in rainfalls were produced.

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Anthony R. Olsen and William L. Woodley

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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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William L. Woodley and John A. Flueck

Abstract

No abstract available.

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William L. Woodley, Anthony Barnston, John A. Flueck, and Ron Biondini

Abstract

FACE-2 was a confirmatory weather modification experiment conducted in south Florida in the three summers of 1978, 1979 and 1980. The results of the FACE-2 replicated and pre-specified confirmatory rainfall analyses are presented and discussed in this paper. These results are interpreted in terms of the published criteria for confirmation of the FACE-1 rainfall results. FACE-2 did not confirm the results of FACE-1, although there are indications that seeding did increase the rainfall over the FACE target area. Some explanations for the lack of confirmation are suggested.

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