Search Results
You are looking at 1 - 10 of 35 items for
- Author or Editor: William L. Woodley x
- Refine by Access: All Content x
Abstract
No abstract available.
Abstract
No abstract available.
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.
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.
A statistical study has been conducted investigating the possible cloud growth effects due to heat released when supercooled water is converted to ice. Calculations are made of the heights to which cloud tops would rise assuming that the supercooled water is converted to ice at − 10C (the seeded case) or at − 30C (the unseeded case). The clouds suffer a loss in buoyancy due to entrainment and the weight of the condensate. Sixty-two cases are treated for soundings from St. Martins Island during August of 1962. The results of this study are compared to those of a similar study for Flagstaff, Arizona, during the summers of 1961, 1962 and 1963. The results imply that spectacular height increases due to seeding can be expected, but that such cases are relatively infrequent. Specific observations of cloud growth due to seeding are noted, but can only be considered as consistent with these concepts rather than verifying them. The factors important for the seeding effect and the conditions which produce them are also examined. Despite the simplicity of the model, it is felt that the computed height differences qualitatively represent a reasonable measure of the potential for cloud dynamics changes from seeding.
A statistical study has been conducted investigating the possible cloud growth effects due to heat released when supercooled water is converted to ice. Calculations are made of the heights to which cloud tops would rise assuming that the supercooled water is converted to ice at − 10C (the seeded case) or at − 30C (the unseeded case). The clouds suffer a loss in buoyancy due to entrainment and the weight of the condensate. Sixty-two cases are treated for soundings from St. Martins Island during August of 1962. The results of this study are compared to those of a similar study for Flagstaff, Arizona, during the summers of 1961, 1962 and 1963. The results imply that spectacular height increases due to seeding can be expected, but that such cases are relatively infrequent. Specific observations of cloud growth due to seeding are noted, but can only be considered as consistent with these concepts rather than verifying them. The factors important for the seeding effect and the conditions which produce them are also examined. Despite the simplicity of the model, it is felt that the computed height differences qualitatively represent a reasonable measure of the potential for cloud dynamics changes from seeding.
Abstract
Spaceborne inferences of cloud microstructure and precipitation-forming processes with height have been used to investigate the effect of ingested aerosols on clouds and to integrate the findings with past cloud physics research. The inferences were made with a method that analyzes data from National Oceanic and Atmospheric Administration Advanced Very High Resolution Radiometer (NOAA AVHRR) and Tropical Rainfall Measuring Mission Visible and Infrared Scanner (TRMM VIRS) sensors to determine the effective radius of cloud particles with height. In addition, the TRMM Precipitation Radar (PR) made it possible to measure the rainfall simultaneously with the microphysical retrievals, which were validated by aircraft cloud physics measurements under a wide range of conditions. For example, the satellite inferences suggest that vigorous convective clouds over many portions of the globe remain supercooled to near −38°C, the point of homogeneous nucleation. These inferences were then validated in Texas and Argentina by in situ measurements using a cloud physics jet aircraft.
This unique satellite vantage point has documented enormous variability of cloud conditions in space and time and the strong susceptibility of cloud microstructure and precipitation to the ingested aerosols. This is in agreement with past cloud physics research. In particular, it has been documented that smoke and air pollution can suppress both water and ice precipitation-forming processes over large areas. Measurements in Thailand of convective clouds suggest that the suppression of coalescence can decrease areal rainfall by as much as a factor of 2. It would appear, therefore, that pollution has the potential to alter the global climate by suppressing rainfall and decreasing the net latent heating to the atmosphere and/or forcing its redistribution. In addition, it appears that intense lightning activity, as documented by the TRMM Lightning Imaging Sensor (LIS), is usually associated with microphysically highly “continental” clouds having large concentrations of ingested aerosols, great cloud-base concentrations of tiny droplets, and high cloud water contents. Conversely, strongly “maritime” clouds, having intense coalescence, early fallout of the hydrometeors, and glaciation at warm temperatures, show little lightning activity. By extension these results suggest that pollution can enhance lightning activity.
The satellite inferences suggest that the effect of pollution on clouds is greater and on a much larger scale than any that have been documented for deliberate cloud seeding. They also provide insights for cloud seeding programs. Having documented the great variability in space and time of cloud structure, it is likely that the results of many cloud seeding efforts have been mixed and inconclusive, because both suitable and unsuitable clouds have been seeded and grouped together for evaluation. This can be addressed in the future by partitioning the cases based on the microphysical structure of the cloud field at seeding and then looking for seeding effects within each partition.
This study is built on the scientific foundation laid by many past investigators and its results can be viewed as a synthesis of the new satellite methodology with their findings. Especially noteworthy in this regard is Dr. Joanne Simpson, who has spent much of her career studying and modeling cumulus clouds and specifying their crucial role in driving the hurricane and the global atmospheric circulation. She also was a pioneer in early cloud seeding research in which she emphasized cloud dynamics rather than just microphysics in her seeding hypotheses and in her development and use of numerical models. It is appropriate, therefore, that this paper is offered to acknowledge Dr. Joanne Simpson and her many colleagues who paved the way for this research effort.
Abstract
Spaceborne inferences of cloud microstructure and precipitation-forming processes with height have been used to investigate the effect of ingested aerosols on clouds and to integrate the findings with past cloud physics research. The inferences were made with a method that analyzes data from National Oceanic and Atmospheric Administration Advanced Very High Resolution Radiometer (NOAA AVHRR) and Tropical Rainfall Measuring Mission Visible and Infrared Scanner (TRMM VIRS) sensors to determine the effective radius of cloud particles with height. In addition, the TRMM Precipitation Radar (PR) made it possible to measure the rainfall simultaneously with the microphysical retrievals, which were validated by aircraft cloud physics measurements under a wide range of conditions. For example, the satellite inferences suggest that vigorous convective clouds over many portions of the globe remain supercooled to near −38°C, the point of homogeneous nucleation. These inferences were then validated in Texas and Argentina by in situ measurements using a cloud physics jet aircraft.
This unique satellite vantage point has documented enormous variability of cloud conditions in space and time and the strong susceptibility of cloud microstructure and precipitation to the ingested aerosols. This is in agreement with past cloud physics research. In particular, it has been documented that smoke and air pollution can suppress both water and ice precipitation-forming processes over large areas. Measurements in Thailand of convective clouds suggest that the suppression of coalescence can decrease areal rainfall by as much as a factor of 2. It would appear, therefore, that pollution has the potential to alter the global climate by suppressing rainfall and decreasing the net latent heating to the atmosphere and/or forcing its redistribution. In addition, it appears that intense lightning activity, as documented by the TRMM Lightning Imaging Sensor (LIS), is usually associated with microphysically highly “continental” clouds having large concentrations of ingested aerosols, great cloud-base concentrations of tiny droplets, and high cloud water contents. Conversely, strongly “maritime” clouds, having intense coalescence, early fallout of the hydrometeors, and glaciation at warm temperatures, show little lightning activity. By extension these results suggest that pollution can enhance lightning activity.
The satellite inferences suggest that the effect of pollution on clouds is greater and on a much larger scale than any that have been documented for deliberate cloud seeding. They also provide insights for cloud seeding programs. Having documented the great variability in space and time of cloud structure, it is likely that the results of many cloud seeding efforts have been mixed and inconclusive, because both suitable and unsuitable clouds have been seeded and grouped together for evaluation. This can be addressed in the future by partitioning the cases based on the microphysical structure of the cloud field at seeding and then looking for seeding effects within each partition.
This study is built on the scientific foundation laid by many past investigators and its results can be viewed as a synthesis of the new satellite methodology with their findings. Especially noteworthy in this regard is Dr. Joanne Simpson, who has spent much of her career studying and modeling cumulus clouds and specifying their crucial role in driving the hurricane and the global atmospheric circulation. She also was a pioneer in early cloud seeding research in which she emphasized cloud dynamics rather than just microphysics in her seeding hypotheses and in her development and use of numerical models. It is appropriate, therefore, that this paper is offered to acknowledge Dr. Joanne Simpson and her many colleagues who paved the way for this research effort.
Abstract
No abstract available.
Abstract
No abstract available.
Abstract
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.
Abstract
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.
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.
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.
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.
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.
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.
