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William L. Woodley and John A. Flueck

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

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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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Abraham Gagin, Daniel Rosenfeld, William L. Woodley, and Raul E. Lopez

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Volume scan radar studies incorporating the use of an elaborate method of defining and tracking convective rain cells through their lifetime have been used to

a) Explore and verify, in quantitative terms, the basic tenet of the technique of cloud seeding aimed at producing dynamic effects. This technique relates increases in the depth of convective cells, assumed to occur due to this type of seeding, to corresponding increases in the treated cells' rainfall intensity, area and duration of precipitation and, consequently, to the total yield of rainfall volume.

b) Employ the data gathered on the gross properties of rainfall of convective cells, namely their heights, intensities, precipitation areas and their durations and total rain volume, to estimate the effect of seeding, if any, on their properties.

These studies suggest that seeding convective cells for dynamic effects affected the preceding properties of these cells in a manner that resulted in increases in their total rainfall and that the positive changes in these properties could be predicted from the changes in maximum cell height following seeding.

The effect of seeding appears to be strongest for cells treated early in their life cycle with a substantial amount of AgI (i.e., more than 600 g). Seeding effects of 22% increases in cell heights and over 100% increases in cell rain volume are indicated under such seeding conditions. The significance levels of these results are found to be 2.1% and 0.6%, respectively.

The positive effects produced by seeding on the AgI treated cells may have resulted in a compensating negative effect on the smaller untreated clouds forming in the vicinity of these treated cells.

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JoséG. Meití, William L. Woodley, and John A. Flueck

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The second phase of the Florida Area Cumulus Experiment (FACE-2) has been completed and an exploratory analysis has been conducted to investigate the possibility that cloud seeding may have affected the rainfall outside the intended target. Rainfall was estimated over a 3.5×105 km2 area centered on the target using geosynchronous, infrared satellite imagery and the Griffith-Woodley rain estimation technique. This technique was derived in South Florida by calibrating infrared images using raingage and radar observations to produce an empirical, diagnostic (a posteriori), satellite rain estimation technique. The satellite rain estimates for the extended area were adjusted based on comparisons of raingage and satellite rainfall estimates for the entire FACE target (1.3×104 km2). All daily rainfall estimates were composited in two ways: 1) in the original coordinate system and 2) in a relative coordinate system that rotates the research area as a function of wind direction. After compositing, seeding effects were sought as a function of space and time.

The results show more rainfall (in the mean) on seed than no seed days both in and downwind of the target but lesser rainfall upwind. All differences (averaging 20% downwind and −10% upwind) are confined in space to within 200 km of the center of the FACE target and in time to the 8 h period after initial treatment. In addition, the positive correlation between untreated upwind rainfall and target rainfall is degraded on seed days, suggesting possible intermittent negative effects of seeding upwind. Although the development of these differences in space and time suggests that seeding may have been partially responsible for their generation, the results do not have strong inferential (P-value) support.

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Joanne Simpson, William L. Woodley, Anthony Olsen, and Jane C. Eden

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Randomized dynamic cumulus seeding programs were executed in 1968 and 1970 on isolated clouds and beginning in 1970 on groups of clouds, to promote mergers in a 4000 mi2 target area in south Florida. With the single clouds, 26 seeded and 26 control cases comprise an adequate sample. In the area experiment, 1970 1971 and 1972 produced only 7 random seed, 5 random control, and 5 non-random control cases. The experiments involve a multi-pronged approach to documentation of seeding effects, emphasizing numerical simulation, ground and airborne measurements, and the application of diverse statistical tools, including but not confined to Bayesian analysis, the main subject of this report.

Rain volumeswere calculated with calibrated 10-cm radars, checked and corrected by gage networks. The single cloud rainfalls, both seeded and control, were well fitted by a gamma distribution with the shape parameter invariant under seeding. Preliminary indications with the area data suggest carryover to the multiple cloud experiment, for both the total target rainfall and that of the “floating target” which moves with seeded complexes.

Bayes equation is formulated for the posterior (after data) probability distribution of the seeding factor f defined as the ratio by which dynamic seeding increases the rain. Prior probabilities are mainly diffuse, with results insensitive to their choice. Natural distributions are specified by control sample means (and the appropriate gamma shape parameter). Sensitivity tests show greater dependence on the former, for which the effects of sampling errors are examined. Finally, results are used to estimate the number of cases required to resolve various magnitudes of seeding factor.

Results for the single clouds are virtually conclusive that seeding increased rainfall, by a factor of about 1.7, in good agreement with published results from classical statistics. For the “floating target” indications are strong, but not conclusive, that dynamic seeding had a positive effect, with the expected value about 3. The tentative estimate of total target seeding factor is about 1.7, with too high standard deviation (−σ0.5) and not firm enough depiction of natural variability for confidence.

If the total target seeding factor were in this approximate vicinity, however, two conclusions would follow: 1) continuation of the experiment is readily justified for practical benefit-cost as well as scientific reasons, and 2) about 50 pairs of cases may be required to resolve the result conclusively.

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Cecilia Girz Griffith, John A. Augustine, and William L. Woodley

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A satellite rain-estimation technique, derived in Florida for convective rainfall, was used to estimate areal rainfall in the U.S. High Plains. Raingages in dense and sparse networks provided the verification data. Unadjusted satellite-inferred rainfalls exceeded the corresponding gage estimates by a factor of 3–5, depending on the area size. This was expected and it is the result of treating convective clouds in arid regions as tropical clouds.

Two objective methods were derived to adjust the technique for use in the High Plains. The first involved gage and satellite comparisons for a small area and then extrapolation of this comparison to satellite rain estimates for large areas. The second involved calculation of an adjustment factor using the output of a one-dimensional cumulus cloud model. Accuracy of the adjusted rainfalls are discussed in terms of bias, mean error factor, root mean square error and linear regression analyses.

These preliminary results suggest that the satellite convective rain estimation technique can provide rain estimates of considerable utility once the estimates are adjusted for regional differences.

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William L. Woodley, Anthony R. Olsen, Alan Herndon, and Victor Wiggert

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Gage and radar methods of convective rain measurement are compared in the context of the continuing multiple cloud seeding experiment of the Experimental Meteorology Laboratory. An optimal system, combining the best features of both, is recommended.The nature of the Florida convective rainfall to be measured is documented using measurements from a dense raingage mesonet (about 3 km2 per gage over 570 km2) that was operated for a total of 93 days in 1971 and 1973, and the gaging requirements for detection and measurement of 24 h rainfalls in the mesonet are determined using the full complement of gages as the standard. For the measurement of areal convective rainfall greater than 0.25 mm within a factor of 2 on 90, 70 and 50% of the days, gage densities of 31, 91 and 208 km2 per gage, respectively, are required.Radar performance in estimating convective rainfall over south Florida is determined using two collocated, calibrated 10 cm radars (UM/10-cm of the University of Miami and WSR-57 of the National Hurricane Center). In all cases, the radar estimates of rainfall are compared with the rainfall as determined by raingages (densities 3 to 8 km2 per gage) in cluster arrays. The relative performances of the two radars are compared.In 1973, WSR-57 radar-derived rainfalls were computed by hand as in 1972 and by computer using taped radar observations. On a daily basis, 80% of the radar estimates were within a factor of 2 of the cluster standard. The combined accuracy of the WSR-57 radar in 1972 and 1973 in estimating convective rainfall approximated that which one would obtain with a gage density of 65 km2 per gage over an area the size of the mesonet.The daily representation of rainfall by the radar improves if one adjusts it using gages. In the mean, adjustment produced a statistically significant 15% improvement (<1% level with two-tailed “t” test) in radar accuracy. The adjusted radar measurements then had an approximate gage density equivalence of 25 km2 per gage.The gaging requirements for the estmation of area mean rainfall for an area the size of the EML target (1.3 × 104 km2) is inferred using the digitized radar observations. To meet a specification that the area-mean rainfall be measured to within a factor of 2 of the true value 99% of the time requires 143 km2 per gage, compared to a requirement of at least 13 km2 per gage for the mesonet.An optimum method of rain measurement is suggested. For the measurement of the rainfall from individual showers anywhere, the gage-adjusted radar is far superior to gages alone. For measurement in a fixed area the size of the mesonet, gages are superior to the radar. To measure rainfall over the EML target either gages alone, or a radar adjusted by gages, can accomplish the task. About 90 evenly spaced gages in the EML target should provide area rain measurements within a factor of 2 of the true value 99% of the time. The radar estimates adjusted by gages should be as accurate as those provided by the network of 90 gages. The final choice as to the measurement system will probably be determined by other considerations such as budget, personnel and terrain over which the measurements are to be made.

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Stephanie P. Browner, William L. Woodley, and Cecilia G. Griffith

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An analysis of 16 days from eight Atlantic storms, two in 1974 and six in 1975, objectively quantified a suspected diurnal oscillation of tropical storm cirrus cloud cover. The oscillation shows a maximum area at approximately 1700 local mean solar time and a minimum area at 0300 local mean solar time. The average ratio of the maximum area to the minimum area is 1.65.

SMS infrared imagery was analyzed with a scanning false-color densitometer to obtain area measurements of the cloudiness associated with the storms. These measurements were made approximately every 1½ h at three temperature thresholds: 253, 239 and 223 K.

Two tests were performed to rule out the possibility of the oscillation being due either to the satellite sensor or to image processing. Measurement of the ocean surface temperature, was made with SMS-I to determine whether the sensor detected a constant ocean temperature. The second test compared simultaneous area measurements obtained by SMS-I and SMS-II. The results of these tests support the storm oscillation detected.

Two other related phenomena were also observed: 1) the amplitude of the area oscillation is apparently inversely proportional to the intensity of the storm, and 2) a time-dependent, shorter period oscillation is superimposed on the daily oscillation. Inferences of causality are made.

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William L. Woodley, Daniel Rosenfeld, and Bernard A. Silverman

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Randomized, cold-cloud, rain-enhancement experiments were carried out during 1991–98 in the Bhumibol catchment area in northwestern Thailand. Exploratory experimentation in 1991 and 1993 suggested increases in rainfall from seeding. A demonstration experiment followed in which the design was specified in advance (i.e., a priori) to determine the potential of on-top silver iodide (AgI) seeding for the enhancement of area (1964 km2) rainfall. It was conducted in accordance with a moving-experimental-unit design as discussed in the design document and summarized herein. The evaluation of the demonstration experiment until its scheduled termination in 1998 involved both cell and unit analyses. The S-band project radar collected 5-min volume-scan data to be used to evaluate cell and unit properties and to determine the mean cell motion vector for the advection of the experimental unit with time. The cell dataset comprises 353 AgI-seeded cells and 289 control cells that received simulated AgI (glaciogenic) treatment. All cells were tracked using the Rosenfeld long-track procedures. The proportional effect of seeding on cell rain volume as estimated by radar is 35% with a one-sided P value of 0.139, which falls short of the P-value threshold of 0.025 that is required for statistical significance. The lower and upper bound of the corresponding 90% confidence interval is −14% and +111%, respectively. Analysis of the unit sample was limited to those cloud systems that postanalysis retracking revealed had been treated and had remained within the boundaries of the moving unit. The proportional effect of seeding on unit rainfalls at 300 min after unit qualification for the sample of 62 experimental units (31 seeded and 31 nonseeded) is 46% with a one-sided P value of 0.107. Thus, the effect of seeding on unit rainfalls also fell short of statistical significance at the threshold P value of 0.025. The 90% confidence interval for the strength of the seeding effect ranges from −11% to +142%, and the one-sided probability that the seeding effect is ≥0%, ≥5%, and ≥10% is 90%, 86%, and 83%, respectively. Regression analysis to account for the potential impact of prequalification unit rainfall biases favoring the seeded sample had no effect on the results of the evaluation. Although the demonstration experiment did not reach statistical significance, much is to be learned about the potential effects of cold-cloud seeding in Thailand from exploration of the full cell and unit demonstration samples, which is done in a companion paper ().

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Daniel Rosenfeld, Duncan Axisa, William L. Woodley, and Ronen Lahav

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It is shown here that hygroscopic seeding requires two orders of magnitude more hygroscopic agent than can be delivered by flare technology for producing raindrop embryos in concentrations to detect by cloud physics aircraft the microphysical signature of rain initiation. An alternative method of finely milled salt powder is shown to be capable of achieving this goal. During field experiments the use of a sulfur hexafluoride (SF6) gas tracer to identify the exact seeded cloud volume and to quantify dilution of the seeding agent showed that the seeding agent dilutes to the order of 10−10 of its released concentration in updrafts at a height of ≥1 km above cloud base. This means that the theoretically expected changes in the cloud drop size distribution (DSD) would not be detectable with a cloud droplet spectrometer in a measurement volume collected during the several seconds that the seeded volume is traversed by an aircraft. The actual measurements failed to identify a clear microphysical seeding signature from the burning of hygroscopic flares within the seeded convective clouds. This uncertainty with respect to hygroscopic flare–seeding experiments prompted an experimental and theoretical search for optimal hygroscopic seeding materials. This search culminated in the production of a salt powder having 2–5-μm-diameter particle sizes that are optimal according to model simulations, and can be distributed from a crop duster aircraft. Such particles act as giant cloud condensation nuclei (GCCN). Any potential broadening of the DSD at cloud base by the competition effect (i.e., when the seeded aerosols compete with the natural ambient aerosols for water vapor) occurs when the seeding agent has not been substantially diluted, and hence affects only a very small cloud volume that dilutes quickly. Therefore, the main expected effect of the GCCN is probably to serve as raindrop embryos. The salt powder–seeding method is more productive by two orders of magnitude than the hygroscopic flares in producing GCCN that can initiate rain in clouds with naturally suppressed warm rain processes, because of a combination of change in the particle size distribution and the greater seeding rate that is practical with the powder. Experimental seeding of salt powder in conjunction with the simultaneous release of an SF6 gas tracer produced strong seeding signatures, indicating that the methodology works as hypothesized. The efficacy of the accelerated warm rain processes in altering rainfall amounts may vary under different conditions, and requires additional research that involves both observations and simulations.

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