Effects Of Cloud Seeding Research Articles

On the Radar Detection of Cloud Seeding Effects in Wintertime Orographic Cloud Systems

Abstract Recent studies from the Seeded and Natural Orographic Wintertime Clouds: The Idaho Experiment (SNOWIE) demonstrated definitive radar evidence of seeding signatures in winter orographic clouds during three intensive operation periods (IOPs) where the background signal from natural precipitation was weak and a radar signal attributable to seeding could be identified as traceable seeding lines. Except for the three IOPs where seeding was detected, background natural snowfall was present during seeding operations and no clear seeding signatures were detected. This paper provides a quantitative analysis to assess if orographic cloud seeding effects are detectable using radar when background precipitation is present. We show that a 5-dB change in equivalent reflectivity factor Ze is required to stand out against background natural Ze variability. This analysis considers four radar wavelengths, a range of background ice water contents (IWC) from 0.012 to 1.214 g m−3, and additional IWC introduced by seeding ranging from 0.012 to 0.486 g m−3. The upper-limit values of seeded IWC are based on measurements of IWC from the Nevzorov probe employed on the University of Wyoming King Air aircraft during SNOWIE. This analysis implies that seeding effects will be undetectable using radar within background snowfall unless the background IWC is small, and the seeding effects are large. It therefore remains uncertain whether seeding had no effect on cloud microstructure, and therefore produced no signature on radar, or whether seeding did have an effect, but that effect was undetectable against the background reflectivity associated with naturally produced precipitation. Significance Statement Operational glaciogenic seeding programs targeting wintertime orographic clouds are funded by a range of stakeholders to increase snowpack. Glaciogenic seeding signatures have been observed by radar when natural background snowfall is weak but never when heavy background precipitation was present. This analysis quantitatively shows that seeding effects will be undetectable using radar reflectivity under conditions of background snowfall unless the background snowfall is weak, and the seeding effects are large. It therefore remains uncertain whether seeding had no effect on cloud microstructure, and therefore produced no signature on radar, or whether seeding did have an effect, but that effect was undetectable against the background reflectivity associated with naturally produced precipitation. Alternative assessment methods such as trace element analysis in snow, aircraft measurements, precipitation measurements, and modeling should be used to determine the efficacy of orographic cloud seeding when heavy background precipitation is present.

Evaluation of hygroscopic cloud seeding in warm-rain processes by a hybrid microphysics scheme using a Weather Research and Forecasting (WRF) model: a real case study

Abstract. To evaluate the hygroscopic cloud seeding in reality, this study develops a hybrid microphysics scheme using a Weather Research and Forecasting (WRF) model, WDM6-NCU (WDM6 modified by National Central University), which involves 43 bins of seeded cloud condensation nuclei (CCN) in the WDM6 bulk method scheme. This scheme can describe the size distribution of seeded CCN and explain the process of the CCN imbedding and cloud and raindrop formation in detail. Furthermore, based on the observational CCN size distribution applied in the modelling, a series of tests on cloud seeding were conducted during the seeding periods of 21–22 October 2020 with stratocumulus clouds. The model simulation results reveal that seeding in in-cloud regions with an appropriate CCN size distribution can yield greater rainfall and that spreading the seeding agents over an area of 40–60 km2 is the most efficient strategy to create a sufficient precipitation rate. With regard to the microphysical processes, the main process that causes the enhancement of precipitation is the strengthening of the accretion process of raindrops. In addition, hygroscopic particles larger than 0.4 µm primarily contribute to cloud-seeding effects. The study results could be used as references for model development and warm-cloud-seeding operations.

Indications of Downwind Cloud Seeding Effects in Utah

Estimation of effects on precipitation downwind (or “extra-area effects”) of a long-standing winter operational snowpack augmentation project in central and southern Utah mountainous areas was originally conducted in a 2003 study. The study utilized a target/control linear regression technique, which has been used to estimate the seasonal effects of cloud seeding within the program’s intended target area. The results of the original study have been updated through the 2018 water year. Seeded target area analyses of December-March high elevation (NRCS SNOTEL) precipitation data for this program indicate an overall season-average increase of about 12% for 41 seeded seasons. Estimations of downwind seeding effects were made for individual stations and various distance bands downwind as far as 100+ miles. The analyses suggest increases of similar percentages to those for the target, expressed as ratio values to the natural precipitation amounts, extending as far as 100 miles downwind (approximately the Utah/Colorado border area). At approximately 100 miles downwind, the area-averaged ratio values (observed divided by predicted values) approach 1.0, suggesting a lack of any significant seeding effect at these distances. Expressed as average-depth precipitation amounts, the target area precipitation increase from seeding is likely about 1.3” of additional water, with much lesser total precipitation increases in the much drier downwind area within 100 miles, trending to 0 beyond this distance. Although variability of the individual downwind station results remains high, the updated results are in good overall agreement with those of the original (2003) study.

Artificial Seeding Effects of Convective Clouds on the Opening Day of Beijing 2008 Summer Olympics

Using the radar reflectivity and intensive rainfall data, artificial seeding effects of convective clouds in Beijing on 8 August 2008, the opening day of the 29th Summer Olympics, were analyzed. The results indicate that, cloud seeding at single operation site for convective clouds invading from southwest direction may sharply mitigate the rainfall observed at leeward automatic weather stations within 5 - 10 min, while enhances the precipitation at a later stage about 10 - 20 min. Cloud seeding effects of operation sites Yuegezhuangxi, Changgouzhen, and Zhoukou, which are placed along the main moving routes or localized developing convective clouds in the west and center parts of Fangshan district, are very conspicuous. Combining the operation sites distribution and radar echoes, it is found that the site Changgouzhen, which is very close to the convective core, plays an essential role in suppressing the growth of convective cloud, reducing the coverage area of intense echoes classified as 45 - 60 dBZ, as well as mitigating the precipitation from neighboring automatic weather stations. Based on radar reflectivity and rainfall data, we find that the clouds over lots of operation sites in eastern Fangshan district are not cold enough to favor glaciogenic seeding with silver iodide, meanwhile, there is not too much precipitation observed.

Implementation of a Silver Iodide Cloud-Seeding Parameterization in WRF. Part I: Model Description and Idealized 2D Sensitivity Tests

AbstractA silver iodide (AgI) cloud-seeding parameterization has been implemented into the Thompson microphysics scheme of the Weather Research and Forecasting model to investigate glaciogenic cloud-seeding effects. The sensitivity of the parameterization to meteorological conditions, cloud properties, and seeding rates was examined by simulating two-dimensional idealized moist flow over a bell-shaped mountain. The results verified that this parameterization can reasonably simulate the physical processes of cloud seeding with the limitations of the constant cloud droplet concentration assumed in the scheme and the two-dimensional model setup. The results showed the following: 1) Deposition was the dominant nucleation mode of AgI from simulated aircraft seeding, whereas immersion freezing was the most active mode for ground-based seeding. Deposition and condensation freezing were also important for ground-based seeding. Contact freezing was the weakest nucleation mode for both ground-based and airborne seeding. 2) Diffusion and riming on AgI-nucleated ice crystals depleted vapor and liquid water, resulting in more ice-phase precipitation on the ground for all of the seeding cases relative to the control cases. Most of the enhancement came from vapor depletion. The relative enhancement by seeding ranged from 0.3% to 429% under various conditions. 3) The maximum local AgI activation ratio was 60% under optimum conditions. Under most seeding conditions, however, this ratio was between 0.02% and 2% in orographic clouds. 4) The seeding effect was inversely related to the natural precipitation efficiency but was positively related to seeding rates. 5) Ground-based seeding enhanced precipitation on the lee side of the mountain, whereas airborne seeding from lower flight tracks enhanced precipitation on the windward side of the mountain.

Separation between Cloud-Seeding and Air-Pollution Effects

Abstract Enhancement of precipitation by cloud-seeding operations has been reported in many studies around the world in the last several decades. On the other hand, suppression of rain and snow by urban and industrial air pollution recently has been documented and quantified. Here it is shown that the two effects are the opposite sides of the same coin, demonstrating the sensitivity of clouds to anthropogenic aerosols of different kinds. This is done by analyzing the rainfall amounts in northern Israel during the last 53 years and explaining the changes there as the combined opposite effects of precipitation suppression by air pollution and enhancement by glaciogenic cloud seeding. Time series based on precipitation from rain gauges were analyzed for seeded and nonseeded days and periods in the experimental control and the target areas. The response variable is Ro, the orographic enhancement factor, which is the ratio of gauge-measured rainfall in inland hilly areas (500–1000 m) to the rainfall at the upwind coasts and plains. The results show that for the whole period of 1950–2002 the Ro of the hilly areas decreased by 15%. In the early nonseeded period (1950–60) Ro was found to be higher than the nonseeded days of the following period, which was the randomized experimental period (1961–74). This result apparently shows the effect of the increasing pollution. Factor Ro had an identical decreasing trend during the seeded days of the experimental period and through the subsequent fully operationally seeded period (1975–2002). However, the trend line of Ro was shifted upward by 12%–14% for the seeded rain time series in comparison with the unseeded time series. Thus, the opposite effects of air pollution and seeding appear to have nearly canceled each other in recent years, leading to the false impression that cloud seeding is no longer effective. However, the findings here suggest that if the operational seeding were to stop, Ro would decrease further by about 12%–14%. The sensitivity of Ro to both seeding and pollution effects was greatest in the areas with the greatest natural orographic enhancement factor and was practically nonexistent in areas in which Ro is near unity. This result suggests that the orographic clouds are the most sensitive to air pollution as well as to cloud-seeding effects on clouds and precipitation, in agreement with the large susceptibility of precipitation from such short-living shallow clouds to aerosols. Based on previous studies and on the results of this paper, it is suggested that the proposed mechanism is the most likely explanation to the observations, and no alternative explanations such as long-term trends in the cross-mountain moisture flux were found probable. It is certain that additional research is required.

An Exploratory Analysis of Crop Hail Insurance Data for Evidence of Cloud Seeding Effects in North Dakota

The basis for the cloud seeding operations of the North Dakota Cloud Modification Project (NDCMP) is first outlined. Then the multiresponse permutation procedures are applied in an analysis of crop hail insurance data for the NDCMP target area and for an upwind control area in eastern Montana. A historical analysis of the annual hail insurance loss ratios for the target area indicates lower hail-loss experience during the NDCMP operational years 1976‐88. A corresponding analysis for the control area shows no indication of a difference during those years, suggesting the absence of any significant climatological variation. Analysis of a target‐ control scatterplot of the loss ratios also indicates that the target area experienced relatively smaller hail losses during the NDCMP period. An inference that the difference can be attributed to the NDCMP seeding operations appears to be justified, and the reduction in hail insurance loss ratios in the target area during the NDCMP years is estimated to be about 45%.

Ice Nuclei, Rainwater Chemical Composition, and Static Cloud Seeding Effects in Israel

This study analyzed the temporal and spatial variation of natural ice nuclei (IN), total suspended particles (TSP), and rainwater chemical composition (RCC) in Israel. This research is complementary to the statistical analyses of cloud seeding, which have shown significant positive seeding effects only in northern Israel, together with detrimemtal effects of desert dust. It was observed that the concentration of continental components dissolved in rainwater increases by about an order of magnitude from northern to southern Israel. High values of IN, TSP, and continental origin matter dissolved in rainwater were coincident with geographic location and synoptic conditions that are indicative of the presence of desert dust. These conditions are coincident with the situations in which cloud seeding did not result in significant rain enhancement. According to geochemical ratios, a major fraction of the dust found in the rainwater originated at the Sahara Desert. These findings support the hypothesis that desert dust has a detrimental effect on cloud seeding in Israel, due to its cloud condensation nuclei and/or ice nucleating activity.

A Review of Cloud Modeling in Weather Modification

The numerical modeling of clouds has a history almost as long as the modern-day concepts of the seeding of clouds. The various models and theoretical developments emerging from the model results are reviewed in this paper. Significant advances have been made in understanding cloud-seeding effects, which bodes well for the continued application of this form of weather modification in the future.

Tests for Persistent Effects of Cloud Seeding in a Recent Australian Experiment

Abstract An analysis of cloud seeding experiments in Australia prior to 1984 that used silver iodide dispensed from aircraft as the seeding agent suggested that there were systematic aftereffects of seeding at least one month after a given seeded day, as revealed by mean target–control precipitation ratios a given number of days after a seeded day. Confirmation has been obtained from a new experiment in southeastern Australia in which both precipitation and ice nuclei were measured. The timing of the aftereffects on precipitation agreed closely with those of the earlier experiments, were confined to the regions that could be expected to have silver iodide deposited on them in rainwater, and were dependent on the amount of silver iodide used in the individual events. Correlations with the persistence time series of earlier work were significant at levels better than 0.1%. However, when unseeded days that were suitable for seeding formed the basis of the persistence time series for the new experiment, there...

A new concept for assessing silver iodide cloud-seeding effects in snow by physical and chemical methods

A new concept for assessing silver iodide cloud-seeding effects in snow by physical and chemical methods

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. Ad...

Effects of Cloud Seeding in West Texas

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.

Persistent Effects of Cloud Seeding with Silver Iodide

A statistical examination of precipitation records in and near areas where cloud seeding experiments have taken place in Australia strongly suggests the delayed effects of seeding. The most conspicuous effect is an increase in precipitation 1 to 3 weeks after a seeded day, which would have caused the conventional statistical estimates of success to have been misleading. Through comparison with earlier findings and some observations of prolonged increases in ice nucleus concentrations following the application of silver iodide to the ground, it is argued that secondary ice nuclei are involved in the persistent effects of seeding. If this is so, then they must have been more effective in enhancing precipitation than the silver iodide, itself; this leads to the possibility that there may be better ways of stimulating precipitation than those used so far.

Comments on “Delayed Effects of Cloud Seeding with Silver Iodide”

Comments on “Delayed Effects of Cloud Seeding with Silver Iodide”

Analysis of Recording Raingage Data for the Israeli II Experiment. Part I: Effects of Cloud Seeding on the Components of Daily Rainfall

Earlier Published analyses of the second Israeli randomized experiment (1969–75) were restricted to 24-h data; this paper provides more details which are based on continuous time data from recording raingages. The present analyses confirm that when cloud tops were warmer than −21°C, seeding increased the efficiency of precipitation. In the −21° to −11°C window, both amount and duration of rainfall increased by some 50%, but no extra rain events appeared. Extra rain events were apparently initiated by seeding when cloud-top temperatures were warmer (−11°C and above); however, this did not significantly increase the amount of rainfall. No effect of seeding was found when cloud tops were colder than −21°C. It appears that seeding makes the existing process of rain formation more effective and also inducts precipitation formation in some clouds that would not have precipitated naturally.

Analysis of Recording Raingage Data for the Israeli II Experiment. Part II: Differential Day and Night Effects of Cloud Seeding

This paper reports separate analyses of daytime and nighttime precipitation based on data from recording raingages of the second Israeli randomized experiment. These analyses seemed important because there are a number of hypotheses on the differential effects of AgI seeding during the day and at night, and because about half of the seeding of this experiment was done at night. Our findings are unfortunately equivocal because of the large variability of the date which had to be broken down by categories of modal cloud-top temperatures and 12-b (day/night) periods. The increase of precipitation efficiency that had been noticed (on 24-h data) to occur in the −12° to −21°C window appears to have been larger at night, but the difference is not significant. The increase in the number of rain events for warmer clouds may have been only a daytime effect, but again, the present data are not conclusive.

Delayed Effects of Cloud Seeding with Silver Iodide

Abstract Apparent delayed effects of aerial cloud seeding with silver iodide are demonstrated in Smith et al.'s experiment in Tasmania. These effects reached a maximum nine days after a seeded day and were most unlikely to have arisen by chance. A possible cause of these and even more delayed effects of seeding is shown to be the enhancement of concentrations of airborne ice-nucleating bacteria by small amounts of silver iodide applied to vegetation.

Effects of Cloud Seeding, Latent Heat of Fusion, and Condensate Loading on Cloud Dynamics and Precipitation Evolution: A Numerical Study

Abstract This study attempts to isolate the dynamic and microphysical effects of seeding. A two-dimensional, time-dependent cloud model has been used to simulate silver iodide (AgI) seeding of convective clouds. Two major dynamic effects (latent heat of fusion and condensate loading) are separated through a sequence of differential processes to examine their individual effect. A High Plains sounding is used for the tests. The effects of condensate loading and latent heat of fusion are due to natural processes as well as to cloud seeding. Separate discussions and comparisons are made of both processes. Condensate loading has the greater influence on cloud development. A method of differencing the results from different cases is used to illustrate the overall seeding effects and to isolate those portions of the latent heat of fusion and loading effects which are due solely to ice-phase cloud seeding. The results indicate significant fusion and loading effects due to seeding, but at 10 min or so after the se...

Reexamination of Cloud-Top Temperatures Used as Criteria for Stratification of Cloud Seeding Effects in Experiments on Winter Orographic Clouds

A reexamination of cloud-top temperatures used in the evaluation of winter orographic cloud seeding experiments is made. Cloud-top temperatures derived from rawinsonde data are found to be typically about 20°C too warm during daytime in the years 1965–72, and larger errors existed in prior years. From these findings it is concluded that temperature stratifications of seeding effects and the resulting conclusions in several studies are inappropriate.