Ground-based Generation Research Articles

Variations in Rainfall and Insured Crop-Hail Losses Associated with Operational Cloud Seeding in South Dakota

A brief review of operational cloud seeding programs in South Dakota is given, followed by an evaluation on the basis of rainfalldata and insured crop-hail loss data for the years 1948-1978. All of the projects used silver iodide as the seeding agent, but a wide variety of generator types, seeding rates, and delivery methods was used. In years w.ith aircraft seeding over small areas, both rainfall and hail damage tended to be larger in target than in non-target counties. The data available do not show such a trend for years when ground-based generators were employed. In years when 40%or more of the state was seeded, the results for target and non-target counties becameindistinguishable in terms of both rainfall and crop hail damage. However, increases in the area included in cloud seeding programs, using either ground based or airborne generators, were associated with an apparent net hail suppression effect, extending over the entire state. The rainfall data do not display such a trend.

A Confirmatory Snowfall Enhancement Project in the Snowy Mountains of Australia. Part I: Project Design and Response Variables

AbstractThe Snowy Precipitation Enhancement Research Project (SPERP) was undertaken from May 2005 to June 2009 in the Snowy Mountains of southeastern Australia with the aim of enhancing snowfall in westerly flows associated with winter cold fronts. Building on earlier field studies in the region, SPERP was developed as a confirmatory experiment of glaciogenic static seeding using a silver-chloroiodide material dispersed from ground-based generators. Seeding of 5-h experimental units (EUs) was randomized with a seeding ratio of 2:1. A total of 107 EUs were undertaken at suitable times, based on surface and upper-air observations. Indium (III) oxide was released during all EUs for comparison of indium and silver concentrations in snow in seeded and unseeded EUs to test the targeting of seeding material. A network of gauges was deployed at 44 sites across the region to detect whether precipitation was enhanced in a fixed target area of 832 km2, using observations from a fixed control area to estimate the natural precipitation in the target. Additional measurements included integrated supercooled liquid water at a site in the target area and upper-air data from a site upwind of the target.

The Fresnel Imager: learning from ground-based generation I prototype

The “Generation I Fresnel Imager Prototype” is a ground-based prototype of a Fresnel Imager (Koechlin et al., Astron Astrophys 443:709–720, 2005), reduced in size (optical elements have a dimension of a few cm) but which features all the elements of an operational Fresnel Imager, and integrated in a clean room. Its design has started in October 2004, the first images were obtained early 2006, the key optical element: a cophased Fresnel Zone Lens, was integrated during summer 2006, and since then it has been used to evaluate new types of target shapes, of Fresnel Array cuttings... . It has been decommissioned end of 2008, some constitutive elements becoming parts of the so-called “Generation II Fresnel Imager Prototype”. We present the constitutive elements and the achieved results, and the lessons learned from this prototype.

An evaluation of eleven operational cloud seeding programs in the watersheds of the Sierra Nevada Mountains

A target–control statistical evaluation of 11 operational cloud seeding programs carried out in watersheds of the Sierra Nevada Mountains was conducted using Monte Carlo permutation (re-randomization) analysis. The water year (October–September) streamflow served as the response variable in the evaluations. The evaluation estimated the effect of seeding on unimpaired streamflow at each of the Sierra targets using the controls that give the most precise evaluation results possible with the available data. It was found that operational cloud seeding succeeded in increasing the streamflow in 6 of the 11 major watersheds in the Sierra Nevada Mountains. All 6 major watersheds indicating a positive seeding effect are on the western (upwind) side of the Sierra Nevada Mountain range. There was insufficient statistical evidence to reject the null hypothesis of no seeding effect in the other 5 major watersheds that were evaluated. It is noteworthy that the 5 watersheds whose evaluation was inconclusive include the 3 watersheds on the eastern (downwind) side of the Sierra Nevada Mountain range. The results of these evaluations and, in particular, those for the San Joaquin, Upper American and Carson–Walker operational cloud seeding programs illustrate the complexities involved in the transport and dispersion of silver iodide plumes from ground-based generators in mountainous terrain. The results suggest that aircraft seeding, either by itself or as a supplement to ground seeding, was able to affect targets that could not be affected by ground seeding alone. There was a statistically significant, positive seeding effect at the West Walker River Near Coleville target that was most likely due to contamination from an upwind seeding program, most likely the Mokelumne operational seeding program. Although contamination may have been present at the other seeding targets, it was not strong enough to affect the statistical results. Follow-on physical-statistical studies are needed to identify and understand the physical reasons for the statistical results of this study. In the opinion of this author, a comprehensive set of silver iodide tracer studies would contribute most to our understanding of the results, especially the dichotomy of seeding results for the operational seeding programs on the western and eastern watersheds of the Sierra Nevada Mountains.

Numerical Comparison of Two Ice Crystal Formation Mechanisms on Snowfall Enhancement from Ground-Based Aerosol Generators

Two mechanisms of ice crystal formation, contact freezing and very rapid condensation freezing, were applied to numerical simulations of ground-based seeding with the Guide Model, an orographic cloud model, to study whether different mechanisms of ice crystal formation substantially affect precipitation patterns and intensities. Although the numerical model has limitations, it was expected to indicate how different ice crystal formation rates lead to differences in precipitation patterns and intensities between the two mechanisms. Numerical simulations of two case studies are presented. One is characterized by moderate wind speeds and colder cloud temperatures, the other by stronger winds and warmer cloud temperatures. The moderate wind field and colder cloud temperatures yielded nearly half an order of magnitude more precipitation than the strong wind field and warmer cloud temperatures. Sensitivity analysis showed that snowfall as a result of forced condensation freezing is strongly dependent on the ambient temperature at the ground-based generator site, while generator site temperature had less effect on the precipitation as a result of contact freezing. Snowfall resulting from contact freezing, however, was found to be strongly dependent on the cloud drop concentration. Liquid water content did not significantly affect the precipitation resulting from ice crystal formation by either mechanism. Precipitation rates induced by forced condensation freezing are about two orders of magnitude greater than those induced by contact freezing in the cases simulated, over the Sierra crest, because of the limited time available for ice particles to grow and precipitate.

Weather modification: hail suppression in Spain

Hail damage to agricultural and horticultural crops alone costs the equivalent of millions of dollars annually in Spain, as it does in many other hail-prone countries. The use of radar and computer technology facilitates hailstorm studies and enables the probability of hail formation to be predicted from mathematical cloud models. Modern aircraft give a much better chance of success to cloud-seeding techniques formerly dependent on rockets and ground-based generators.

Feasibility of ground-based generation of artificial micropulsations

The magnetic field generated by a ground-based horizontal frame antenna operating in the Pc 1 frequency range is calculated for different ionospheric conditions. The generated signal can propagate laterally in the ionospheric waveguide and, as a result of mode conversion in the lower ionosphere, upward into the magnetosphere along the field lines above the source. For (nighttime) waveguide propagation, generation of fields of the order of 10 mγ at distances of thousands of kilometers requires the magnetic moment of an antenna of the order of 100 km in radius carrying a current of the order of 1 ka. A magnetic moment of the same order will produce upward-going fields of the order of tens of milligammas at the top of the ionosphere, for either daytime or nighttime conditions. Generation of micropulsations by a ground-based rotating superconducting magnet is briefly considered and is found to be well outside the range of current feasibility.

Continuous Ice Nuclei Measurements Made During Pulsed Cloud Seeding

In a cloud seeding program, it is necessary to know what meteorological conditions are controlling the travel of artificial ice nuclei. While it is obvious that they will travel with the air currents, it is difficult to determine if the currents will travel to the desired area, and how much time is taken to transport the nuclei from the generator to the target area. Silver iodide particles were generated in a cyclic manner from ground-based generators, while continuous observations of ice nuclei were made on the surface in the target area. Observations show the surface wind velocity at the generator site to be the primary factor controlling the particle travel. However, nuclei concentrations have been observed to remain low even though the wind velocity was favorable for detection. This may be due to the silver iodide particles nucleating the cloud to form ice crystals, which do not enter the nucleus counter.

Cloud seeding trials at Delhi during monsoon months, July to September (1957-59)

A programme of rainmaking trials during monsoon months (July to September), using the technique of dispersal in air of salt seeds of appropriate sizes from ground-based generators has been in operation at Delhi since1957. The note presents the design and programme of the seeding trial, and the results obtained and experience gained in the course of the first three years of experimentation.