WSP13034 - Laserfiche WebLink

Home Browse Search

. unnoticed, or have been dismissed as inconsequen- tial just a few years ago, are now often found to have broader ramifications. Some of these changes are quite subtle; for example, increased cloudiness asso- ciated with condensation trails from jet aircraft may modify the radiation budget at the ground. Others, such as acid rain are more obvious; structures, veg- etation, and lake water quality have all been adversely affected. Air quality and visibility are often locally degraded by increased anthropogenic pollutants, and urban effects on temperature, humidity, wind, and precipitation have been well documented. 2. Status of planned weather modification' There is growing evidence that glaciogenic seeding (the use of ice-forming materials) can, under certain weather conditions, successfully modify supercooled fog, some orographic stratus clouds, and some con- vective clouds. Recent research results, utilizing both in situ and remote measurements in summer and winter field programs, provide dramatic though limited evidence of success in modifying shallow cold oro- graphic clouds and single-cell convective clouds. Field studies are beginning to define the frequencies with which responsive clouds occur within specific meteo- rological regimes. Successful treatment of any suitable cloud requires that sufficient quantities of appropriate seeding mate- rials must enter the cloud in a timely, well-targeted fashion. As the need for stringent spatial and temporal targeting has been established, it has become appar- ent that problems with seeding plume delivery in many early experiments may in part accountfor the failure of such programs to produce significant results. a. Fog and stratus removal Operations employing glaciogenic seeding todissi- pate supercooled fogs and low stratus have become routine at some airports. The ability to admit more solar radiation to reduce heating requirements through the dissipation of such clouds and fogs appears promising, The dissipation of warm (nonsupercooled) fogs can often be accomplished by more expensive thermal techniques, but this has proven cost effective at only a few major airports. More reliable and economical warm-fog dissipation techniques have not yet been established. b. Precipitation increase There is considerable evidence that, under certain conditions, precipitation from supercooled orographic 332 e clouds can be increased with existing techniques. Statistical analyses of precipitation records from some long-term projects indicate that seasonal increases on the order of 10% have been realized. The cause and effect relationships have not been fully documented; however, the potential for increases of this magnitude is supported by field measurements and numerical model simulations. Both ~how that SL W exists in amounts sufficient to produce the observed precipi- tation increases and could be tapped if proper seeding technologies were applied. The processes culminat- ing in increased precipitation have recently been directly observed during seeding experiments con- ducted over limited spatial and temporal domains. While such observations further support statistical analyses, they have to date been of limited scope, and thus the economic impact of the increases cannot be assessed. Although some present precipitation augmentation efforts are reportedly suc- cessful, more consistent results would probably be obtained if some basic im- provements in seedingmethodology were made. Recent experiments continue to suggest that pre- cipitation from single-cell and multicell convective clouds may be increased, decreased, and/or redistrib- uted. The response variability is not fully understood, but appears to be linked to variations in targeting, cloud selection criteria, and assessment methods. Heavy glaciogenic seeding of some warm-based convective clouds (bases about + 1 OOC or warmer) can stimulate updrafts through added latent heat release (a dynamic effect) and consequently increase precipitation. However, convincing evidence that such seeding can increase rainfall over economically significant areas is not yet available. Seeding to enhance coalescence or affect other warm-rain processes within clouds having summit temperatures warmer than about OOC has produced statistically acceptable evidence of accelerated pre- cipitation formation within clouds, but evidence of rainfall change at the ground has not been obtained. Although some present precipitation augmentation efforts are reportedly successful, more consistent results would probably be obtained if some basic improvements in seeding methodology were made, Transport of seeding materials continues to be uncer- tain, both spatially and temporally, Improved delivery techniques and better understanding of the subse- Vol. 73, No, 3. March 1992 .' 4 f \' \' ~ I " 1 ., < j) \\- I' , , ,\.. \ :" J 1/,"--'\ I, 1/,'1. J)' ,