Review of Downwind Extra-Area Effects of Precipitation Enhancement

Abstract

This paper discusses a) findings to date about downwind effects of cloud seeding and b) research methodologies that may be applied to advance our understanding of the subject. An important segment of the present report provides the intellectual foundation of any research effort aimed at understanding and assessing downwind effects of a precipitation enhancement project. The body of scientific knowledge presented here tells us by example what can be done to further the understanding of downwind effects. Downwind effects of cloud seeding demonstrate a strong signal according to the U.S. National Academy of Sciences (1973). There was evidence of effects at long distances well beyond where errors in seeding targeting might be expected and little evidence for decreases in precipitation downwind. Two mechanisms for downwind effects have been hypothesized. They consist of a) downwind transport of ice nuclei and ice crystals from the seeding source, and b) dynamic invigoration of clouds by release of latent heat of freezing. Research and verification of these mechanisms in any precipitation enhancement project are required. This activity involves the tracking and observation of seeding effects into a downwind area. There will be microphysical observations of a) detrainment of ice nuclei and ice crystals from clouds seeded for the primary target area and their environs, b) transport and dispersion of the ice nuclei and ice crystals downwind, c) total budget of these traveling ice nuclei and crystals including destination sinks and residence times, d) entrainment of these particles into clouds downwind, and e) development of cloud and precipitation in the downwind clouds. The investigation may simultaneously be concerned with dynamic seeding effects including observations of a) the dynamics in and around the clouds being seeded for the primary target area, b) the interaction of these dynamics with other clouds, c) the dynamic development of these other clouds followed by d) the development of cloud and precipitation in the other clouds. It will be important to establish the physical context of both the microphysicai and dynamical studies in terms of the cloud physical events occurring up to and including the precipitation being enhanced for the primary target area. The research will use well-developed scientific tools. They include rawinsondes, instrumented aircraft, radar, precipitation gauges, satellite cloud imagery, microwave radiometry, and precipitation trace chemistry. Numerical models are important for incorporating relevant theory and would include terrain effects, synoptic and mesoscale meteorology, (gravity) wave motions, seeding material transport and dispersion, and cloud and precipitation microphysics. In order to raise the entire level of knowledge of downwind cloud seeding effects, the research of the form outlined here and in the main text should be undertaken.