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MODELS OF THE DEVELOPMENT OF THE ELECTRICAL STRUCTURE OF CLOUDS 131 10 Models of the Development of the Electrical Structure of Clouds Zev Levin Tel Aviv University Israel Tzur National Center for Atmospheric Research INTRODUCTION Thunderstorms are highly variable in their intensity, dimensions, composition, and electrical structure. Some generalizations can be made about them, however. The lightning activity follows strong vertical air currents and precipitation. As a consequence of this correlation, lightning is most frequently observed in cumulus clouds, rarely in stratus clouds, and never in isolated cirrus clouds. Both satellite and ground observations reveal lightning activity at all latitudes between 60Â° N and 60Â° S with the most frequent occurrence at low latitudes and over land. The high occurrence rate over land is believed to be related to the more convectively unstable conditions normally present over land. In high latitudes, the lightning frequency decreases because of the reduced convection from colder surfaces and the reduced absolute humidity. Most thunderstorms contain both water drops and ice crystals, they usually have mass contents (water + ice) greater than 3 g/m3, and they have precipitation rates (involving particles larger than 100 Âµm) in excess of 20 mm/h. Although lightning has been observed most often in clouds containing both ice and water, there have been a few observations of lightning from all-water clouds (e.g., Lane-Smith, 1971). Lightning has been observed in clouds that are completely at temperatures below 0Â°C, but these clouds usually contain both supercooled water droplets and ice. The complexity of the processes leading to the development of both the precipitation and electrical structure in the clouds makes it impossible to construct or validate theories of cloud electrification from simple field experiments. It is only through the complementary efforts of laboratory experimentation, field observations, and mathematical simulations that we can hope to understand the physical processes involved in thunderstorms. A recent review by Latham (1981) summarized some of the main observations of thunderstorm electrification in a coherent fashion, and we refer the interested reader to it. Improved understanding of the major processes leading to the buildup of strong electrical fields and their mutual interaction with precipitation can lead to better forecasting of thunderstorm activity for use in aviation and protection of forested areas, to the development of methods for artificially modifying lightning activity, and even to the development of more efficient rain-enhancement operations. As an ultimate test of the various theories of how electrical charge separates in thunderclouds, it would be necessary to design a model that simulates as accurately as possible the three-dimensional and time-dependent nature of the cloud and its environment, including the