A STUDY OF ELECTRIFICATION AND PRECIPITATION GROWTH IN THUNDERCLOUDS

Abstract

Chapter I deals with the review of studies carried out by other investigators on the structure of thunderclouds and measurements of various parameters associated with electric field and precipitation intensity in thunderclouds. This review consiaQys +v;e_charge and size of precipitation particles in thunderclouds. Various charging mechanisms which are considered responsible for electrification of thunderclouds have been discussed in detail, and their merits and demerits based on theoretical and experimental approaches are reviewed. Electrical conditions prevailing in, around and out side the thunderclouds are discussed. This is followed by a critical review of studies concerning electric field build-up at the ground surface beneath a thundercloud. The characteristics of intracloud lightning discharges have also been discussed in brief. This chapter sets the sc_ey,efor further investigations reported in the following chapters. Chapter II is devoted to studies related to the effect of corona space charge on electric field build-up inside a thundercloud and at the ground surface beneath a thundercloud. The corona space charge is produced from the ground irregularities (elevated objects etc.) near the ground surface when ground electric field is intense beneath thunder-clouds, and is transported into thundercloud by an updraft. The production rate of corona space charge is a rapidly iv increasing function of the ambient electric field at the ground surface. The phenomenaaft' cl formulation of the electric field development inside a thundercloud, incorporating a comprehensive account of corona space charge, has been presented. It is shown that corona space charge transported from the ground surface enhances growth rate and maximum value of electric field inside a thundercloud in its mature stage: The time lag in production and transport of corona space charge from the ground to cloud leads to changes in electrical activity of thundercloud only in later stages. This numerical simulation also assesses the effect of corona space charge on ground electric field beneath a thundercloud, and provides more accurate values of ground electric field. It has been found that corona space charge reduces ground electric field severely. Further, the time behaviour of electric field growth at the ground surface is also affected by corona space charge. The magnitudes of ground electric fields obtained including corona space charge effect are in good agreement with the experimental observations.