Airborne studies of the electrical properties of large convective clouds

Abstract

AbstractStudies have been made using instrumented aircraft of the electrical and microphysical properties of cumulus and cumulonimbus clouds over Flagstaff, Arizona. Measurements were made throughout the life‐cycle of these clouds either by making horizontal penetrations through the cloud at intervals of 2 Centigrade degrees or by entering the cloud at the highest level attainable and spiralling down to cloud base. The aircraft could attain an altitude of around 32,000 ft (9·7 km), corresponding to a temperature ceiling of approximately −30°C. Continuous measurements were made of vertical electric field strength, F, the charge carried on larger hydrometeors, Q, the concentrations, types and dimensions of cloud particles, liquid‐water content, temperature and other meteorological parameters.F was measured using a rotating cylindrical field‐mill of the type devised by Kasemir (1964), which enabled fields ranging from about 100 Volt m−1 to corona values to be measured with an accuracy of about 10 per cent. Q was measured using a double‐sheath induction technique which permitted charges ranging in magnitude from about 10−2 to 1·0 e.s.u. to be measured to within 10 per cent. A series of careful subsidiary experiments established the range of conditions over which these electrical instruments provided reliable results. The concentrations, types and dimensions of cloud particles were measured using the continuous particle sampler described by MacCready (1962).The measured electrical characteristics of the clouds studied varied markedly from cloud to cloud but it was possible to isolate certain recurrent patterns and correlations. The measured values of Q were found generally to be carried on rimed aggregates of ice crystals. The degree of electrification, represented by the values of F and Q, was highest in clouds where these aggregates coexisted with appreciable quantities of supercooled water droplets and ice crystals. At all levels within the clouds positive and negative values of Q were obtained, but the proportion of positively charged larger hydrometeors increased with increasing temperature within the cloud. Small clouds or clouds in their early stages of development usually possessed a simple electrical structure, with positive fields existing throughout most of their volume. Larger, more mature clouds possessed extremely complex electrical structures and a much higher degree of electrification. Sharp reversals of the dominant sign of Q were often observed to occur once or twice during the horizontal penetration of a precipitation shaft below cloud base at temperatures above 0°C.Lack of vital information, particularly concerning the cloud dynamics, rendered it impossible to assess the measurements in terms of all current theories of thunderstorm electrification. However, it can be shown that both the Reynolds‐Brook process and the Müller‐Hillebrand mechanism probably contributed significantly to the electrification of the clouds studied. It is difficult to distinguish the respective roles of these processes. The evidence suggests, however, that clouds possessing a simple structure, or clouds in the early stages of their maturity, have an electrical structure that is largely compatible with the predictions of the inductive process, whereas the gross electrical characteristics of more complex clouds are often explicable in terms of the Reynolds‐Brook theory.