The electrical structure of nimbostratus clouds

Abstract

Simultaneous records of precipitation current density, potential gradient and wind speed have been obtained with mobile equipment mounted on a Land-Rover and with fixed equipment located at a field station in the Pennines. Measurements were made in quiet precipitation with the two sets of equipment separated by horizontal distances of up to 8 km in a direction in line with the cloud movement. Maximum cross-correlation between the precipitation current density records at the two stations was usually for time lags corresponding to the time of travel of the clouds between the stations. On one occasion of low wind speed, simultaneous precipitation current correlation was obtained at the two stations when separated by 5 km. It was deduced that the time variations of electrical parameters observed at a ground station would normally be the result of movement of the cloud system rather than its electrical development. Analysis of the effects of wind speed has shown that rain electrification is more intense at sites more exposed to the wind and that the effects may be due to a process operating from ground level up to a height of several hundred metres. The persistence of potential gradient and of precipitation current density has been shown to be inversely proportional to cloud speed. The horizontal dimensions of cloud across which persistence exists at a given point in time has been shown to be independent of wind speed and to be of the order of several kilometres. It is suggested that these dimensions may be a characteristic of cloud type and may be used as a criterion for their identification. A theoretical model to explain the electrical structure of nimbostratus clouds has been proposed, and it has been shown to be consistent with observations. The model satisfactorily explains the inverse relation between precipitation current density and potential gradient and it explains observed differences in phase between precipitation current-time and potential gradient-time curves in terms of the periodicity of the electrical changes within the cloud.