On the Feasibility of Precisely Measuring the Properties of a Precipitating Cloud with a Weather Radar
In this paper the results of an investigation are presented that are concerned with the feasibility of employing a weather radar to make precise measurements of the properties of a precipitating cloud. A schematic cloud is proposed as a model for interpreting the interaction of the radar energy with the cloud. Point values of the liquid-water concentration are estimated from measurements of the received power. The measurements were made under conditions which minimized errors arising from attenuation of the radar signal and a radar beam which is not completely filled with raindrops. A continuity equation for liquid-water concentration is developed. The vertical speeds at the core of convective clouds are related to the spatial and temporal variations of the liquid-water content by means of this equation. The version of the continuity equation developed in this study represents an improvement over forms used previously. The new version accounts for the downward development of a radar echo at speeds faster than the fall speed of raindrops. This echo development is caused by the coalescence mechanism. An error analysis is performed and it indicates that the percentage error of the measurements of the liquid-water concentration may be as much as 102.4%. The fractional error of the vertical speeds is + 1391.4% which results from the compounding of the experimental errors of the terms in the continuity equations. To check the estimated magnitudes of the experimental errors a case study was performed. The echoes of 23 convective clouds were studied and 695 observations of liquid-water concentration were obtained. The observed magnitudes indicate that these estimates are of the correct order. The values of vertical speeds also indicate that the estimated error of this quantityis indeed large. Possible methods for reducing the experimental errors are considered. This examination indicates that reasonably accurate measurements of liquid-water concentration can be made if high experimental standards are maintained. The use of calibrating instruments which are very accurate together with good experimental control may permit a reduction of the percentage error to less than 2O%. However, this study indicates that attempts to measure vertical speeds accurately by use of the continuity equation may not be too successful.
Runnels, R.C. (1967). On the Feasibility of Precisely Measuring the Properties of a Precipitating Cloud with a Weather Radar. Texas Water Resources Institute. Available electronically from