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Previous authors have reported significant correlations between the horizontal extent of convective showers or storms and the volume of rain they produce. This paper employs that idea to develop a simplified method for estimating convective rainfall by considering only the horizontal extent and duration of the precipitation. The present analysis is based on rain gage and radar data from an area in western North Dakota. A synoptic adjustment is applied to the radar rain volume estimates. A quantity called the integrated rainfall coverage can be calculated from either gage or radar data and is found to be well correlated with the rain volumes. The maximum echo area during any one scan in one hour seems to be the hourly radar measurement best correlated with the rain volumes. This limited study suggests that the accuracy of the simplified method approaches that of methods using radar reflectivity data and may have operational value in some special situations. Many authors have shown that variations in latitude andin geographical, orographical, and synoptic conditions result in differences in radar estimated rainfall [e.g., Byers, 1948; Stout and Mueller, 1968; Estoque and Fernandez-Partagas, 1974]. In addition, there are complicating effects due to microwave attenuation, evaporation of the raindrops, vertical air motions, particle shape and fall speed variations, and advection of the rain while falling from the radar's sampling volume to the ground. The combined effects may limit the accuracy of radar estimates of areal rainfall to not better than 50% [Atlas and Chmeta, 1957; Hildebrand et at., 1979]. Nevertheless, radar can map precipitation much more completely in space and time than can any feasible network of rain gages on the surface. This capability is so valuable that experimenters in atmospheric physics, weather modification, and hydrology are • Now with the Research and Development Branch of the Office of Atmospheric Resources Research, U.S. Bureau of Reclamation, Denver, Colorado 80225.

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