Rainfall measurements by weather radar: Applications to hydrology

Surface precipitation measurements are of prime importance in studies of hydrology, climatology, and meteorology. These measurements can be improved by using both weather radar and conventional rain and snow gages. Recent advances in digital radar data management make possible improved precipitation estimates for hydrological applications such as flood forecasting. A project undertaken in Alberta provides areal precipitation data for operational flood forecasting. Maps depicting rainfall intensities are generated at specified time intervals from the Alberta Research Council's S band weather radar. Transmitted 150 km by telecopier to the Flow Forecasting Branch, Alberta Environment, the real‐time radar‐derived areal maps of precipitation are used with point surface observations to specify more precisely the precipitation events that lead to floods. During precipitation events, weather radar data provide an indication of the applicability of point surface observations, leading to better operational flow forecasts. In one example the radar showed that high rainfall amounts observed at a conventional surface rain gage fell over a localized area. Using only surface gage information, a flood forecast likely would have been issued. However, radar intelligence suggested otherwise, so the forecast was not issued. The potential benefit of weather radar to hydrologists is assessed using rainfall within a representative subbasin. The precipitation was estimated from radar observations and from an independent gage network used by operational hydrologists. Throughout a stormy 6‐day period, the accumulated precipitation for a subbasin estimated by the two methods agreed to within 15%. Shortcomings of typical rain gage networks in assessment of areal rainfall are evident. In one case, weather radar data revealed that although rain fell on the network gages, the subbasin itself received little rain, so the gage estimate was high. In another example, the weather radar demonstrated that rain fell in the subbasin but missed the network gages. The spatial variability of precipitation, evident in weather radar observations, is generally not observed by gage networks. Therefore radar measurements of rainfall intensity and rainfall amount can be of considerable benefit to operational and applied hydrologists.