Sampling errors in radar estimates of rainfall

The relatively slow rate of application of radar rainfall to operational hy drology is partially due to concerns about the measurement errors. The erro rs that result in a bias in the field mean have been studied extensively an d can to some extent be treated, but errors that manifest themselves as mor e or less white noise become important when considering what scale is appro priate for spatially distributed hydrological modeling. This paper evaluate s the errors that arise: in radar estimates of rainfall as a result of temp oral sampling, spatial averaging, measuring the field at some distance abov e the ground, and recording the reflectivity data with a limited radiometri c resolution. By far the most significant source of error was found to be d ue to measuring the field at some height above the ground. The mean standar d difference in rainfall rate between fields separated by 1 km in height at 1 km spatial resolution was found to be of the order of 100% of the mean r ainfall rate. When the spatial resolution is reduced to 5 km the mean stand ard difference between the fields with the same 1 km vertical separation fe ll to about 50% of the mean rainfall rate. Temporal sampling was found to b e quite sensitive to the intermittency of the rain field being sampled. The mean standard error caused by 2-min sampling for 10-min accumulations decr eased from 14% for scattered rainfall to 8% for widespread rainfall.