Sampling errors in radar estimates of rainfall

Abstract

The relatively slow rate of application of radar rainfall to operational hydrology is partially due to concerns about the measurement errors. The errors that result in a bias in the field mean have been studied extensively and can to some extent be treated, but errors that manifest themselves as more or less white noise become important when considering what scale is appropriate for spatially distributed hydrological modeling. This paper evaluates the errors that arise in radar estimates of rainfall as a result of temporal sampling, spatial averaging, measuring the field at some distance above the ground, and recording the reflectivity data with a limited radiometric resolution. By far the most significant source of error was found to be due to measuring the field at some height above the ground. The mean standard 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 rainfall rate. When the spatial resolution is reduced to 5 km the mean standard difference between the fields with the same 1 km vertical separation fell to about 50% of the mean rainfall rate. Temporal sampling was found to be quite sensitive to the intermittency of the rain field being sampled. The mean standard error caused by 2‐min sampling for 10‐min accumulations decreased from 14% for scattered rainfall to 8% for widespread rainfall.