Long-term evaluation of gauge-adjusted precipitation estimates from a radar in Norway

Abstract

The implementation of weather radars in Norway by the Norwegian Meteorological Institute (met.no) has made radar a potential tool to improve hydrologic predictions through the use of distributed precipitation input. Met.no supplies gauge-adjusted quantitative hourly radar precipitation estimates. A key concern regarding the use of radar precipitation estimates in hydrology is their accuracy. In this study, the precipitation estimates from the Rissa radar in Norway were evaluated through a comparison with observations from 112 gauges used in the adjustment (dependent) and 15 gauges not included in the adjustment (independent). The comparison with daily measurements from the dependent gauges showed a decline in the radar's detection probability beyond a range of about 140 km, with a more severe decline in winter. The deviations between radar- and gauge-conditional mean precipitation were significantly higher in summer than in winter. There was an overestimation at most of the gauge locations during summer, while there were more underestimations during winter. A dependence of accuracy on range was identified from the spatial distribution of the Efficiency Index and mean absolute difference. The evaluation against the independent gauges revealed trends mostly similar to the ones obtained from comparison with the dependent gauges. The radar estimates exhibited better agreement with gauge measurements during winter. The main reasons for the errors remaining in the gauge-adjusted precipitation estimates are the absence of correction for the vertical profile of reflectivity, the use of average monthly adjustment factors, derivation of these factors using data from previous years and the use of a single reflectivity–precipitation rate (Z–R) relation.