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. The use of ground-based precipitation measurements in radar precipitationestimation is well known in radar hydrology. However, the approach of usinggauged precipitation and near-surface air temperature observations to improveradar precipitation estimates in cold climates is much less common. In coldclimates, precipitation is in the form of snow, rain or a mixture of thetwo phases. Air temperature is intrinsic to the phase of the precipitationand could therefore be a possible covariate in the models used to ascertainradar precipitation estimates. In the present study, we investigate the useof air temperature within a non-parametric predictive framework to improveradar precipitation estimation for cold climates. A non-parametric predictivemodel is constructed with radar precipitation rate and air temperature aspredictor variables and gauge precipitation as an observed response using a k nearest neighbour ( k -nn) regression estimator. The relative importanceof the two predictors is ascertained using an information theory-basedweighting. Four years (2011–2015) of hourly radar precipitation rates fromthe Norwegian national radar network over the Oslo region, hourly gaugedprecipitation from 68 gauges and gridded observational air temperatures wereused to formulate the predictive model, hence making our investigationpossible. Gauged precipitation data were corrected for wind-inducedunder-catch before using them as true observed response. The predictive modelwith air temperature as an added covariate reduces root-mean-squareerror (RMSE) by up to 15 % compared to the model that uses radarprecipitation rate as the sole predictor. More than 80 % of gauge locationsin the study area showed improvement with the new method. Further, theassociated impact of air temperature became insignificant at more than 85 %of gauge locations when the near-surface air temperature was warmer than10  ∘ C, which indicates that the partial dependence of precipitationon air temperature is most useful for colder temperatures.

Estimating radar precipitation in cold climates: the role of air temperature within a non-parametric framework