Urban-Catchment-Scale Evaluation of Rainfall–Runoff Predictions Based on Weather Radar Nowcasts

The performance of weather radar nowcast for use in urban hydrology is often evaluated based on observed weather radar rainfall. This may not, however, accurately reflect the performance of urban hydrological rainfall–runoff predictions that rely on weather radar nowcasts. This study highlights the significant role of both spatial scale and evaluation methodology in assessing the performance of radar-based rainfall nowcasts for urban hydrological applications. We investigate whether the weather radar nowcast performance at mesoscales corresponds to that at smaller urban catchment scales and if the weather radar observation-based evaluation of nowcast performance aligns with the hydrological runoff prediction performance. We apply the COTREC nowcasting technique in both a small Danish catchment and a larger spatial domain, which corresponds to a mesoscale, and provide evidence that nowcasting performance at the mesoscale is not indicative of its performance at the catchment scale. When mesoscale representativeness is assumed for event-accumulated rainfall depths, nowcast performance at the catchment scale tends to be overestimated during low-intensity rainfall events and underestimated during high-intensity events. Conversely, time series evaluations show less pronounced differences across spatial scales. By comparing weather radar observation-based nowcast evaluations with hydrologically based evaluations, the performance of hydrological predictions is further demonstrated to differ from that of weather radar rainfall predictions. For lead times ≤ 10 min , the weather radar observation-based evaluation overestimates the hydrological prediction skill. However, the hydrological prediction skill is underestimated during high-intensity rainfall events if weather radar observation-based evaluations are assumed representative when evaluating time series.