Abstract
Abstract. This study explores the limits of radar-based forecasting for hydrological runoff prediction. Two novel radar-based ensemble forecasting chains for flash-flood early warning are investigated in three catchments in the southern Swiss Alps and set in relation to deterministic discharge forecasts for the same catchments. The first radar-based ensemble forecasting chain is driven by NORA (Nowcasting of Orographic Rainfall by means of Analogues), an analogue-based heuristic nowcasting system to predict orographic rainfall for the following eight hours. The second ensemble forecasting system evaluated is REAL-C2, where the numerical weather prediction COSMO-2 is initialised with 25 different initial conditions derived from a four-day nowcast with the radar ensemble REAL. Additionally, three deterministic forecasting chains were analysed. The performance of these five flash-flood forecasting systems was analysed for 1389 h between June 2007 and December 2010 for which NORA forecasts were issued, due to the presence of orographic forcing. A clear preference was found for the ensemble approach. Discharge forecasts perform better when forced by NORA and REAL-C2 rather then by deterministic weather radar data. Moreover, it was observed that using an ensemble of initial conditions at the forecast initialisation, as in REAL-C2, significantly improved the forecast skill. These forecasts also perform better then forecasts forced by ensemble rainfall forecasts (NORA) initialised form a single initial condition of the hydrological model. Thus the best results were obtained with the REAL-C2 forecasting chain. However, for regions where REAL cannot be produced, NORA might be an option for forecasting events triggered by orographic precipitation.
Highlights
To issue early warnings about flash floods, information about the spatial and temporal distribution of precipitation is crucial
First we show how the spread of the two ensembles NORA and REAL-C2 generally develops over lead time
NORA is an analoguebased ensemble forecast for orographic precipitation, consisting of 12 members, initialised with the initial conditions derived from a four-day nowcast with deterministic radar quantitative precipitation estimate (QPE)
Summary
To issue early warnings about flash floods, information about the spatial and temporal distribution of precipitation is crucial. Forecasting for flash-flood events would help to extend the time available to issue warnings and implement safety measures. Producing such forecasts is, a very challenging task. In an experimental framework Zappa et al (2011) used a radar ensemble to generate ensembles of initial conditions and showed that the uncertainty from initial conditions decays within the first 48 h of the forecast. Another very challenging issue is the uncertainty about the distribution and intensity of the main triggering variable, the precipitation.
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