The potential of radar-based ensemble forecasts for flash-flood early warning in the southern Swiss Alps

Abstract

Abstract. This study explores the limits of radar-based forecasting for hydrological runoff prediction. Two novel radarbased 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.

Flood triggering in Switzerland: the role of daily to monthly preceding precipitation

“…A modeling study by Paschalis et al (2014) showed that soil saturation can play a paramount role in mediating the discharge response of a small Pre-Alpine catchment. The initial conditions also significantly affect flash flood forecasting in the Southern Swiss Alps (Liechti et al, 2013). However, Norbiato et al (2009) found that the impact of initial moisture conditions on the runoff coefficient during floods is important only for catchments with intermediate subsurface water storage capacity; i.e., the role of initial moisture conditions is negligible for catchments with either very large or very small storage capacity.…”

How can expert knowledge increase the realism of conceptual hydrological models? A case study based on the concept of dominant runoff process in the Swiss Pre-Alps

“…Scherrer, 1997). The third mapping approach we used is based on the experimentalist approach introduced by Schmocker-Fackel et al (2007) and Margreth et al (2010), which has already been used in, for instance, Antonetti et al (2016Antonetti et al ( , 2017. Basically, the approach consists of the following steps.…”

“…The initial conditions can significantly affect simulation results, especially in a forecasting context (Liechti et al, 2013). For example, in a study about the uncertainties involved in operational flood forecasting chains in an alpine Swiss catchment, Zappa et al (2011) found that uncertainty in initial conditions lasts for the first 48 h, but is almost negligible compared with the uncertainty originating from meteorological data.…”

A Conceptual Flash Flood Early Warning System for Africa, Based on Terrestrial Microwave Links and Flash Flood Guidance

“…Fast responding catchments are most prone to flash floods [7]. As pointed out by [8], accurate knowledge of the spatial and temporal distribution of rainfall is essential for flash flood early warning systems. The spatial scale should not exceed that of the catchments of interest or the typical convective storm within the area of interest.…”