Flash-flood forecasting in two Spanish Mediterranean catchments: a comparison of distinct hydrometeorological ensemble prediction strategies

Abstract

Abstract. Hydrological Ensemble Prediction Systems (HEPSs) are becoming more and more popular methods to deal with the meteorological and hydrological uncertainties that affect discharge forecasts. These uncertainties are particularly difficult to handle when dealing with Mediterranean flash-flood forecasting as many hydrological and meteorological factors take place and precipitation comes from small scale convective systems. In this work, the performances of distinct HEPS are compared for two heavy precipitation events that affected two different semi-arid Spanish Mediterranean catchments : the 5 cases of the 03 November 2011 on the Llobregat River in Catalonia, and the 28 September 2012 on the Guadalentín River near in Murcia. The latter case corresponds to the eighth Intense Observing Period (IOP8) of HYMEX field campaign. The uncertainty on quantitative precipitation forecasting is sampled by using two different convection-permitting meteorological ensemble generation strategies. The first EPS strategy consists in dynamically downscaling the ECMWF-EPS directly by means of the WRF model, whereas the second is based on the AROME-WMED model. Its deterministic QPFs are perturbed based 10 on a previous rainfall forecast error climatology and by using the probability density functions of the errors, in term of total amounts and location of the heaviest rainfalls. The population of both ensembles is of 50 members, which are used to drive the semi-distributed and conceptual HEC-HMS and the fully distributed and physically-based ISBA-TOP hydrological models.For each HEPS, the performance is assessed in term of the quantitative discharge forecasts. The results point out the benefits of using (i) a hydrological model when evaluating highly-variable and convective-driven precipitation fields and (ii) an EPS 15 to better encompass these uncertainties arising from the different elements of the HEPS. Issues about the optimal number of ensemble members and impact of the ensemble forecasting lead time are addressed for optimal flash-flood forecasting purposes as well.

Hydrometeorological analysis of a flash flood event in an ungauged Mediterranean watershed under an operational forecasting and monitoring context

“…The substantial role of NWP modelling and nowcasting‐related products in flash flood prediction has been highlighted by a variety of retrospective experiments in the Mediterranean basin during the past recent years, especially with the use of the weather research and forecasting (WRF) mesoscale atmospheric model (Ferrari et al, 2020; Hally et al, 2015; Silvestro et al, 2019; Vincendon & Amengual, 2017). In Greece and neighbouring countries in particular, Papaioannou et al (2019) used the WRF model, its hydrological extension package (WRF‐Hydro), and the Hydrologic Engineering Center‐River Analysis System (HEC‐RAS) software to produce inundation maps for the 9 October 2006 event in Volos, Central Greece.…”