Comment on nhess-2022-182

Abstract

This paper presents a methodological framework designed for the event-based evaluation of short-range hydro-meteorological ensemble forecasts, in the specific context of an intense flash-flood event characterized by high spatio-temporal variability. The proposed evaluation adopts the point of view of end-users in charge of the organization of evacuations and rescue operations at a regional scale. Therefore, the local exceedance of discharge thresholds should be anticipated in time and accurately localized. A step-by-step approach is proposed, including first an evaluation of the rainfall forecasts. This first step helps to define appropriate spatial and temporal scales for the evaluation. The anticipation of hydrological responses (discharge thresholds) is then analyzed, with a focus on the flood rising limb period, and using simulated flows and zero-future rainfall forecasts as references. Based on this second step, several gauged sub-catchments are selected, at which a detailed evaluation of the forecast hydrographs is finally achieved. This methodology is tested and illustrated on the October 2018 flash-flood, which affected part of the Aude River basin (south-eastern France). Three ensemble rainfall now casting research products recently proposed by Météo-France are evaluated and compared. The results confirm the added value of these ensembles to improve discharge threshold detection and anticipation, but at the price of a significant increase of false alarms. Finally, even if the evaluation of ensemble hydro-meteorological forecasts based on a low number of documented flood events remains challenging due to the limited statistical representation of the available data, the evaluation framework proposed herein should contribute to draw robust conclusions about the usefulness of newly developed rainfall ensemble forecast approaches, and about their limits and possible improvements.