Ensemble flood forecasting based on operational forecasts of the regional Eta EPS in the Taquari-Antas basin

Vinícius Alencar Siqueira,Fernando Mainardi Fan,Sin Chan Chou,Walter Collischonn

doi:10.1590/2318-0331.011616004

Vinícius Alencar Siqueira, Fernando Mainardi Fan + Show 2 more

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https://doi.org/10.1590/2318-0331.011616004

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Abstract

ABSTRACT Hydrological Ensemble Prediction Systems (HEPS) play an important role on operational flood forecasting. Unlike in deterministic approach, which relies on a single prediction of future river flows, these systems can represent the forecast uncertainty and provide a better detection of extreme hydro-meteorological events. In this context, the present study aimed to assess both the quality of ensemble flood forecasts on Taquari-Antas basin and its potential to provide additional information to a local Flood Alert System. The hydrological model MGB-IPH was coupled to the high-resolution meteorological EPS Eta model with five members of different parameterization schemes and boundary conditions, as well as to the deterministic version of Eta regional model. On a single event evaluation, the peak discharge was reasonable well predicted by at least one ensemble member, in nearly all forecasts, with a good prediction of the flood timing for the considered lead times. In a comparison with deterministic forecasts, the ensemble ones showed higher accuracy and higher probability of detection (POD) for the reference thresholds, preserving false alarm rates at reasonably low levels. An overall tendency of underestimation was also identified, with most observations falling between the higher ranks of the ensemble. Furthermore, the combination of previous forecasts (t-12h) with the recent ones leads to a slight increase of ensemble spread and POD, despite the performance reduction in terms of accuracy and bias for the ensemble mean. Results suggest that there is a benefit in having hydrological ensemble forecasts obtained from the high resolution EPS Eta model, which can be used as a complementary information to a local Flood Alert System supporting pre-alert issues and Civil Defense internal planning actions.

Highlights

Hydrological forecasting is a key component of early flood warning systems
The MGB-IPH model was coupled to the short-range, operational Eta-5 km Ensemble Prediction Systems (EPS) in order to assess the quality of the hydrological ensemble forecasts, as well as its potential to provide additional information to a local Flood Alert System
In a single event evaluation, the hydrological forecasts were more sensitive to the convective parameterization of Eta EPS, which demonstrates that coupling a hydrological model to an EPS with different physical representations can be useful to capture forecast uncertainty

Summary

Introduction

Hydrological forecasting is a key component of early flood warning systems. The selection of the forecast approach usually depends on the desired lead time and characteristics of the basin, such as rainfall-runoff response and flood wave travel times. Several studies have reported inadequate flow estimates when coupling QPF with hydrological models, usually caused by errors in timing, position and amount of predicted rainfall (e.g. BENOIT et al, 2003; HABETS; LEMOIGNE; NOILHAN, 2004; COLLISCHONN et al, 2005; VERBUNT et al, 2006; KRUK; VENDRAME; CHOU, 2013). This can be explained by the fact that precipitation is one of the most difficult variables to be forecasted, since it is very sensitive to both initial state of the atmosphere and representation of physical processes in numerical weather models (STENSRUD; BAO; WARNER, 2000; GOLDING, 2000; EBERT, 2001). Even with finer resolution models, processes that occur in sub-grid scales such as cloud microphysics or deep convection must be simplified using parameterization schemes, associated uncertainty can propagate to weather phenomena with characteristics much larger than model truncation scales (PALMER, 2000)

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