Inclusion of potential vorticity uncertainties into a hydrometeorological forecasting chain: application to a medium size basin of Mediterranean Spain

Abstract

Abstract. The improvement of the short- and mid-range numerical runoff forecasts over the flood-prone Spanish Mediterranean area is a challenging issue. This work analyses four intense precipitation events which produced floods of different magnitude over the Llobregat river basin, a medium size catchment located in Catalonia, north-eastern Spain. One of them was a devasting flash flood – known as the "Montserrat" event – which produced 5 fatalities and material losses estimated at about 65 million euros. The characterization of the Llobregat basin's hydrological response to these floods is first assessed by using rain-gauge data and the Hydrologic Engineering Center's Hydrological Modeling System (HEC-HMS) runoff model. In second place, the non-hydrostatic fifth-generation Pennsylvania State University/NCAR mesoscale model (MM5) is nested within the ECMWF large-scale forecast fields in a set of 54 h period simulations to provide quantitative precipitation forecasts (QPFs) for each hydrometeorological episode. The hydrological model is forced with these QPFs to evaluate the reliability of the resulting discharge forecasts, while an ensemble prediction system (EPS) based on perturbed atmospheric initial and boundary conditions has been designed to test the value of a probabilistic strategy versus the previous deterministic approach. Specifically, a Potential Vorticity (PV) Inversion technique has been used to perturb the MM5 model initial and boundary states (i.e. ECMWF forecast fields). For that purpose, a PV error climatology has been previously derived in order to introduce realistic PV perturbations in the EPS. Results show the benefits of using a probabilistic approach in those cases where the deterministic QPF presents significant deficiencies over the Llobregat river basin in terms of the rainfall amounts, timing and localization. These deficiences in precipitation fields have a major impact on flood forecasts. Our ensemble strategy has been found useful to reduce the biases at different hydrometric sections along the watershed. Therefore, in an operational context, the devised methodology could be useful to expand the lead times associated with the prediction of similar future floods, helping to alleviate their possible hazardous consequences.

Classifying severe rainfall events over Italy by hydrometeorological and dynamical criteria

“…On one hand, efforts have been devoted to the assessment of the predictive ability of meteorological models with respect to severe rainfall events (Petroliagis et al, 1997;Romero et al, 2005;Hohenegger et al, 2006Hohenegger et al, , 2008Davolio et al, 2007;Schär, 2007a, 2007b;Argence et al, 2008;Lebeaupin et al, 2008;Amengual et al, 2009;Lapeyre and Talagrand, 2009): their goal is to evaluate how closely a particular model can predict the true state of a certain severe rainfall scenario as a function of observational data quality and availability, parametrization of the subgrid processes, model coupling, model approximations and resolution. On the other hand, some studies have investigated the dynamical, kinematical and microphysics properties of observed systems (Leckebusch and Ulbrich, 2004;Grazzini, 2007;Miglietta and Regano, 2008;Sanchez-Gomez et al, 2008).…”

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

“…The automatic procedure uses as objective function the root mean square error weighted according to the peak; and as search algorithm, the univariate-gradient method (USACE-HEC, 2000). A complete description of the HEC-HMS set-ups and the calibration and validation tasks for both catchments can be found in (Amengual et al, 2007(Amengual et al, , 2009(Amengual et al, , 2015. …”

“…This fact highlights the highly nonlinear nature of the rainfall-runoff transformation in semi-arid basins. (Table 3 ; for further information about the values of the Qp T s, see Amengual et al (2009Amengual et al ( , 2015.…”

“…Guadalentín basins, respectively. The Guadalentín and Llobregat are an archetype of Spanish Mediterranean catchments that are recurrently affected by flooding (for further details, see Amengual et al (2007Amengual et al ( , 2009Amengual et al ( , 2015 ). The Guadalentín river basin is located in Murcia while the Llobregat watershed is located in Catalonia, south and north-eastern Spain respectively( Fig.…”

“…4) that is better with HEC-HMS model. Note that HEC-HMS was previously calibrated and validated by using observed streamflow and precipitation data for both river basins Amengual et al (2007Amengual et al ( , 2009Amengual et al ( , 2015, while ISBA-TOP has been run without any previous calibration task as described by Vincendon et al (2016). Thus, ISBA-TOP has been initialized by using an spatial estimation of the initial soil moisture.…”