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

摘要

The improvement of the short- and mid-range numerical runoff forecasts overthe flood-prone Spanish Mediterranean area is a challenging issue. This workanalyses four intense precipitation events which produced floods of differentmagnitude over the Llobregat river basin, a medium size catchment located inCatalonia, north-eastern Spain. One of them was a devasting flash flood –known as the "Montserrat" event – which produced 5 fatalities and materiallosses estimated at about 65 million euros. The characterization of theLlobregat basin's hydrological response to these floods is first assessed byusing rain-gauge data and the Hydrologic Engineering Center's HydrologicalModeling System (HEC-HMS) runoff model. In second place, the non-hydrostaticfifth-generation Pennsylvania State University/NCAR mesoscale model (MM5) isnested within the ECMWF large-scale forecast fields in a set of 54 h periodsimulations to provide quantitative precipitation forecasts (QPFs) for eachhydrometeorological episode. The hydrological model is forced with these QPFsto evaluate the reliability of the resulting discharge forecasts, while anensemble prediction system (EPS) based on perturbed atmospheric initial andboundary conditions has been designed to test the value of a probabilisticstrategy versus the previous deterministic approach. Specifically, aPotential Vorticity (PV) Inversion technique has been used to perturb the MM5model initial and boundary states (i.e. ECMWF forecast fields). For thatpurpose, a PV error climatology has been previously derived in order tointroduce realistic PV perturbations in the EPS. Results show the benefits ofusing a probabilistic approach in those cases where the deterministic QPFpresents significant deficiencies over the Llobregat river basin in terms ofthe rainfall amounts, timing and localization. These deficiences inprecipitation fields have a major impact on flood forecasts. Our ensemblestrategy has been found useful to reduce the biases at different hydrometricsections along the watershed. Therefore, in an operational context, thedevised methodology could be useful to expand the lead times associated withthe prediction of similar future floods, helping to alleviate their possiblehazardous consequences.