Flood predictions based on quantitative precipitation forecasts (QPFs)provided by deterministic models do not account for the uncertainty in theoutcomes. A probabilistic approach to QPF, one which accounts for thevariability of phenomena and the uncertainty associated with a hydrologicalforecast, seems to be indispensable to obtain different future flowscenarios for improved flood management. A new approach based on a searchfor analogues, that is past situations similar to the current one underinvestigation in terms of different meteorological fields over WesternEurope and East Atlantic, has been developed to determine an ensemble ofhourly quantitative precipitation forecasts for the Reno river basin, amedium-sized catchment in northern Italy. A statistical analysis, performedover a hydro-meteorological archive of ECMWF analyses at 12:00 UTC relativeto the autumn seasons ranging from 1990 to 2000 and the correspondingprecipitation measurements recorded by the raingauges spread over thecatchment of interest, has underlined that the combination of geopotentialat 500 hPa and vertical velocity at 700 hPa provides a better estimation ofprecipitation. The analogue-based ensemble prediction has to be considerednot alternative but complementary to the deterministic QPF provided by anumerical model, even when employed jointly to improve real-time floodforecasting. In the present study, the analogue-based QPFs and theprecipitation forecast provided by the Limited Area Model LAMBO have beenused as different input to the distributed rainfall-runoff model TOPKAPI,thus generating, respectively, an ensemble of discharge forecasts, whichprovides a confidence interval for the predicted streamflow, and adeterministic discharge forecast taken as an error-affected "measurement"of the future flow, which does not convey any quantification of the forecastuncertainty. To make more informative the hydrological prediction, theensemble spread could be regarded as a measure of the uncertainty of thedeterministic forecast.
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