The use of meteorological analogues to account for LAM QPF uncertainty

Flood predictions based on quantitative precipitation forecasts (QPFs) provided by deterministic models do not account for the uncertainty in the outcomes. A probabilistic approach to QPF, seems to be indispensable to obtain different future flow scenarios for improved flood management. A new approach based on analogues has been developed to determine an ensemble of hourly quantitative precipitation forecasts for the Reno river basin, a medium-sized catchment in northern Italy. A statistical analysis has underlined that the combination of geopotential at 500 hPa and vertical velocity at 700 hPa provides improved precipitation estimates. The analogue-based ensemble prediction has to be considered not as an alternative but complementary to the deterministic QPF provided by a numerical model, even when employed jointly to improve real-time flood forecasting. In the present study, the analogue-based QPFs and the precipitation forecast provided by the Limited Area Model LAMBO have been used as different input to the distributed rainfall runoff model TOPKAPI, thus generating, respectively, an ensemble of discharge forecasts, which provides a confidence interval for the predicted streamflow, and a deterministic discharge forecast taken as an error-affected “measurement” of the future flow, which does not convey any quantification of the forecast uncertainty. To make more informative the hydrological prediction, the ensemble spread could be regarded as a measure of the uncertainty of the deterministic forecast.