Probabilistic hydrological nowcasting on Mediterranean small catchments: from theoretical approaches to operational applications

The Mediterranean area in last century was affected by very intense rainfall events concentrated over small portion of the territory generating flash-floods and landslides. These events caused serious damages to urban areas and in the worst events led to human losses. The temporal scale of these events has been observed to be strictly linked to the size of the catchments involved. Considering the presence of a great number of catchments with small drainage area (order of some km2) and related response time of few hours, a forecast at short lead time is essential for this kind of events. Nowcasting models, covering the time interval of the following two hours starting from the observation try to extend the predictability limits of the forecasting models in support of real-time flood alert system operations. This research project points to the realization of an integrated hydrological nowcasting chain, coupling existing nowcasting techniques (PhaSt, a spectral-based nowcasting procedure) and hydrological model (Continuum, a continuous distributed hydrological model). A work of enhancement of the nowcasting technique has been firstly performed to extend the forecast horizon a modification of the algorithm has been inserted in order to take into account the mechanism of growth and decay of the precipitation structure. Then the blending with the meteorological models that could allow to integrate the prediction at short lead time of the nowcasting technique (0-2 hours) with the longer lead time of the meteorological models. A parallel work has been done in collaboration with the Centre of Applied Research in Hydrometeorology on the comparison of two probabilistic nowcasting technique and the effect of the propagation of the error of the rainfall forecast in the hydrological nowcasting chain. 9 The work focuses not only on the enhancement of the predictive ability of the single elements of the chain but is trying also to understand how each element can integrate in order to give a result that is reachable but also satisfying from an operational point of view and that can be used as a support in the decisional process for the warning system.