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Sea state can influence the turbulent air–sea exchanges, especially the momentum flux, by modifying the sea-surface roughness. The high-resolution non-hydrostatic convection-permitting model MESO-NH is used here to investigate the impact of a more realistic representation of the waves on heavy precipitation during the Intense Observation Period (IOP) 16a of the first HyMeX Special Observation Period (SOP1). Several quasi-stationary mesoscale convective systems developed over the western Mediterranean region, two of them over the sea, and resulted in heavy precipitation on the French and Italian coasts on 26 October 2012. Three different bulk parametrizations are tested in this study: a reference case (NOWAV) without any wave effect, a parametrization taking into account theoretical wave effects (WAV) and a last one with realistic wave characteristics from the MFWAM analyses (WAM). Using a realistic wave representation in WAM significantly increases the roughness length and the friction velocity with respect to NOWAV and WAV. The three MESO-NH sensitivity experiments of the IOP16a show that this surface-roughness increase in WAM generates higher momentum fluxes and directly impacts the low-level dynamics of the atmosphere, with a slowdown of the 10 m wind, when and where the wind speed exceeds 10 m s−1 and the sea state differs from the idealized one. The turbulent heat fluxes are not significantly influenced by the waves, these fluxes being controlled by the moisture content rather than by the wind speed in the simulations. Although the convective activity is globally well reproduced by all the simulations, the difference in the low-level dynamics of the atmosphere influences the localization of the simulated heavy precipitation. Objective evaluation of the daily rainfall amount and of the 10 m wind speed against the observations confirms the positive impact of the realistic wave representation on this simulation of heavy precipitation.

The e-Science environment developed in the framework of the EU-funded DRIHM project was used to demonstrate its ability to provide relevant, meaningful hydrometeorological forecasts. This was illustrated for the tragic case of 4 November 2011, when Genoa, Italy, was flooded as the result of heavy, convective precipitation that inundated the Bisagno catchment. The Meteorological Model Bridge (MMB), an innovative software component developed within the DRIHM project for the interoperability of meteorological and hydrological models, is a key component of the DRIHM e-Science environment. The MMB allowed three different rainfall-discharge models (DRiFt, RIBS and HBV) to be driven by four mesoscale limited-area atmospheric models (WRF-NMM, WRF-ARW, Meso-NH and AROME) and a downscaling algorithm (RainFARM) in a seamless fashion. In addition to this multi-model configuration, some of the models were run in probabilistic mode, thus giving a comprehensive account of modelling errors and a very large amount of likely hydrometeorological scenarios (> 1500). The multi-model approach proved to be necessary because, whilst various aspects of the event were successfully simulated by different models, none of the models reproduced all of these aspects correctly. It was shown that the resulting set of simulations helped identify key atmospheric processes responsible for the large rainfall accumulations over the Bisagno basin. The DRIHM e-Science environment facilitated an evaluation of the sensitivity to atmospheric and hydrological modelling errors. This showed that both had a significant impact on predicted discharges, the former being larger than the latter. Finally, the usefulness of the set of hydrometeorological simulations was assessed from a flash flood early-warning perspective.

Within the framework of FLOODsite, specific attention is dedicated to improve our understanding of flash flood. Flash floods differ markedly from floods occurring on larger basins with larger time characteristics. Flash floods are produced by rain accumulations of typically more than 200 mm during less than 6 hours over natural watersheds ranging in area from 25 to 2500 km2. The rising rate of waters of several m.h-1 and the flow velocities of several m.s-1 make these floods extremely dangerous for human lives. Research on flash floods requires a mobilisation of activities and resources at the European level for at least four reasons: i. Flash floods are rare event. In Europe one or two flash floods per year have dramatic consequences. Probably ten times more cases of storms of comparable severity occur during the same period. ii. Flash floods are ill documented events. Each country has its own investigation and archiving rules frequently separating the meteorological and hydrological aspects. iii. The forcing meteorological situations and their climatic trend develop at the continental scale. iv. Socio-economic short and long term strategies mitigating flash floods need to be harmonized across Europe. Within this general context, this report aims at presenting the main results obtained within Action 1.1 of Task 1. Here, the main objective is the understanding of the meteorological processes that govern flash-flood driven storms. Three cases typical of flash-flood (described section 2) occurring in Western Mediterranean are simulated using the local research model MesoNH (section 3). After identifying synoptic scale factors that favour development of quasi-stationary mesoscale convective systems, sensitive experiments together with Lagrangian trajectory analyses have been performed to identify the role of the orography, of the low-level jets and of the convection itself in maintaining stationarity on the simulated storms and its resulting cumulated surface precipita, Floodsite