Numerical simulations of heavy precipitation events in Liguria (Italy): sensitivity to microphysics, SST and natural aerosols

Mediterranean coastal regions are regularly affected by localized heavy precipitation events, resulting in very dangerous flash floods, often of limited predictability. Due to its position, exposed to southerly moist flows from the Mediterranean Sea, and the steep orography near the coasts, one of the most affected areas is Liguria region in northwestern Italy. Extreme precipitation is usually observed between late summer and midautumn, when heat and moisture fluxes from the Mediterranean Sea are the highest. The typical synoptic situation responsible for such events are also associated with desert dust transport towards the Mediterranean and occasionally the Alpine region and Central Europe. Four different case studies corresponding to severe rainfall events occurred over Liguria between October 2010 and October 2014 have been considered and numerical experiments have been performed with the Weather Research and Forecasting (WRF) model to investigate the sensitivity of predicted precipitation to horizontal resolution, microphysics schemes and SST initialization. In particular, two different SST initializations are considered: a coarse field provided by a global atmospheric model and a high-resolution multisatellite analysis. The response of heavy precipitation to SST is a complex one, which involves the modification of PBL and low-level flow characteristics and its interaction with topography. The impact of satellite-derived SST on total predicted precipitation appears beneficial especially for simulations initialized the day before the event, due to the relatively slow adjustment of atmospheric fields to the higher-resolution initial condition. Conversely, such an impact is generally neutral or even negative for 24 h forecasts. The analysis of the most recent episode (October 2014) suggests that a satellite-retrieved SST initial field can improve the quantitative precipitation forecast only when the large-scale forcing is accurate enough. Finally, some preliminary simulations have been performed with the online-coupled meteorology-chemistry model WRF-Chem to investigate the possible role of natural aerosols (dust and sea salt) in the modulation of severe precipitation events in Liguria.