Large-scale atmospheric patterns associated with mesoscale features leading to extreme precipitation events in Northwestern Italy

The synoptic and large-scale atmospheric conditions for heavy rainfall events in Northwestern Italy are diagnosed through the joint analysis of surface precipitation gauges and reanalysis atmospheric fields. Quantiles of local surface gauge precipitation observations are used to estimate the much larger-scale composite maps (conditional mean fields) of vertically integrated moisture flux, low-level winds, sea-level pressure, and 500 hPa height across the Atlantic and European domains. Remarkably, coarse-resolution reanalysis data show distinct synoptic conditions for heavy precipitation in localized regions that are below the resolution of the reanalysis. In this paper the key attributes of the new approach that is based on the joint analysis of gridded reanalysis and station data are presented. The application of the methodology are used to establish supporting evidence for hydrometeorological processes that lead to extreme precipitation across Northwest Italy. The results confirm the role of large-scale flow features that interact with regional topography in producing localized extreme precipitation. Whereas previous studies were based on a few case studies (modeled or observational), in this study the approach to producing a large ensemble of cases and composite statistics are introduced.