Links between precipitation, circulation weather types and orography in central Italy

Recent analyses of satellite and surface observations reported a negative annual rainfall trend in central Italy. The complex orography of the Apennines and the strong influence of climate change in the Mediterranean basin complicates the explanation of such trends and their spatial variability. This work aims at describing the link between circulation weather types, orography and precipitation patterns observed in central Italy, as a first step towards the understanding of such climate trends. Using ERA5 reanalysis data from 1951 to 2019, four weather types are identified as most responsible for the spatial variability of rainfall in central Italy. They are associated with cyclonic circulations characterized by high water vapour transport coming from west, south-west, south-east and north-east. The analysis of wind speed and precipitation climatology for the period 1951-2019, as derived from both surface observations and reanalysis, confirms a strong influence of moist south-westerly fluxes over all the domain, while the effect of north-easterly fluxes remains confined to the Apennines and the Adriatic coast. ERA5 overestimates annual rainfall in most parts of the region, except on the north-central Apennines where the underestimation reaches average values of 300 mm. The analysis of circulation weather types and their associated precipitation variability demonstrates that this deficit can be ascribed to a wrong representation of the orographic precipitation component as seen by ERA5 in correspondence of cyclonic north-easterly and westerly flows impinging towards the north-central Apennines. Indeed the precipitation associated with north-easterly and westerly fluxes shows the strongest modulation by orography. This interaction and its possible modification by climate change effects should be considered in future studies investigating recent climate trends in central Italy.