Analysis of the development mechanisms of a large-hail storm event, on the Adriatic Sea using an atmosphere-ocean coupled model (COAWST)

On the morning of 10 July 2019, an intrusion of relatively cold and dry air, over the Adriatic Sea, through a "bora jet", gave rise to a frontal structure at the ground, which moved rapidly from the Northern to the Southern Adriatic. The intense thermal gradient (together with a high positive sea surface temperature anomaly), the interaction of the jet with the complex topography of Apennines  and the coastal boundary, generated a storm structure that moved parallel to the central Italy coast. In particular, between 8UTC and 12UTC, a supercell developed along the coast to the north of Pescara city (middle Adriatic), producing rainfall that reached 130 mm in 3 hours, and a violent hailstorm (estimated diameter greater than 10 cm). 

In this work, the frontal dynamics and the genesis of the thunderstorm are studied using the numerical system COAWST. Local polarimetric radar observations are also used to check the consistency of the simulations in the mature phase of the supercell. Numerical experiments are performed using a 1 km grid over central Italy, initialized using the ECMWF IFS analysis/forecasts. The sensitivity study investigates the role of the orography, the sea surface temperature (SST) and the coupling between ocean and atmosphere. Orography tests include simulations where the relevant peaks of the Apennine range  (such as Gran Sasso and Picentini) are removed as well as cases where their peaks are modified compared to their real values. In terms of SST, we employ, using an uncoupled approach, the ECMWF SST dataset, the MFS-CMEMS Copernicus dataset at 4 km, 0.01°C Satellite SST, and we investigate the role of the SST anomaly (adding +1°C and +2°C to the real field). The role of the ocean-atmosphere interaction is tested using the COAWST numerical model using an ocean model numerical grid at 1 km resolution over the whole Adriatic Sea. 

The preliminary results show that the topography and in particular  the interaction with the peaks of the Apennine range plays a fundamental role in the dynamics of the cold pool that trigger the convective system. Also, the SST anomaly is found to play an important role in the development of the supercell. In particular, we observed that the simulations forced with MFS-CMEMS SST and the COAWST model runs produce a very realistic SST, in terms of spatial and temporal distribution, but colder by about 1.5 °C in absolute value if compared to observed satellite data. This difference generates lower heat fluxes, less evaporation, weaker precipitations and smaller hail than using warmer SSTs.