Recent trends in sounding-derived indices and thunderstorm-related observations: a wider perspective over the Po Valley.

Recent analyses have shown that in the eastern part of the Po Valley the atmospheric potential for more frequent and/or intense thunderstorms has been increasing in the last years. From the analysis of high-resolution soundings in Udine-Rivolto (Northeastern Italy) we found that atmospheric instability has significantly increased during the period 1992-2022. Moreover, the abundance of water vapour in the atmosphere shows a large increase, even higher than what could be expected from the Clausius-Clapeyron law. Surprisingly, the ground observations related to thunderstorms do not reflect the significant trends seen in potential instability and absolute moisture: no significant trend in the last years is identified for short-term intense rainfall, lightning activity and hail occurrence. Only the mean dimension of the hailstones has significantly increased. These outcomes are quite surprising, because they do not agree with the warming climate framework in which it is common to expect an increase in extreme events. To support these results and to verify their validity not only on a limited region, it is necessary to replicate the same analysis on a wider area, starting from the other side of the Po Valley. To this purpose, we have analysed high-resolution soundings from the site of Cuneo-Levaldigi (Piedmont, western part of the Po Valley) from 2005 to 2024. From a preliminary analysis we can conclude that the increase in potential instability and water vapour availability are consistent over the two sides of the Po Valley: for example, in both locations, most unstable CAPE is increasing at rate of about +100 J/kg per decade, while total column vapour (i.e. precipitable water) is increasing at a rate of about +1.6 mm per decade. In general, this result corroborates the conclusion that the environment is becoming potentially more favourable to storm development and intensification in the Po Valley. The upcoming analyses will investigate observational datasets in Northwestern Italy (e.g., short-term intense rainfall from regional weather station network, lightning detection networks, weather radar products) to determine if, in this area, an increase in observed thunderstorm activity can be identified, or if the mismatch between increasing thunderstorm potential and the actual observed activity shows a more general validity.