Trends of sounding-derived indices and observations in NE Italy in the last 30 years

High vertical resolution soundings observed at 00 and 12 UTC during the months of April to September of 30 years (from 1992 to 2021) are analyzed to characterize the potential  instability, dynamic conditions and moisture conditions of the atmosphere above NE Italy. Their climatological trends are studied, finding that the potential instability and moisture (including the precipitable water) show a robust and statistically significant increasing trend, producing a more favorable environment for storm development. Winds do not substantially change, apart from the meridional component of the low level wind, that causes a positive trend also in the wind shear.

Despite these significant increases in instability and moisture variables, the corresponding observations, in terms of rainfall (from a network of rain gauges), hail (from a network of hailpads) and cloud to ground lightning flashes (from the EUCLID lightning detection network), do not show similar trends. Instability indices describing pre-convective environment are often assumed to trace the storm probability, so that their climatological change is used as a proxy for the probability of storm occurrence and/or intensity. Our findings cast doubts on the validity of this approach, at least locally, because to a strong increase of sounding derived indices (toward a more unstable environment) does not correspond an equally robust increase of the observed storm activities.