Lapse rate inversions in the Po Valley : a 30-year overview

In this study some features of lapse rate inversions in the lower layers of the atmosphere over the Northern Italy area of the Po Valley have been investigated. Radiosonde data available at 12 UTC from four meteorological stations (Cuneo Levaldigi, Milano Linate, San Pietro Capofiume and Rivolto) located in this area have been taken into account considering a 32-year period from 1985 to 2016; Milano Linate’s data have been found to be the most complete in terms of data coverage, so they have been used for a consistent part of the following evaluations. In particular, parameters like the number of occurrence of days with thermal inversion observed, the number of occurrence of “inversion events” (i.e. time periods formed by consecutive “inversion days) and their temporal features (i.e. time-span distribution) and the vertical structure of the temperature lapse rate on inversion days have been investigated; these evaluation has been carried on both annual and seasonal basis. Some analysis have shown that there are common features through this air basin, while there significant differences in other aspects; in addition, assessments on trend evolution (on annual and seasonal basis) of some phenomena have shown that there are different behaviours depending on the season considered. An empirical model for the distribution of thermal inversion events’ duration has been developed; at last, a comparison between data from radiosonde and radon measurements made by the University of Milan has been carried on in order to evaluate if the two different methods are useful and consistent to determine the free mixing height generated from subsidence inversions.