INFLUENCE OF CLIMATE CHANGE ON THE FREQUENCY OF LAPSE RATE INVERSION AND STAGNATION EVENTS IN THE PO VALLEY: HISTORICAL TREND AND FUTURE PROJECTIONS

Abstract

This work analyzes the variation of the frequency of occurrence of days characterized by diurnal lapse rate inversion and by air stagnation events in the Po valley between 1950-2090, based on past observations at San Pietro Capofiume and Milano Linate and of future projection according to CCLM 4-8-19 regional climatic model (Med-CORDEX project). Lapse rate inversion, typically occurring during the cold season, strongly limits atmospheric dispersion, thus favoring the stagnation of the pollutants and leading to high-concentration events. For the period from 1987 to 2006, the analysis showed an increase in days characterized by lapse rate inversion approximately estimated in +1,2 days/year according to San Pietro Capofiume data, whereas a +1,1 days/year increase has been observed for Milano Linate data (period 1999-2012). For future estimates, a new model for the prediction of days with lapse rate inversion is presented. The model, based on the comparison of G850 (temperature gradient between ground level and standard geopotential height of 850hPa) and a coefficient dependent on surface pressure, has been developed based on radio sounding data from San Pietro Capofiume between 1987 and 2006 and validated on data from Milano Linate between 1999-2012. For future projections, considering a RCP4.5 medium emission scenario, it is estimated a further increase in frequency of lapse rate inversion in the period from 2010 to 2100 with a grow rate approximately equal to +1,2 days/decade. In this scenario, the years of the 2091-2100 decade will be characterized by an increase of 12 days (approximately 10%) with lapse rate inversion with respect to 1986-2005 average, concentrated especially in the warm period. In a RCP8.5 high-emission scenario it is estimated a lower grow rate of lapse rate inversion, equal to +0,6 days/decade for period from 2010 to 2100. Finally, the projections indicate that the frequency of air stagnation events, days characte-rized by moderate winds and no rain, is also expected to rise: for the 2091-2100 decade an increase of 13 days and 11 days is estimated with respect to 1986-2005 a-verage, respectively in RCP4.5 and RCP8.5 scenarios.