The problem of Elevation-Dependent Warming (EDW) in the Greater Alpine Region (GAR)

For some years now we have been increasingly concerned about the phenomenon of global warming and its consequences. In particular, some studies have shown the extent to which some areas of the world exhibit higher sensitivity to the global temperature increase, both in terms of enhanced warming rates and of the associated impacts. Among these so called hot spot regions are high-elevation mountain areas: they are showing, on average, a higher warming rate (0.3°C/decade) with respect to the globally-averaged temperature increase (0.2°C/decade). This amplification has been often referred to as Elevation-Dependent-Warming (EDW), which literally means a statistically significant dependence of warming rates on elevations (not necessarily an amplification). There are several possible EDW drivers identified in the literature, especially making use of global and regional climate models: the snow-albedo feedback, the water vapour modulation of long wave radiation, changes in aerosol and clouds and their effects on shortwave and longwave radiation, and interactions among all them: this makes the analysis of the EDW mechanisms particularly difficult. This work aims at studying EDW in a specific mountain area, the Greater Alpine Region (GAR). Initially, an ensemble of observation-based datasets has been analyzed with the aim of comparing them and demonstrating their ability in reproducing the climatology and temporal trends of the temperature in the area, as well as the dependence of the trends on the elevation. The datasets used are E-OBS and HISTALP (gridded datasets based on the interpolation of in-situ station data), and ERA5 and MERRA-2 (reanalysis datasets). As for the analysis of the EDW drivers, the methods already applied in previous papers have been considered. Besides the expected temperature bias among the gridded datasets based on interpolated in-situ station data and reanalysis data, the comparison showed a surprisingly high variability in the EDW signals. This could suggest a possible role of (at least) the spatial resolution and the consequent ability to adequately reproduce the orography of the region (and the processes dependent on the elevation). Potential EDW drivers include the changes in albedo, in downward longwave and shortwave radiation, in water vapour and clouds, in aerosol particles. Actual EDW drivers are, among those variables, the ones whose dependence on elevation is physically consistent with the elevational gradient of warming rate and those which, independently of elevation, still spatially correlate with the temperature change. As a preliminary study, in this thesis we explore the possible role of aerosol particles on EDW in the Alps, exploiting tha availability of aersol data in the MERRA dataset. This would constitute an important contribution to the understanding of EDW in the Alps. In agreement with previous EDW studies on mid-latitude mountains, the changes or temporal trends in albedo, surface downward longwave radiation and specific humidity are the main EDW drivers identified by ERA5. In winter, when the dataset found negative EDW, the shortwave radiation seems to not play a role again. Further analysis aims to better study the probable influence of atmospheric aerosol on EDW in the Alps