Impact of climate change on wintertime persistent inversions in the Grenoble valley during the 21st century

When anticyclonic conditions persist over mountainous regions, inversion layers can develop in the valleys and persist from a few days to a few weeks, especially in winter. During inversion episodes, the atmosphere inside the valley is stable and vertical mixing is prevented, promoting the accumulation of pollutants below the inversion layer and affecting the air quality of the valley.

Mountainous areas are experiencing a warming rate twice stronger than the global atmospheric temperature, thus, they are particularly sensitive to climate change. In the 21^st century, the valley circulation will undergo unknown modifications due to climate change, and this concerns also the fate of wintertime persistent inversions, which could be reinforced or weakened with consequences on pollutant trapping.

This work addresses the issue of climate change impact on persistent inversions in the Grenoble valley, which is the most populated city in the Alps. The long-term projections of the regional climate model MAR (Modéle Atmosphérique Régional) forced by the global climate model MPI (developed by the Max Planck Institute) are first used to perform a statistical study of the inversions over the 21^st century. The main characteristics of the inversions, e.g. duration, frequency, intensity, and their trends, are investigated for two different scenarios (SSP2-4.5 and SSP5-8.5). The intensity and the frequency of the inversions show a statistically significant decreasing trend in the 21^st century for the worst-case scenario.

The detailed structure of the inversion (atmospheric circulation, vertical temperature profiles, and inversion top) is next investigated by comparing two persistent episodes in the past and around 2050. For this purpose, the WRF (Weather Research and Forecasting) model, forced by MAR, is used at high resolution (111 m). In order to perform a fair comparison of the episodes, and given the influence of the orography on the valley circulation, the episodes are selected in such a way to satisfy common large-scale characteristics of a wintertime anticyclonic regime over Grenoble.