Evaluation of air stagnation periods using regional climate models over Europe

Due to its high population density and strong industrialisation, Europe is subject to a high degree of air pollution. Reliable information on current and future air quality (AQ) is therefore necessary to develop policies. This information can be based on regional climate models (RCMs) such as used in CORDEX where weather-related uncertainties are estimated using RCM ensemble. Air polution peaks often occur during stagnant atmospheric conditions. We validate EURO-CORDEX RCMs to reproduce stagnant periods characterized by the Horton atmospheric stability index. We first prove the index's relation with both average and extreme air pollutant concentrations. The spatio-temporal features of air stagnant periods over continental Europe are then compared with reanalysis data from ERA5 for 25 RCM models. Overall a satisfactorily agreement is found for the stagnant periods despite a systematically underestimated frequency and stagnation duration. This bias is tracked back to the behavior of a large group of models over orographically complex regions. We show how bias correction can be used to improve the average, the variability and the duration of stagnation periods.