Evaluation of air stagnation in China by CMIP5 models

The change of air stagnation days is closely related with the variation of air pollutant concentration, while evaluating the performance of simulating air stagnation days will provide a basis for projection. In this paper, we calculated the number of air stagnation days during 1961-2005 by employing the outputs from 15 Coupled Model Intercomparison Project phase 5 (CMIP5) models and observation data. Then the models' ability to simulate air stagnation days was assessed. The statistical approach we used is listed as follows: standard deviation, root-mean square error, time evolution of each sectors, trend pattern and EOF (Empirical Orthogonal Function). We found that the performance of multi-model ensemble (MME) is high, but that of individual models varies widely. The CanESM2, IPSL-CM5B-LR and MME work well over the most parts of China. And MME can reproduce the spatial pattern of observed air stagnation days in summer and winter well. While the observed time evolutions of spatial averaging fluctuate greater than the simulated ones over China and its eight sub-regions, and MME values of most sectors in China are higher than the corresponding observed values. In winter, MME values are higher than the observed ones over most sectors. There is a good agreement in air stagnation trends pattern between the observational and model data over the Eastern part of China and most part of Xinjiang, but the spread of simulated trends is smaller than the one of observed trends. MME has the ability to capture the spatial distribution of EOF1 and EOF2; however, it fails to reproduce the observed time coefficient series of EOFs.