Backward Trajectory and Circulation Features in Different Haze Processes in Winter of Zhejiang
Based on available observation data of Zhejiang Province for the period from 1981 to 2013, temporal and spatial distribution characteristics of winter haze in Zhejiang Province were analyzed. The results show that haze days have increased significantly since the mid1980s. Haze days in the northern and western part of the province are much more than the south and the east, also more in the basin and plain than on the hills and islands. The division of the haze distribution pattern is obtained according to the Kmeans clustering analysis, which shows the pattern can be divided into two categories. The first category accounts for 62% of the total haze, most of which happens in December, while most of the second category usually appears in January and December. In the end, according to the NCEP/NCAR reanalysis data, the different atmospheric circulation features, including the strength of cold high, vertical velocity and the distribution of water vapor, are discussed for the two kinds of haze patterns respectively. It is found that Zhejiang Province is controlled by cold high from northern China and the sinking airflow in the first haze pattern. In the second haze pattern, however, the cold high has transformed, and the sinking flow is also weakened, which makes air move slowly, so it is hard for pollutants to dissipate.