Precipitation Characteristics and Correlation Analysis During an Air Pollution Episode
Precipitation (5 mm) characteristics and NCEP/NCAR reanalysis data of a pollution process in Dalian on 16 November 2015 were analyzed. The results showed that the synoptic situation was the typical highlevel trough before the precipitation in Dalian in autumn and winter. The atmospheric stratifications were stable with inversion layers during the early stage of pollution and the late stage of precipitation. There was ascending motion before the precipitation for more than 10 hours, which means the fine particles (PM2.5) around the surface could be transported into the upper air continually with the rising air, and part of the particles would transform into ice nuclei as supplement of ice nucleus in the precipitating cloud. In addition, three more processes of precipitation (<5 mm) in Dalian in autumn and winter during 2015-2016 were analyzed. Cloud liquid water content, amount of precipitation and concentration of fine particles (PM2.5) were discussed in detail. The results showed that there are less ice nuclei leading to smaller rainfall with low concentration of PM2.5, which is suitable for artificial precipitation operation to increase the ice nuclei in clouds and the surface precipitation. Otherwise, there are more or overdosed ice nuclei leading to sufficient precipitation or lower precipitation with high concentration of PM2.5, especially during the air pollution episode. In such cases during artificial precipitation operation working dose or times should be reduced according to the actual situation. Comprehensive analysis also showed that there are two kinds of pollution sources in Dalian. One is local source, and the other is external source. A certain amount of precipitation could reduce pollution through wet deposition by local source, with no obvious effect by external source.