[Meteorological mechanism for the formation of a serious pollution case in Beijing in the background of northerly flow at upper levels].

There is a kind of serious air pollution case occurring in the situation of northerly flow at the levels above 850 hPa in Beijing area. Meteorological data, NCEP reanalysis data and PM2.5 concentration survey were used to analyze the roles of meteorological mechanism in this kind of air pollution case. The results showed that the stable stratification did not exist all the time during the gradual increase of PM2.5 concentration. The favorable meteorological conditions in PM2.5 accumulation came from the layered structure of vertical speed and convergence in vertical direction. From the surface to the middle level of troposphere, the vertical velocity showed an updraft-downdraft-updraft distribution. And the divergence displayed a convergence-divergence-convergence structure. The convergence at lower levels led to the gathering of the polluted air from the surrounding and the ascending current brought it upwards. However, the polluted air could not go further up due to the prolonged downdraft layer above. As the result, PM2.5 near the surface was concentrated gradually. The vertical speed in layer structure was attributed to the high level northerly flow without invading into the boundary layer. In such case, the wind was still in southerly direction or kept calm at the surface. The weak cold air or no cold air at all was the main reason that the northerly flow could not hit the surface. The developed downdraft layer was relative to the convergence above which was resulted from the wind speed pulses in the northerly flow at upper levels. In conclusion, the air dynamical effect played a key role in the pollution type of northerly flow at upper levels. Therefore, it will be beneficial to enhance our capability in the analysis and forecasting of this kind of pollution case by focusing on 24 hours temperature variation at the middle and lower troposphere as well as the vertical distribution of vertical velocity and divergence.