京津冀地区一次强沙尘天气过程的成因及特征

Causes and characteristics of a dust weather process in the Beijing-Tianjin-Hebei Region

摘要:

利用常规污染物监测资料、卫星资料和再分析资料等，对京津冀地区在2017年春季遭遇的一次强沙尘天气过程进行分析。结果表明，此次过程是由地面冷锋过境，高空槽后冷空气持续补充引起，沙源地主要位于巴丹吉林、腾格里沙漠，随后以西北路径输送至京津冀地区。前期沙源地感热通量迅速增大，与中低层冷平流叠加，导致不稳定层结增强，助于起沙；高空强风速带加强并向下延伸，中低层次级环流发展，不但使沙尘传输并下降至地面，而且使高层高动量和高位涡冷空气下传，促进低空急流形成、低层系统发展，使大风及沙尘天气维持；沙尘过境时，地面至4 km高度存在沙尘型气溶胶，PM_2.5和PM₁₀浓度变化趋势较一致并达到重度污染水平，且气溶胶光学厚度（Aerosol Optical Depth，AOD）与空气质量指数（Air Quality Index，AQI）具有较好的时空匹配关系，重污染时段AOD值大于1，污染减弱时AOD值降至0.6以下。

关键词:沙尘;感热通量;高低空急流;动量下传;AOD

Abstract:

In this study,based on the routine pollutant monitoring data,satellite data,reanalysis data,backward trajectory models,etc.,the causes and characteristics of a strong dust weather process which occurred in the Beijing-Tianjin-Hebei region in 2017 was analyzed.The study results showed that the synoptic situation of the cold front was what had mainly led to this process,along with the continuous supplement of cold air after the trough.The major sand-dust sources were located in the Badan Jilin Desert and Tengger Desert.Backward trajectory simulation suggested that the contribution to the increasing dust in the Beijing-Tian-Hebei region originated from the northwest.In addition,the cities surrounding the northwest long-distance trajectories suffered from different levels of pollution.The sensible heat flux increased rapidly in the sand-dust sources,which combined with the cold advection in the middle and lower layers,in turn enhancing the unstable stratification and promoting the sediment discharge.The strong,high-level wind speed strengthened and continued to extend downward.The secondary circulation in the middle and low levels developed,which not only transferred and descended the dust particles,but also caused the high momentum and high potential vorticity of the upper level to pass downward.The low-level jet and systems were formed and developed,thereby maintaining the strong wind and dust weather.The negative horizontal helicity had a high correspondence with the occurrence area of dust weather.When the dust weather was passing through,the dust type aerosol appeared between the surface and a height of 4 km.In addition,the concentrations of PM_2.5 and PM₁₀ exhibited high consistence in variation,and led to the level of severe pollution.Furthermore,the values of aerosol optical depth (AOD) and air quality index (AQI) had a high correlation in both the spatial and temporal distributions.The temporal and spatial distributions showed that the value of AOD exceeded 1 during the high pollution period,and dropped below 0.6 when the extent of pollution gradually decreased.

Key words:dust;sensible heat flux;high-and low-level jet;momentum down propagation;AOD