Mechanism For Synoptic And Lntra-Seasonal Variability Of Low-Visibility Events In Typical Regions Of Eastern China In Winter

Low visibility events caused by haze pollution have a serious impact on economy and health.The Synoptic variability and intra-seasonal variability of visibility are the most important theoretical basis in air pollution prediction.Based on the observation data and reanalysis data from 1980 to 2016 winter,the synoptic variability and intraseasonal variability of visibility were extracted,and the atmospheric circulation anomalies affecting the synoptic and intra-seasonal variability of visibility were investigated.Results show that the local anomalous low-level southerly wind is the key factor for the low visibility on both synoptic scale and intra-seasonal scale.The activity of Rossby wave in mid latitude is the main factor for synoptic visibility variability over the Beijing Tianjin Hebei(BTH)region,Shanghai(SH)region and Guangzhou(GZ)region in winter.Over all of these three regions,synoptic low-visibility events occurs when local southerly wind anomaly occurs,associated with an anomalous cyclone on the west and an anomalous anticyclone on the east.The anomalous southerly wind in the mid-lower troposphere plays a key role in reducing the visibility,possibly by weakening the wind speed of the prevailing northwesterly winter monsoon,enhancing the stability in the boundary layer,and increasing relative humidity.The essential atmospheric circulation anomalies affecting the intra-seasonal variability of visibility in the BTH and SH region are the intra-seasonal seesaw between Mongolian High(MH)and Aleutian Low(AL),and the intra-seasonal variability of visibility in GZ region is controlled by the intra-seasonal variability of MH.An anomalous southerly wind appears over the BTH and SH region when the MH and the AL are weaker at intra-seasonal time scale,and it results in intra-seasonal low-visibility events possibly through reduced total wind speed,enhanced static stability,and enhanced relative humidity as well.When the MH is weak,the cold air-mass burst decreases,and southwesterly wind anomaly prevails in the GZ region.The southwesterly wind anomaly mainly affects the visibility of GZ by reducing the wind speed and increasing the relative humidity.Based on the HYSPLIT model,the backward trajectories of SH and GZ were analyzed to track the source of air-parcels in the boundary layer.Results show that the regional transportation from the northwest has an important impact on the low-visibility events in SH at both synoptic scale and intra-seasonal scale,and the low visibility events in the GZ region are mainly caused by accumulation of local pollutants under stagnant meteorological conditions.