Mechanism for synoptic and intra-seasonal oscillation of visibility in Beijing-Tianjin-Hebei region

Based on the station observation data from 1980 to 2016, the atmospheric circulation anomalies affecting the synoptic and intra-seasonal oscillation (ISO) of visibility in the Beijing-Tianjin-Hebei (BTH) region were investigated. Results show that the synoptic variability of visibility over the BTH region was affected by Rossby waves moving from west to east. When the BTH region is on the eastern flank of a trough or the western flank of a ridge in the mid-troposphere, the anomalous southerly wind in the mid- and low troposphere played 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 continuous low-visibility days usually began when surface high-pressure system moved eastward from East Asia continent into the sea, and it terminates when the next cold high-pressure system invades the BTH region from the west. The essential atmospheric circulation anomalies affecting the intra-seasonal variability of visibility in the BTH region are the intra-seasonal seesaw between Siberian High (SH) and Aleutian Low (AL). An anomalous southerly wind appears over the BTH region when the Siberian High and the Aleutian Low 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. No propagating feature of the intra-seasonal atmospheric circulation anomalies is identified.

Automated summary

Based on station observation data from 1980 to 2016, the study investigated the atmospheric circulation anomalies affecting visibility in the Beijing-Tianjin-Hebei (BTH) region, finding that Rossby waves and the intra-seasonal seesaw between the Siberian High and the Aleutian Low play crucial roles. Wind speed, stability, and relative humidity are identified as important factors influencing synoptic and intra-seasonal variability of visibility in the region.