Seasonal variation of local atmospheric circulations and boundary layer structure in the Beijing‐Tianjin‐Hebei region and implications for air quality

Abstract

AbstractThe Beijing‐Tianjin‐Hebei (BTH) region experiences frequent heavy haze pollution in fall and winter. Pollution was often exacerbated by unfavorable atmospheric boundary layer (BL) conditions. The topography in this region impacts the BL processes in complex ways. Such impacts and implications on air quality are not yet clearly understood. The BL processes in all four seasons in BTH are thus investigated in this study using idealized simulations with the WRF‐Chem model. Results suggest that seasonal variation of thermal conditions and synoptic patterns significantly modulates BL processes. In fall, with a relatively weak northwesterly synoptic forcing, thermal contrast between the mountains and the plain leads to a prominent mountain‐plain breeze circulation (MPC). In the afternoon, the downward branch of the MPC, in addition to northwesterly warm advection, suppresses BL development over the western side of BTH. In the eastern coastal area, a sea‐breeze circulation develops late in the morning and intensifies during the afternoon. In summer, southeasterly BL winds allow the see‐breeze front to penetrate farther inland (∼150 km from the coast), and the MPC is less prominent. In spring and winter, with strong northwesterly synoptic winds, the sea‐breeze circulation is confined in the coastal area, and the MPC is suppressed. The BL height is low in winter due to strong near‐surface stability, while BL heights are large in spring due to strong mechanical forcing. The relatively low BL height in fall and winter may have exacerbated the air pollution, thus contributing to the frequent severe haze events in the BTH region.