Barrier effect of terrain on cold air and return flow of dust air masses

Abstract

The occurrence and development of dusty weather are closely related to cold air and geographical conditions. This study employed regular and special weather and aerosol observation data and the simulation results of the Weather Research and Forecasting model to assess dusty weather processes that occurred during March 2018 in Beijing, China. Specifically, the vertical structures of the aerosol, temperature, pressure, and wind fields and the causes of the abnormalities in the movement paths of dust air masses were analyzed. The results indicated that the mountainous terrain, atmospheric stratification, and basic flow in the region were key factors that affected the thermodynamic properties and vertical structures of airflow over the mountains. The air retention zone resulted in a vertical structure, with anthropogenic aerosols in the lower layer of the atmosphere and foreign dust aerosols in the upper layer of the atmosphere during the early stage of the dust storm. The warming effect of subsidence prevented cold air masses carrying dust aerosols from further permeating the lower atmosphere, and the dust air mass was inhibited from directly moving into the plains region through the regular northward path. When the orthogonal component of the basic flow and mountains was relatively small, the dust air mass could not directly cross the mountains. Instead, the dust air mass moved eastward, reaching the downstream region with a lower terrain, where the air mass formed a bypass easterly wind flow under the force of the stronger pressure gradient of the near-surface layer. Finally, the dust air mass that had moved away from Beijing reversed and traveled back toward the plains region of Beijing, forming sandy and dusty weather and leading to significant increases in near-surface PM10 concentrations.