Impacts of synoptic forcing and cloud inhibition on aerosol radiative effect and boundary layer structure during winter pollution in Sichuan Basin, China

Abstract. Winter persistent aerosol pollution frequently occurs in the Sichuan Basin (SCB) due to its unfavorable weather conditions, such as low wind, wetness, and cloudiness. Based on long–term observational data analyses from 2015–2021, it was found that the four representative stations in the SCB often simultaneously experienced PM_2.5 pollution accompanied by variations in meteorological conditions above 850 hPa, which indicates a connection between regional winter air pollution in the SCB and large–scale synoptic patterns. The dominant 850 hPa synoptic patterns of winter SCB were classified into six patterns using T–model principal component analysis. Pattern 2, characterized by an east high west low (EHWL) pressure system, and Pattern 5, featuring a low trough (LT), were identified as key synoptic patterns for the beginning and accumulation of pollution processes. Pattern 1, characterized by a strong high pressure in the north, was the cleanest pattern associated with reduced PM_2.5 concentrations. The EHWL and LT patterns were associated with a remarkably high cloud liquid content, which was attributed to upper southerly winds that introduced humid air and converted aerosols into fog/cloud drops. Clouds reduce solar radiation through reflection and scattering, resulting in more stable stratification and aerosol accumulation. This cloud radiation interaction (CRI) is more pronounced in the LT pattern due to denser isobaric lines and stronger southerly winds than in the EHWL pattern. Numerical simulation experiments using WRF-Chem showed afternoon upper–level heating and morning surface cooling forced by the aerosol radiation interaction (ARI) and evening strong surface cooling influenced by valley winds in the SCB. With wet and cloudy synoptic forcing, CRI directly affects the stability of the boundary layer and is modulated through ARI inhibition. For example, Chongqing showed lower PM_2.5 concentrations and stronger ARI than the western and southern SCB due to thinner cloud liquid content and weaker CRI inhibition on ARI. The CRI inhibition caused a 50 % reduction in solar radiation and boundary layer height during the daytime under the LT pattern, which was larger than that under the EHWL pattern. This study comprehensively analyzed the cloud inhibition on ARIs and their impacts on the boundary layer structure under typical synoptic forcing during pollution processes, emphasizing the significant role of CRI inhibition in wet and cloudy regions.