Influence of boundary layer-cloud coupling on cloud microphysics based on aircraft observations in the North China...

Aerosol-cloud interactions significantly influence atmospheric dynamics and energy balance. This study investigated the vertical and spectral distributions of aerosols and cloud microphysical variables using 35 aircraft observations over the North China Plain (NCP) from 2019 to 2021. Aerosol number concentration (Na) was highest in winter under both clear sky and cloudy conditions, but lowest in summer in clear sky and in spring in cloudy conditions. Na and black carbon mass concentration (M_BC) decreased with altitude before becoming uniform in all conditions. The altitude where aerosols became uniform varied, reaching 3000 m (winter) and 2000 m (other seasons) in clear sky conditions, and 3500 m in cloudy conditions. Cloud droplet number concentration (Nc) and liquid water content (LWC) exhibited unimodal vertical profiles in all seasons, whereas ice crystal number concentration (Ni) displayed seasonal differences without a distinct vertical trend. The cloud droplet size distribution (dNc/dD) peaked at 6.5 μm, and the ice distribution (dNi/dD) had bimodal peaks at 125 and 1500 μm. Among the 21 observations in cloudy conditions, 15 were decoupled and 6 coupled, forming the basis for analyzing the boundary layer-cloud coupling mechanism. Decoupled clouds were more influenced by downward air masses outside NCP, while coupled clouds were more affected by upward air masses within NCP. In spring, dNc/dD with a diameter greater than 8.5 μm was relatively low in decoupled clouds, while in coupled clouds, dNc/dD with a diameter greater than 27 μm was higher. In autumn, dNi/dD of all diameters was relatively higher in decoupled clouds. The aerosol size distribution characteristics inside and outside the cloud showed no significant change due to the boundary layer-cloud coupling conditions.