Regulation of cloud particle spectra and vertical structure by sub-cloud aerosols: insights from aircraft observations and numerical simulations

Abstract. Aerosol–cloud interactions are key processes affecting cloud microphysics, precipitation formation, and climate system energy balance. Using aircraft observations from Hebei and parcel model simulations, the spectral characteristics, vertical structures, and response of cloud particles to aerosols are systematically analysed. Observations show that in liquid and mixed-phase clouds, droplet number concentrations exhibit bimodal peaks at diameters of 5–7 µm and 14–15 µm, while ice and mixed-phase clouds display ice crystal peaks near 125 and 1550 µm. Under high aerosol loading, ice crystal number concentrations increase at both small and large sizes but decrease at intermediate sizes. Under high sub-cloud aerosol loading, effective droplet diameters in the lower part (H<0.3, H is normalized cloud height) of liquid and mixed-phase clouds are 2.4 and 2.8 µm larger, respectively, than those in the upper part (H>0.7). The lower cloud layers also exhibit higher droplet number concentrations and broader size distributions. Under low-aerosol loading, the upper part of liquid and mixed-phase clouds contains droplets that are 2.7 and 4.4 µm larger than those in the lower part, respectively. The droplet number concentrations are higher in the upper layers for both cloud types. In ice clouds, high aerosol loading yield decreasing ice crystal number concentrations with height and effective radius that first increase but then decrease. Sensitivity experiments reveal that increasing aerosol concentrations cause a monotonic increase in droplet number concentrations. High aerosol concentrations amplify vapor competition and latent heat release, increasing temperature and reducing supersaturation, reducing the effective radius. These findings provide improved constraints for representing aerosol–cloud interactions in weather and climate models.