High effective supersaturation offsets low aerosol hygroscopicity to promote orographic cloud formation over the southern Tibetan Plateau

The Tibetan Plateau, known as Asian water tower, plays a crucial role in regional water cycles and climate. However, limited in-situ cloud observations have hindered a comprehensive understanding of cloud microphysical processes over this region. To address this, a ground-based in-situ experiment was conducted in the southern Tibetan Plateau (STP) to investigate aerosol-cloud-precipitation interactions. Utilizing extensive microphysical measurements of aerosols and clouds, this study derived in-cloud water vapor supersaturation (SS) and examined its role in aerosol activation and cloud development. Results revealed that orographic clouds over the STP exhibit notably high SS levels, with an average SS of 0.36% and cloud case mean SS ranging from 0.1% to 1.27%. Such high SS compensates for the low hygroscopicity of Aitken-mode aerosols, facilitating their activation into cloud droplets. The resulting microphysical changes include increased droplet number concentrations, broader droplet spectra, and enhanced cloud liquid water content, which could promote collision-coalescence processes and precipitation formation. Moreover, these SS-induced microphysical changes may enhance cloud albedo and influence regional radiative forcing, potentially impacting atmospheric circulation and monsoon dynamics over the Tibetan Plateau. These findings highlight the critical role of high SS in cloud formation under conditions of weak aerosol activation potential, offering new insights into orographic cloud processes in high-altitude environments.