Theoretical framework for measuring cloud effective supersaturation fluctuations with an advanced optical system

Abstract. Supersaturation is crucial in cloud physics, determining aerosol activation and influencing cloud droplet size distributions, yet its measurement remains challenging and poorly constrained. This study proposes a theoretical framework to simultaneously observe critical activation diameter and hygroscopicity of activated aerosols through direct measurements of scattering and water-induced scattering enhancement of interstitial and activated aerosols, enabling effective supersaturation measurements. Advanced optical systems based on this framework allow minute- to second-level effective supersaturation measurements, capturing fluctuations vital to cloud microphysics. Although currently limited to clouds with supersaturations below ∼ 0.2 % due to small scattering signals from sub-100 nm aerosols, advancements in optical sensors could extend its applicability. Its suitability for long-term measurements allows for climatological studies of fogs and mountain clouds. When equipped with aerial vehicles, the system could also measure aloft clouds. Therefore, the proposed theory serves as a valuable method for both short-term and long-term cloud microphysics and aerosol–cloud interaction studies.