Diurnal Patterns in the Observed Cloud Liquid Water Path Response to Droplet Number Perturbations

Abstract

A key uncertainty in Aerosol‐cloud interactions is the cloud liquid water path (LWP) response to increased aerosols (λ). LWP can either increase due to precipitation suppression or decrease due to entrainment‐drying. Previous research suggests that precipitation suppression dominates in thick clouds, while entrainment‐drying prevails in thin clouds. The time scales of the two competing effects are vastly different, requiring temporally resolved observations. We analyze 3‐day Lagrangian trajectories of stratocumulus clouds over the southeast Pacific using 2019–2021 geostationary data. We find that clouds with a LWP exceeding 200 g m⁻² exhibit a positive response, while clouds with lower LWP show a negative response. We observe a significant diurnal cycle in λ, indicating a more strongly negative daytime adjustment driven by entrainment‐drying. In contrast, at night, precipitation suppression can occasionally fully counteract the entrainment‐drying mechanism. Overall, λ appears weaker than previously suggested in studies that do not account for the diurnal cycle.