Co-variability drives the inverted-V sensitivity between liquid water path and droplet concentrations

Many studies using climatological data of liquid water path (LWP) and droplet concentration (Nd) find an inverted-V relationship, where LWP increases and then decreases with Nd. Our findings suggest that while these LWP responses to changes in Nd align with proposed causal mechanisms, such as entrainment evaporation feedback and precipitation suppression, the inverted-V is primarily driven by the co-variability between LWP and Nd. This co-variability arises from meteorological conditions and microphysical processes, each independently affecting LWP and Nd in opposite directions. We further demonstrate that the inverted-V relationship reflects the climatological evolution of Stratocumulus clouds (Sc). Therefore, background anthropogenic changes in Nd should, in principle, be manifested in changes across the entire Sc climatology along its evolution. Instantaneous LWP response to Nd derived from ship tracks, or other similar natural experiments, may therefore not accurately represent the climatological LWP response. This is because the local perturbations in Nd may not align with the plausible natural co-variability between LWP and Nd, which varies depending on the cloud state along the Sc evolution.