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

Abstract

Climatological data of the liquid water path (LWP) and droplet concentration (Nd) often reveal an inverted-V relationship, where LWP increases and then decreases as Nd increases. Our findings show that while this LWP response to an increase in Nd aligns with proposed causal mechanisms, such as entrainment evaporation feedback and precipitation suppression, the inverted-V pattern is primarily driven by the co-variability between LWP and Nd. This co-variability arises from (1) large-scale meteorology, which controls both LWP and Nd, causing them to vary in opposite directions simultaneously, and (2) microphysical processes, where an increase in LWP is typically accompanied by a decrease in Nd. Consequently, we suggest that the inverted-V sensitivity should not be used as evidence for positive radiative forcing through LWP adjustments to aerosols as it is largely explained by co-variability. We further demonstrate that the inverted-V relationship essentially reflects the climatological evolution of Stratocumulus clouds (Sc). Therefore, background anthropogenic changes in Nd should, in principle, be reflected in the redistribution of occurrences across the inverted V while maintaining its shape.