Cloud Susceptibility to Aerosols: Comparing Cloud-Appearance vs. Cloud-Controlling Factors Regimes

Clouds can be classified into regimes by the cloud appearance or by the cloud meteorological controlling factors. The cloud appearance regimes inherently include adjustments to aerosol effects, such as transitions between closed and open cells. Therefore, aggregation of cloud susceptibilities to aerosols over the cloud-appearance regimes excludes much of the cloud adjustment component of the susceptibilities. In contrast, aggregating susceptibilities over regimes defined by cloud-controlling factors includes the full effects of cloud adjustments. Here we compared the susceptibilities of the two kinds of cloud regimes and demonstrated this effect. Overall, increasing cloud droplet number concentration (N_d) consistently leads to precipitation suppression, higher cloud fraction (CF), and reduced liquid water path (LWP), regardless of how the regime is defined. However, their susceptibilities to N_daggregated over cloud-appearance regimes are significantly lower than those aggregated over cloud-controlling factors regimes, with lower-tropospheric stability (LTS) serving as an example to define cloud-controlling factors regimes. This underestimation is more pronounced for CF susceptibility, where the susceptibility for cloud appearance regimes is only 1/4 of the susceptibility for cloud controlling regimes. These findings imply that relying solely on cloud-appearance regimes may underestimate the effective radiative forcing produced by cloud adjustment (ERF_aci). Nevertheless, the substantial variability in the magnitude of cloud adjustment across appearance regimes at similar LTS also suggests that a single cloud-controlling factor is not sufficient to fully separate cloud regimes to quantify cloud adjustment. Therefore, identifying a comprehensive set of cloud-controlling factors is essential for accurately quantifying cloud adjustments in future studies.