Beyond discrete stratocumulus regimes: A ternary continuum of morphology reveals within-regime variability in cloud susceptibilities

Abstract

We introduce a new framework for defining marine stratocumulus cloud morphologies using a ternary diagram. A ternary diagram is a triangular representation of three components, with each vertex corresponding to 100% of one component, and any point within the triangle representing a mixture of all three that sums to 100%. We use cloud optical thickness (τc) as the diagnostic physical variable and accordingly define three correspondingτc classes. Different combinations of the threeτc classes define different cloud morphologies, which vary continuously within the ternary space. The method is applied to one year of satellite observations of stratocumulus clouds and reveals the frequency of occurrence of the different morphologies across the ternary space. Large-eddy simulations complement the satellite analysis and show that cloud evolution tends to follow preferred paths across the ternary morphology space, explaining why the observations are concentrated within a limited range of morphologies. We further investigate the susceptibility of cloud liquid water path (LWP), cloud albedo, and cloud fraction to variations in droplet number concentration, conditioned on cloud morphology. We find that susceptibilities vary strongly with cloud morphology, yet in the most frequently occurring scenes, LWP and cloud albedo susceptibilities largely offset each other, resulting in a near-zero global in-cloud albedo response. We also find that cloud fraction susceptibility can be negative in low-LWP morphologies, presumably due to strong negative LWP adjustments. These findings have important implications for marine cloud brightening, whose effectiveness needs to be evaluated in a morphology-dependent framework to achieve the intended outcomes.