Morphology-Conditioned Susceptibility of Marine Stratocumulus Clouds Suggests Weak Marine Cloud Brightening Potential

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 corresponding τc classes. Different combinations of the three τc 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 for the most frequent observed morphologies, LWP and cloud albedo susceptibilities largely offset each other, resulting in a net in-cloud albedo response close to zero. The cloud fraction susceptibility is found to be positive in precipitating morphologies and negative in non-precipitating morphologies. 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.