Dissecting cirrus clouds: navigating effects of turbulence on homogeneous ice formation

Abstract

Turbulent air motions determine the local environment in which cloud ice crystals form. Homogeneous freezing of aqueous solution droplets is the most fundamental pathway to nucleate ice crystals in cirrus. Lack of knowledge about the role of turbulence in cirrus ice formation limits our understanding of how uncertainties in small-scale cloud processes affect the climatological radiative effect of cirrus. Here we shed first light on how turbulent fluctuations in temperature and supersaturation interact with probabilistic homogeneous freezing. We show that spatial model resolution substantially below 1–10 m is needed to properly simulate homogeneous freezing events. Importantly, microscale turbulence generates large variability in nucleated ice crystal number concentrations. Previous research ascribed this variability to mesoscale dynamical forcing due to gravity waves alone. The turbulence-generated microphysical variability has macrophysical implications. The wide range of predicted cloud radiative heating anomalies in anvil cirrus due to turbulence-ice nucleation interactions, comparable to typical mean values, is potentially large enough to affect the response of tropical cirrus cloud systems to global warming. Our results have ramifications for the multiscale modeling of cirrus clouds and the interpretation of in situ measurements.