Ice Crystal Complexity and Link to the Cirrus Cloud Radiative Effect

The aim of this chapter is to present emerging information on ice crystal morphology in the nano- and microscale from various sources, and explore its impact on ice crystal’s single-scattering properties and ultimately on cirrus cloud radiative effect. Increasing amount of observations in the last decade have shown that cirrus ice crystals invariably contain some degree of facet roughness, hollowness, or other morphological complexities that make the crystal shape deviate from that of a pristine hexagonal shape. These morphological complexities are found to be present in a wide variety of environmental conditions. A set of physical and statistical complexity parameters is used to quantify ice crystal complexity depending on the application. Although laboratory studies have shown that complexity parameters tend to increase with increasing crystal growth rate, observations in natural cirrus indicate that complexity parameters only weakly vary at cirrus temperatures below 230 K. Morphological complexities affect the optical properties of ice crystals. Implementing the optical properties of complex ice crystals in climate models has shown that the radiative effect of crystal complexity is a cooling of 1-2 Wm-2 globally. Due to the prevalence of ice crystal morphological complexities and their influence on cirrus radiative properties, the radiative effect of crystal complexity should be taken into account in future climate models. However, work is still needed to transfer the increasing knowledge of the physical nature of ice crystal complexity into optical particle models and eventually into parameterizations used in models.