Radiative influences on the structure and lifetime of cirrus clouds

Our goal in this work is to determine the role of radiation in cirrus clouds. How it impacts the formation and evolution of cloud structure, whether or not it initiates convective instability, and what effect it has on the lifetime of these clouds. In this study, we simulate cirrus clouds using the Met Office large-eddy simulation model with a broad-band solar and infrared ice-specific radiation scheme. We find that radiation can have quite an effect on cirrus clouds. Results for runs with and without radiation show many dramatic differences. In general, radiatively influenced clouds are observed to be much more dynamic and inhomogeneous. The radiation is observed to strongly enhance cellular structure within the cloud layer, and a Fourier analysis of the horizontal ice-water path (IWP) shows that this cellular enhancement gives rise to inhomogeneity length-scales roughly related to the thickness of the layer. The Fourier amplitudes for the radiative cases are usually two or three times larger in magnitude than the non-radiative cases. The inhomogeneity length-scales do appear in the non-radiative cases but they are very weak. The radiatively driven clouds in these simulations often had horizontally averaged IWPs larger in magnitude by more than double compared to the non-radiative cases once the radiation had taken effect, and the cloud lifetime was increased by between 30 minutes and 2 hours. To evaluate if radiation is causing convective instability, we derive and implement a radiation stability number, R sn. For radiative instability to occur, R sn must satisfy the condition 0 <R sn <1. We also present similar stability numbers for latent, and the combined radiation and latent processes. By evaluating these numbers throughout the domain during the simulation, we can determine when these diabatic processes will overcome the thermal stratification of the layer and induce convective instability. These processes are found frequently to induce instability for the radiative inclusive case. We find that radiative and latent instability usually occurs in updraught regions, and that latent heating initially enhances the development of the plumes prior to the radiation. We also find that the magnitude of the latent heating is also strongly coupled to the effects of radiation, since stronger cell structure induced by radiation results in stronger latent heating at cloud top and less precipitation at cloud base.