Radiative influences on the structure of cirrus clouds using a Large Eddy Simulation (LES) model.

This work focuses on the role that radiation takes in initiating and maintaining cloud inhomogeneity in cirrus clouds. Two base runs were performed to generate cirrus clouds with very different scales and magnitudes of inhomogeneities in their ice water contents. The two base runs were generated by a two hour simulation using the Large Eddy Simulation (LES) model. The simulations were then continued for an additional four hours. During this second stage, the cooling of the layer and the initial artificial heat pertubations (used to generate the cirrus) were switched off. For each of the two base runs, in the second stage two runs were performed (four in total) including one with radiative heating and cooling and the other with no radiative effects. It is found that in both cases the radiatively driven clouds last longer (hours) than the nonradiatively driven clouds. In addition, a Fourier analysis of the horizontal inhomogeneities shows that the radiation tends to initiate clouds inhomogeneity with a length scale of approximately one km, independent of the scale of inhomogeneity in the base runs. A length scale of inhomogeneity of 2-3km also becomes dominant as the cloud evolves, which results from plumes combining once they reach a convectively stable upper layer.