Large-Eddy Simulations of the 26 November 1991 FIRE II Cirrus Case

Abstract

Large-eddy simulations (LESs) were performed to study the dynamical, microphysical, and radiative processes in the 26 November 1991 FIRE II cirrus event. The LES model inherits the framework of the RAMS version 3b, developed at Colorado State University. It includes a new two-stream radiation model developed by Harrington and a new subgrid-scale model developed by Kosovic. The LES model successfully simulated a single thin cloud layer for LES-1 and a deep cloud structure for LES-2. The simulations demonstrated that latent heat release can play a significant role in the evolution of thick cirrus clouds. For the thin cirrus in LES-1, the latent heat release was insufficient for the cirrus clouds to become positively buoyant. However, in some special cases such as LES-2, positively buoyant cells can be embedded within the cirrus layers. The updrafts from these cells induced its own pressure perturbations that affected the cloud evolution. Vertical profiles of the total radiative and latent heating rates indicated that for well-developed, deep, and active cirrus clouds, radiative cooling and latent heating could be comparable in magnitude in the cloudy layer. This implies that latent heating cannot be neglected in the construction of a cirrus cloud model. The probability density function (PDF) of the vertical velocity (w) was analyzed to assist in the parameterization of cloud-scale velocities in large-scale models. For the more radiatively driven, thin cirrus case, the PDFs are approximately Gaussian. However, in the interior of the deep, convectively unstable case, the PDFs of w are multimodal and very broad, indicating that parameterizing cloud-scale motions for such clouds can be very challenging.