VALIDITY OF THE TANGENT LINEAR-APPROXIMATION IN A MOIST CONVECTIVE CLOUD MODEL

The validity of the moist tangent linear model (TLM) derived from a ti me-dependent 1D Eulerian cloud model is investigated by comparing TLM solutions to differences between results from a nonlinear model identi cally perturbed. The TLM solutions are found to be highly sensitive to the amplitude of the applied perturbation, and thus the linear approx imation is valid only for a specific range of perturbations. The TLM f ails to describe the evolution of perturbations when the initial varia tion is given on a parameter in the vicinity of a nonlinear regime cha nge, a result that has important implications for many cloud-scale pro cesses. Uncertainty imposed on certain aspects of microphysical proces ses can have a significant influence on the behavior of the TLM soluti ons, and in some cases this behavior can be explained by the particula r discretizations used to solve the equations. The frequency at which the nonlinear basic state is updated in the TLM can also have a profou nd effect on the TLM validity, though this sensitivity is in some case s modulated by the numerical scheme and model configuration used.