Adiabatic and Viscous Simulations of Symmetric Instability: Structure, Evolution, and Energetics

Abstract

This paper reports on simulations of dry-adiabatic and viscous symmetric instability circulations with a two-dimensional research version of the French Weather Service Limited Area Model PERIDOT. The simulations of an idealized two-dimensional case of a dry symmetric instability presented here constitute a preliminary work to future real case studies. As a first step, the capacity of the model to simulate circulations of the dry inviscid symmetric instability that are in agreement with the linear hydrostatic theory has been verified. This has allowed the design and verification of diagnostic tools. In order to use these diagnostics on simulations of real data cases, a spectral method has been developed to separate the basic state from the symmetric instability circulations. For all of our simulations, the energetics of the circulation show that the circulations draw their kinetic energy mainly from the term linked to the basic vertical shear and the correlation of the vertical and longitudinal components of the motion of the perturbation. Adequate horizontal and vertical resolution and diffusion formulation are found to be necessary to simulate a reasonable numerical evolution of the symmetric instability circulations. In particular, the nonlinear evolution of the dry viscous symmetric instability circulations has been studied by the inclusion of a realistic turbulent diffusion: a complete life cycle is obtained, and a quasi neutrality is reached at the end of the integrations. These simulations provide a first approach to the adjustment problem (i.e., how can the neutrality with respect to symmetric instability, which is frequently observed in real bands, be reached?) and of its representation in a numerical model.