Large‐scale non‐turbulent dynamics in the atmosphere

Abstract

AbstractClassical two‐dimensional turbulence theory is often used to understand large‐scale atmospheric flows. However, the equations governing classical two‐dimensional turbulence can only be derived from the governing equations by assuming that the horizontal scale is much smaller than the deformation radius, which is the scale on which baroclinic instability takes place. Typically, the large‐scale quasi‐two‐dimensional disturbances have the vertical depth‐scale of the troposphere and are of larger scale than the baroclinic waves which maintain them. It is therefore more appropriate to study the quasi‐two‐dimensional disturbances with a model appropriate to scales larger than the deformation radius. The semi‐geostrophic model is an accurate model in this regime. It is shown that it does not permit the enstrophy cascades associated with classical two‐dimensional turbulence. This agrees with other results in the literature suggesting that two‐dimensional flow on scales larger than the deformation radius is essentially non‐turbulent. The observed quasi‐permanently unsteady behaviour of the atmosphere thus represents the natural internal dynamics, and does not require explanation by anisotropic forcing. In the ocean, on the other hand, the external deformation radius, which governs the behaviour of two‐dimensional eddies, is much larger than the internal deformation radius, which determines the scale of baroclinic development. It may thus be appropriate to use two‐dimensional turbulence as a model for scales between the internal and external deformation radius. Copyright © 2002 Royal Meteorological Society.