Monday 29 Feb 2016: Horizontal and vertical spectra from the UM, and the effects of stratified turbulence
Prof. Mike Cullen - Met Office
The dynamical behaviour of the Unified Model is studied using the scale dependence of the Rossby and Froude numbers, together with the degree of imbalance. Diagnostics are obtained separately for the tropics and extratropics. These show the expected regime of quasi-geostrophic turbulence on scales larger than 1000km. However, there is little sign of the expected flattening of the spectrum at smaller scales. However, the continued increase of the Froude number as the scale reduces is indicative of the transition to stratified turbulence, rather than to layered two-dimensional turbulence. This would be consistent with the observed -5/3 energy spectrum. Detailed examination of the flow around the tropopause shows large Froude numbers and some turbulence. The use of vertical diffusion to model the turbulence in this region does not seem beneficial. There are also large imbalances in the stratosphere. The main contributors are a lack of small scale rotational motion and vertical propagation of orographic gravity waves. At the highest vertical resolution the vertical propagation of gravity waves is restricted by wave breaking, which may be a useful benefit of increased vertical resolution.