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Effects of large-scale energy dissipation in geostrophic turbulence

Scott, R (St Andrews)
Tuesday 09 December 2008, 10:30-11:00

Seminar Room 1, Newton Institute


We compare the distinct effects of frictional damping and radiative, or thermal, damping on the equilibration of two-dimensional geostrophic turbulence. The spatial distribution of energy in both physical and spectral space is examined with particular attention to the distribution of coherent vortices, which are found to be ubiquitous with either form of large-scale energy dissipation. Consideration of the stochastically forced vorticity equation suggests that in the case of frictional damping, maximum vorticity values depend on the damping coefficient $r$ through \qext$\sim r^{-1/2}$, while in the case of thermal damping \qext is approximately independent of damping coefficient. These are well-supported by numerical experiments. The difference between frictional and thermal damping becomes striking in simulations of forced shallow water turbulence on the sphere. While shallow-water models have been successful in reproducing the formation of robust, and fully turbulent, latitudinal jets similar to those observed on the giant planets, they have to date consistently failed to reproduce prograde (superrotating) equatorial winds. Here it is demonstrated that shallow water models not only can give rise to superrotating winds, but do so very robustly, provided that the physical process of large-scale energy dissipation by radiative relaxation (thermal damping) is taken into account. With appropriate choice of thermal damping rate, equatorial superrotation can be achieved at apparently any deformation radius.


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