Consistent diffusion and dissipation in a GCM

Dissipative heating due to horizontal and vertical diffusion is commonly neglected in GCMs. Inclusion of dissipative heating in a hydrodynamically consistent way has been shown to reduce spurious thermal forcing in a simple GCM and to be consistent with estimates of the Lorenz energy clycle. Also the climate simulated by a GCM can be affected in a statistically significant way. Dissipative heating warms the area around the tropopause and changes the strength of the residual circulation slightly. The implementation of such a consistent scheme requires the replacement of the hyperdiffusion with a harmonic scheme. Therefore a nonlinear harmonic diffusion scheme has been developed which allows for a consistent dissipation while damping more selectively in time and space. This scheme is based on the ideas of Smagorinsky. Using the nonlinear versus the linear diffusion changes the patterns of internal variability. The amount of dissipation due to horizontal diffusion decreases over Europe while dissipation due to vertical diffusion increases, indicating an enhanced level of resolved gravity-wave activity. A modification of the nonlinear scheme is introduced which enhances the diffusion in case of strong divergence and allows the model to simulate a n-3 slope of the energy spectrum also for the highest resolved wavenumbers.