Convection in Lorenz’s global energy cycle with the ECMWF model

Lorenz’s global energy cycle includes the conversion rate C between available potential and kinetic energy. In traditional estimates of C only gridscale processes were evaluated; subgridscale processes were lumped into dissipation. It is argued that this is inadequate; organized subgridscale heat fluxes like deep convection cannot be treated as molecular.

Here both C^grid and C^sub are evaluated from the ECMWF Integrated Forecast System, for a 1-yr forecast in climate mode. The subgridscale fluxes are obtained from the model parametrization and the results tested for consistency; the largest contribution comes from the convection scheme. The integrand of C^sub, the familiar ‘buoyancy flux’ –̅αʹ̅ωʹ, is locally much smaller than its gridscale counterpart –̅α̅ω. However, the buoyancy flux is upward throughout, and thus representative for, the global atmosphere. The global annual means are C^grid = (3.4 ± 0.1) W m⁻² and C^sub = (1.7 ± 0.1) W m⁻². Further, the gridscale generation rate of available potential energy is evaluated independently and found to be G^grid = (3.0 ± 0.2) W m⁻².

These results suggest that (i) the subgridscale processes contribute significantly to the Lorenz energy cycle and (ii) the cycle, represented by the total dissipation of D = (5.1 ± 0.2) W m⁻², is more intense than all earlier gridscale estimates have indicated.