Comparative terrestrial atmospheric circulation regimes in simplified global circulation models: II. energy budgets and spectral transfers

Abstract

The energetics of possible global atmospheric circulation patterns in an Earth-like atmosphere are explored using a simplified GCM based on the University of Hamburg's Portable University Model for the Atmosphere. Results from a series of simulations, obtained by varying planetary rotation rate {\Omega} with an imposed equator-to-pole temperature difference, were analysed to determine the heat transport and other contributions to the energy budget for the time-averaged, equilibrated flow. These show clear trends with {\Omega}, with the most intense Lorenz energy cycle for an Earth-sized planet occurring with a rotation rate around half that of the present day Earth. KE and APE spectra, E_K(n) and E_A(n) (where n is total spherical wavenumber), also show clear trends with \Omega, with n^{-3} enstrophy-dominated spectra around \Omega* = \Omega/\Omega_E = 1, where \Omega_E is the rotation rate of the Earth) and steeper (\sim n^{-5}) slopes in the zonal mean flow with little evidence for the n^{-5/3} spectrum anticipated for an inverse KE cascade. Instead, both KE and APE spectra become almost flat at scales larger than the internal Rossby radius, L_d, and exhibit near-equipartition at high wavenumbers. At \Omega* << 1, the spectrum becomes dominated by KE with E_K(n) \sim 2-3 E_A(n) at most wavenumbers and a slope \sim n^{-5/3} across most of the spectrum. Spectral flux calculations show that enstrophy and APE are almost always cascaded downscale, regardless of {\Omega}. KE cascades are more complicated, however, with downscale transfers across almost all wavenumbers, dominated by horizontally divergent modes, for \Omega* \lesssim 1/4. At higher rotation rates, transfers of KE become increasingly dominated by rotational components with strong upscale transfers (dominated by eddy-zonal flow interactions) for scales larger than L_d and weaker downscale transfers for scales smaller than L_d.