The effects of closure-based eddy diffusion on the climate and spectra of a GCM

Abstract

A recently developed horizontal eddy diffusion parameterization based on closure theory has been implemented in the CSIRO atmospheric global climate model (GCM). Detailed studies of the effects of the parameterization on the simulated atmospheric circulation, transient kinetic energy and kinetic energy spectra have been carried out for different months throughout the year. These diagnostics have been compared with corresponding results from observations and from control simulations using an ad hoc diffusion parameterization also employed in earlier works with this model. The new diffusion parameterization has improved the simulated atmospheric circulation in the following ways. Zonal and total wavenumber spectra now have approximate −3 power laws for ranges of intermediate wavenumbers, as in the observations, while the control simulation spectra are considerably flatter. Peak values of transient kinetic energy near the tropospheric jet cores are considerably larger than in the control and in better agreement with observations, particularly in boreal winter. The strength and location of the tropospheric jets are also improved with the new diffusion parameterization, with monthly averaged zonal mean winds in the Northern Hemisphere and Southern Hemisphere jet cores stronger than for the control by as much as 5 m s−1 in boreal winter and spring. The GCM simulations are also stable with longer timesteps with the new eddy diffusion parameterization. It is suggested that further improvements in GCM simulations may be achievable through parameterizations of the nondiagonal elements of the covariance matrix of eddy fluxes such as the eddy-topographic force. DOI: 10.1034/j.1600-0870.2003.201329.x