Concerning the effect of surface drag on the circulation of a baroclinic planetary atmosphere

Abstract

Extended runs of a multi‐level spectral numerical model of atmospheric flow on a hemisphere have been used to investigate the effect of surface drag variations on the nature of the time‐averaged flow. It is shown that the most drastic effect of reducing the surface drag is to permit strong horizontal, barotropic shears in the zonal mean flow, which inhibit baroclinic instability. This superficially paradoxical result is implicit in many earlier calculations, most clearly in the lifecycle description of nonlinear baroclinic instability, but appears not to be widely appreciated. The kinetic energy spectra are interpreted in terms of the wavenumber of the most baroclinically unstable normal modes KR, the Rhines blocking wavenumber Kβ, and a limiting wavenumber based on surface drag KD. The changes in KR and the smaller changes in Kβ as drag is reduced account satisfactorily for the changes found in the kinetic energy spectra.

Barotropic Impacts of Surface Friction on Eddy Kinetic Energy and Momentum Fluxes: An Alternative to the Barotropic Governor

“…At first glance, these results appear to directly support the barotropic governor mechanism, with increased drag causing the zonal mean winds to decrease, decreasing the barotropic shear and thus increasing the baroclinicity. However, one aspect of the response does not fit with previous work on the barotropic governor: globally averaged eddy kinetic energy decreases with increased drag [opposite to the increase found by James and Gray (1986)]. We therefore seek an alternative explanation for these effects of drag on the eddy fluxes.…”

“…Changes in heat flux therefore are likely not responsible for the lack of change of EKE. Many previous authors have used globally averaged EKE to diagnose the presence of the barotropic governor feedback between the eddies and the mean shear (James and Gray 1986;Robinson 1997;Chen et al 2007b), so the effect of drag on the EKE in the GEOS-5 model integrations suggests that perhaps a mechanism exists other than the conventional barotropic governor that explains this response of the eddies to drag.…”

“…This response is opposite to what might be predicted by the barotropic governor mechanism. Furthermore, the globally averaged EKE defined as done by James and Gray (1986) decreases with increased drag from 5.65 3 10 5 to 5.63 3 10 5 J m 22 . One might think that the mid-to-lower tropospheric heat flux might explain the changes in EKE in GEOS-5.…”

“…James and Gray (1986) and James (1987) first showed that stronger meridional shears in the zonal mean flow can actually reduce the conversion from potential energy to eddy kinetic energy (baroclinic eddy growth) by restricting the structure of the normal modes. This effect was termed the ''barotropic governor'' by James (1987), whereby increased barotropic shear modifies the baroclinic growth rates and thus reduces the eddy kinetic energy.…”

Experiments on the structure of baroclinic waves and zonal jets in an internally heated, rotating, cylinder of fluid

“…He claims that the lateral scale of the Jovian and Saturnian zonal jets ͑and even that of the westerly jet seen in each hemisphere of the terrestrial atmosphere͒ is related to the Rhines scale. James and Gray 9 have argued, however, that the Williams model is only appropriate in situations where there is a large inertial range ͑for the inverse energy cascade͒ separating the baroclinic energy conversion scale ͑Rossby deformation radius͒ and L ␤ , which is not the case in the terrestrial atmosphere. 7 More recently Cho and Polvani 10 have performed a series of numerical simulations of freelyevolving shallow-water turbulence on a sphere, and found that although the number of jets increased with rotation rate as predicted by Rhines, the L ␤ scale could not be used as a quantitative predictor of that number.…”