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1981 .

(4 publications)

C. Basdevant, B. Legras, R. Sadourny, and M. Béland. A Study of Barotropic Model Flows: Intermittency, Waves and Predictability. Journal of Atmospheric Sciences, 38:2305-2326, November 1981. [ bib | DOI | ADS link ]

The régime flows corresponding to the barotropic nondivergent equation with forcing, drag and subgrid-scale dissipation are studied using spectral model on the plane and on the sphere. The flow régimes obtained exhibit clear evidence of the existence of an enstrophy-cascading inertial range, together with a reverse energy cascade toward small wavenumbers. It is shown, however, that the enstrophy cascade is not associated with the k3 spectral slope expected from the Kolmogorov-Kraichnan theory of two-dimensional turbulence; the slopes obtained are significantly steeper. This apparent paradox is tentatively resolved by a phenomenological theory of space-time intermittency in two dimensions; it is further shown that such intermittency associated with steeper spectra also restores locality of the nonlinear transfers in wavenumber space. In contrast to the well-known nonlocality typical of two-dimensional non-intermittent turbulent flows. The effect of differential rotation in connection with Rossby wave propagation is also studied: the reverse energy cascade is actually inhibited, and zonal anisotropy prevails in the large scales as expected from Rhines' theory. But it is shown that this anisotropy is in fact carried down by nonlinearity throughout the enstrophy inertial range. Finally, the predictability properties of our flows are investigated with reference to the Leith-Kraichnan theory. It is shown that the presence of Rossby waves actually increases predictability through several mechanisms: direct inhibition of the nonlinear transfers in the larger scales, concentration of energy in highly predictably large-scale zonal structures, and slowdown of error propagation in the enstrophy inertial range due to the presence of anisotropy at small and intermediate scales.

R. Sadourny and C. Basdevant. A class of operators for modelling two-dimensional turbulent diffusion. Academie des Sciences Paris Comptes Rendus Serie Sciences Mathematiques, 292:1061-1064, April 1981. [ bib | ADS link ]

The problem of defining turbulent diffusion for numerical modelling of two-dimensional flows is considered. Numerical models, limited to a finite number of degrees of freedom, do not represent interactions between explicit scales and scales too small to be resolved, which play an essential role in cascade processes. These interactions must be modelled as turbulent diffusion operators in addition to the explicit interactions. The proposed model resembles prediction-correction methods used in numerical analysis and amounts to calculating vortex advection with a priori small scale vortex modification as a function of vortex movement in the neighborhood of the point considered.

K. Laval, R. Sadourny, and Y. Serafini. Land surface processes in a simplified general circulation model. Geophysical and Astrophysical Fluid Dynamics, 17:129-150, 1981. [ bib | DOI | ADS link ]

The land surface processes as parameterized for the current version of the L.M.D. General Circulation Model are described. The model predicts ground temperature for bare soil, ice and snow; the treatment of ground hydrology involves a prediction of soil moisture and snow depth. The parameterization is tested on a 90-day integration using a sectorial model with artificial modelling of continents and orography; sea surface temperature, surface albedo and ice cover are given assigned values based on climatological data for January. The resulting distributions of hydrological and thermodynamic variables at the Earth's surface are discussed.

K. Laval, H. Le Treut, and R. Sadourny. Effect of cumulus parameterization on the dynamics of a general circulation model. Geophysical and Astrophysical Fluid Dynamics, 17:113-127, 1981. [ bib | DOI | ADS link ]

The purpose of this study is to test a modification of the parameterization of convection in a general circulation model. The analysis is done with a sectorial model. Its resolution is 11 levels and 1625 grid points. In version A of the model, we use a moist convective adjustment (M.C.A.) wherever the air is conditionally unstable and saturated; in version B, we add a convective scheme to M.C.A. in the case of conditionally unstable but not saturated air. This last scheme is based on the parameterization of Kuo (1965). We compare zonal means and energy cycles of the two versions; improvements in version B seem substantial, essentially in latitude-height distribution of energy variables.

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