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- Author or Editor: R. Sadourny x
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Abstract
The effect of potential enstrophy conservation in finite-difference model flows on the stability of large- scale Rossby waves is investigated. It is also shown that cylindrical coordinates can be used successfully in global numerical experiments. Coherent boundary conditions at the poles are derived and tested on Rossby waves allowing cross-polar flow.
Abstract
The effect of potential enstrophy conservation in finite-difference model flows on the stability of large- scale Rossby waves is investigated. It is also shown that cylindrical coordinates can be used successfully in global numerical experiments. Coherent boundary conditions at the poles are derived and tested on Rossby waves allowing cross-polar flow.
Abstract
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 k −3 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.
Abstract
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 k −3 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.
Abstract
Results from 90-day simulations with the LMD GCM are described, where sea surface temperatures of 1987 or 1988 years are respectively prescribed. The initial states correspond to 1 June 1987 and 1 June 1988. The simulated precipitation rates over India show a strong contrast between the two years, with drought occurring during summer 1987 and abundant rainfall during summer 1988. The dry regime simulated during 1987 corresponds to an eastward displacement of the outflow at 200 mb and a weaker westerly flow at the surface as compared with 1988, both features being in agreement with reality. Because it is more difficult for models to simulate rainfall differences than to simulate wind variations between the two years, the changes in simulated rainfall over India are studied in more detail. In particular, more integrations are carried out to test the sensitivity of rainfall variations to initial conditions, and the result is that the decrease of rainfall in 1987 compared to 1988 is a robust feature of the model.
Very early, the importance of evapotranspiration in simulating land rainfall was emphasized. Additional integrations are performed in order to study the impact of the new vegetation scheme introduced in the LMD GCM. It is shown that the contrast in rainfall between the two years is better simulated when the evapotranspiration rate of vegetation cover is represented. When vegetation is not represented in the model, the model does not simulate accurately the interannual variation of the precipitation rates.
Abstract
Results from 90-day simulations with the LMD GCM are described, where sea surface temperatures of 1987 or 1988 years are respectively prescribed. The initial states correspond to 1 June 1987 and 1 June 1988. The simulated precipitation rates over India show a strong contrast between the two years, with drought occurring during summer 1987 and abundant rainfall during summer 1988. The dry regime simulated during 1987 corresponds to an eastward displacement of the outflow at 200 mb and a weaker westerly flow at the surface as compared with 1988, both features being in agreement with reality. Because it is more difficult for models to simulate rainfall differences than to simulate wind variations between the two years, the changes in simulated rainfall over India are studied in more detail. In particular, more integrations are carried out to test the sensitivity of rainfall variations to initial conditions, and the result is that the decrease of rainfall in 1987 compared to 1988 is a robust feature of the model.
Very early, the importance of evapotranspiration in simulating land rainfall was emphasized. Additional integrations are performed in order to study the impact of the new vegetation scheme introduced in the LMD GCM. It is shown that the contrast in rainfall between the two years is better simulated when the evapotranspiration rate of vegetation cover is represented. When vegetation is not represented in the model, the model does not simulate accurately the interannual variation of the precipitation rates.
Abstract
The sensitivity of the interannual variations of the summer monsoons to imposed cloudiness has been studied with a general circulation model using the initial conditions prepared from the European Centre for Medium-Range Forecasts analyses of 1 May 1987 and 1988. The cloud optical properties in this global model are calculated from prognostically computed cloud liquid water. The model successfully simulates the contrasting behavior of these two successive monsoons. However, when the optical properties of the observed clouds are specified in the model runs, the simulations show some degradation over India and its vicinity. The main cause of this degradation is the reduced land–sea temperature contrast resulting from the radiative effects of the observed clouds imposed in such simulations. It is argued that the high concentration of condensed water content of clouds over the Indian land areas will serve to limit heating of the land, thereby reducing the thermal contrast that gives rise to a weak Somali jet. A countermonsoon circulation is, therefore, simulated in the vector difference field of 850-hPa winds from the model runs with externally specified clouds. This countermonsoon circulation is associated with an equatorial heat source that is the response of the model to the radiative effects of the imposed clouds. Indeed, there are at least two clear points that can be made: 1) the cloud–SST patterns, together, affect the interannual variability; and 2) with both clouds and SST imposed, the model simulation is less sensitive to initial conditions. Additionally, the study emphasizes the importance of dynamically consistent clouds developing in response to the dynamical, thermal, and moist state of the atmosphere during model integrations.
Abstract
The sensitivity of the interannual variations of the summer monsoons to imposed cloudiness has been studied with a general circulation model using the initial conditions prepared from the European Centre for Medium-Range Forecasts analyses of 1 May 1987 and 1988. The cloud optical properties in this global model are calculated from prognostically computed cloud liquid water. The model successfully simulates the contrasting behavior of these two successive monsoons. However, when the optical properties of the observed clouds are specified in the model runs, the simulations show some degradation over India and its vicinity. The main cause of this degradation is the reduced land–sea temperature contrast resulting from the radiative effects of the observed clouds imposed in such simulations. It is argued that the high concentration of condensed water content of clouds over the Indian land areas will serve to limit heating of the land, thereby reducing the thermal contrast that gives rise to a weak Somali jet. A countermonsoon circulation is, therefore, simulated in the vector difference field of 850-hPa winds from the model runs with externally specified clouds. This countermonsoon circulation is associated with an equatorial heat source that is the response of the model to the radiative effects of the imposed clouds. Indeed, there are at least two clear points that can be made: 1) the cloud–SST patterns, together, affect the interannual variability; and 2) with both clouds and SST imposed, the model simulation is less sensitive to initial conditions. Additionally, the study emphasizes the importance of dynamically consistent clouds developing in response to the dynamical, thermal, and moist state of the atmosphere during model integrations.
