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1992 .

(4 publications)

F. Hourdin. A new representation of the absorption by the CO2 15-microns band for a Martian general circulation model. Journal of Geophysical Research, 97:18319, November 1992. [ bib | DOI | ADS link ]

A model for absorption by the CO2 15-microns band has been adapted from the Morcrette et al. (1986) terrestrial wide-band model for use in a general circulation model of the Martian atmosphere. The absorption model is validated by comparison with exact line-by-line integrations for a set of atmospheric profiles characteristic of Martian conditions. The Doppler effect is included in a simple way with no significant increase of the computational cost. The model gives accurate results up to 80 km. The Doppler effect, in fact, is shown to be significant only above 50 km for mean Martian conditions. The transmissivities of the wide-band model are fitted to the results of a more accurate statistical narrow-band model. Various formulations of the statistical band model, including accurate representation of the Doppler effect (Fels, 1979; Zhu, 1989) are validated by comparison with line-by-line results.

S. Bony, H. Le Treut, J.-P. Duvel, and R. S. Kandel. Satellite validation of GCM-simulated annual cycle of the Earth Radiation Budget and cloud forcing. Journal of Geophysical Research, 97:18061--18081, November 1992. [ bib | DOI | ADS link ]

Earth Radiation Budget Experiment (ERBE) data are used to validate radiative fluxes and cloud radiative forcing (CRF) simulated by the Laboratoire de Météorologie Dynamique (LMD) general circulation model (GCM). The emphasis of the work is on the development of new tests to obtain more significant elements of comparison between model simulations and satellite observations. These tests are applied to the clear-sky fluxes and the cloud radiative forcing. The validation of the CRF described by a model requires to test the consistency between the solar or shortwave (SW: 0.2 to 5 μm) and longwave (LW: 5 to 50 μm) cloud forcing. For this purpose, we compute the mean cloud perturbation of the planetary albedo as a function of the LW cloud forcing for both model results and ERBE observations. In the SW spectral domain, the consideration of total fluxes does not provide very constraining elements of validation because most of the observed variations are prescribed (incoming solar radiation, solar zenith angle). We therefore distinguish the part of the SW seasonal variations related only to the variation of external parameters (mainly the insolation) from the part which arises from the combined variation of internal climate parameters (mainly cloud albedo and snow/ice cover) with the insolation. Fourier analysis is used to study the seasonal amplitude and phase of the CRF. The seasonal variation of the cloudiness is, respectively, out of phase (in phase) with the insolation in mid-latitudes (in low and high latitudes). We show that this acts to enhance (to reduce) the seasonal amplitude of the absorbed SW flux in mid-latitudes (in low and high latitudes). Finally, we show that the impact of the seasonal variation of the cloudiness on the variation of the net CRF is less than 10 W m-2.

Z.-X. Li and H. Le Treut. Cloud-radiation feedbacks in a general circulation model and their dependence on cloud modelling assumptions. Climate Dynamics, 7:133--139, April 1992. [ bib | DOI | ADS link ]

The general circulation model (GCM) used in this study includes a prognostic cloud scheme and a rather detailed radiation scheme. In a preceding paper, we showed that this model was more sensitive to a global perturbation of the sea surface temperatures than most other models with similar physical parametrization. The experiments presented here show how this feature might depend on some of the cloud modelling assumptions. We have changed the temperature at which the water clouds are allowed to become ice clouds and analyzed separately the feedbacks associated with the variations of cloud cover and cloud radiative properties. We show that the feedback effect associated with cloud radiative properties is positive in one case and negative in the other. This can be explained by the elementary cloud radiative forcing and has implications concerning the use of the GCMs for climate sensitivity studies.

D. A. Randall, R. D. Cess, J. P. Blanchet, G. J. Boer, D. A. Dazlich, A. D. Del Genio, M. Deque, V. Dymnikov, V. Galin, S. J. Ghan, A. A. Lacis, H. Le Treut, Z.-X. Li, X.-Z. Liang, B. J. McAvaney, V. P. Meleshko, J. F. B. Mitchell, J.-J. Morcrette, G. L. Potter, L. Rikus, E. Roeckner, J. F. Royer, U. Schlese, D. A. Sheinin, J. Slingo, A. P. Sokolov, K. E. Taylor, W. M. Washington, R. T. Wetherald, I. Yagai, and M.-H. Zhang. Intercomparison and Interpretation of Surface Energy Fluxes in Atmospheric General Circulation Models. Journal of Geophysical Research, 97:3711--3724, March 1992. [ bib | DOI | ADS link ]

We have analyzed responses of the surface energy budgets and hydrologic cycles of 19 atmospheric general circulation models to an imposed, globally uniform sea surface temperature perturbation of 4 K. The responses of the simulated surface energy budgets are extremely diverse and are closely linked to the responses of the simulated hydrologic cycles. The response of the net surface energy flux is not controlled by cloud effects; instead, it is determined primarily by the response of the latent heat flux. The prescribed warming of the oceans leads to major increases in the atmospheric water vapor content and the rates of evaporation and precipitation. The increased water vapor amount drastically increases the downwelling infrared radiation at the Earth's surface, but the amount of the change varies dramatically from one model to another.

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