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lmd_Li1998_abstracts.html

1998 .

(4 publications)

F. Chevallier, F. Chéruy, N. A. Scott, and A. Chédin. A Neural Network Approach for a Fast and Accurate Computation of a Longwave Radiative Budget. Journal of Applied Meteorology, 37:1385-1397, November 1998. [ bib | DOI | ADS link ]

The authors have investigated the possibility of elaborating a new generation of radiative transfer models for climate studies based on the neural network technique. The authors show that their neural network-based model, NeuroFlux, can be used successfully for accurately deriving the longwave radiative budget from the top of the atmosphere to the surface. The reliable sampling of the earth's atmospheric situations in the new version of the TIGR (Thermodynamic Initial Guess Retrieval) dataset, developed at the Laboratoire de Météorologie Dynamique, allows for an efficient learning of the neural networks. Two radiative transfer models are applied to the computation of the radiative part of the dataset: a line-by-line model and a band model. These results have been used to infer the parameters of two neural network-based radiative transfer codes. Both of them achieve an accuracy comparable to, if not better than, the current general circulation model radiative transfer codes, and they are much faster. The dramatic saving of computing time based on the neural network technique (22 times faster compared with the band model), 106 times faster compared with the line-by-line model, allows for an improved estimation of the longwave radiative properties of the atmosphere in general circulation model simulations.

H. Le Treut, M. Forichon, O. Boucher, and Z.-X. Li. Sulfate Aerosol Indirect Effect and CO2 Greenhouse Forcing: EquilibriumResponse of the LMD GCM and Associated Cloud Feedbacks. Journal of Climate, 11:1673-1684, July 1998. [ bib | DOI | ADS link ]

The climate sensitivity to various forcings, and in particular to changes in CO2 and sulfate aerosol concentrations, imposed separately or in a combined manner, is studied with an atmospheric general circulation model coupled to a simple slab oceanic model. The atmospheric model includes a rather detailed treatment of warm cloud microphysics and takes the aerosol indirect effects into account explicitly, although in a simplified manner. The structure of the model response appears to be organized at a global scale, with a partial independence from the geographical structure of the forcing. Atmospheric and surface feedbacks are likely to explain this feature. In particular the cloud feedbacks play a very similar role in the CO2 and aerosol experiments, but with opposite sign. These results strengthen the idea, already apparent from other studies, that, in spite of their different nature and their different geographical and vertical distributions, aerosol may have substantially counteracted the climate effect of greenhouse gases, at least in the Northern Hemisphere, during the twentieth century. When the effects of the two forcings are added, the model response is not symmetric between the two hemispheres. This feature is also consistent with the findings of other modeling groups and has implications for the detection of future climate changes.

P. Barthelet, S. Bony, P. Braconnot, A. Braun, D. Cariolle, E. Cohen-Solal, J.-L. Dufresne, P. Delecluse, M. Déqué, L. Fairhead, M.-A. Filiberti, M. Forichon, J.-Y. Grandpeix, E. Guilyardi, M.-N. Hqussais, M. Imbard, H. Le Treut, C. Lévy, Z. Xin Li, G. Madec, P. Marquet, O. Marti, S. Planton, L. Terray, O. Thual, and S. Valcke. Simulations couplées globales des changements climatiques associés à une augmentation de la teneur atmosphérique en CO 2. Comptes Rendus de l'Académie des Sciences - Series IIA - Earth and Planetary Science, 326:677-684, May 1998. [ bib | DOI | ADS link ]

Two transient CO 2 experiments using two coupled general circulation models developed by the French GASTON group have been realized using the same methodology. No flux corrections at the air-sea interface were used in these experiments. The main features of the present climate are reasonably well captured by both coupled models in the control simulations, although the biases are not the same, The transient CO 2 simulations show a global warming, ranging between 1.6 and 2.0 degC at the time of CO 2 doubling (+ 70 years). These values, and the main geographical characteristics of climate change, are in agreement with previous studies published by other research groups, using either flux corrected or non-flux corrected models.

Z. X. Li. Measurements of interannual variation of the vertical at Jozefoslaw by astrometric and gravimetric observations. , 129:353-355, April 1998. [ bib | DOI | ADS link ]

Jozefoslaw astronomical and geodetical observatory at Warsaw is the place where parallel observations of astrometric latitude (since 1959) and meridional plumb line variations from gravimetric methods (since 1976) have been carried out continuously over the past 20 years. The observational data at this observatory have been analysed to confirm the reality of the plumb line variation results derived from the astrometric latitude residuals. Cross correlation analyses between the results of the two techniques have demonstrated that the interannual plumb line variations along the meridian detected by the astrometric technique is in good accordance with those from the gravimetric technique. The results shown in the paper can be considered as evidence of the existence of non-tidal plumb line variations at interannual time scales, of which the scale is about 0.02” in the case of Jozefoslaw observatory, and the possibility in measuring them by astrometric techniques.

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