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1994 .

(6 publications)

H. Le Treut, Z. X. Li, and M. Forichon. Sensitivity of the LMD General Circulation Model to Greenhouse Forcing Associated with Two Different Cloud Water Parameterizations. Journal of Climate, 7:1827-1841, December 1994. [ bib | DOI | ADS link ]

The atmospheric general circulation model of the Laboratoire de Météorologic Dynamique is coupled to a slab ocean model and is used to investigate the climatic impact of a C02 doubling. Two versions of the model are used with two different representations of the cloud-radiation interaction. Both of them contain a prognostic equation for the cloud liquid water content, but they differ in the treatment of the precipitation mechanism. The annual and global mean of the surface warming is similar in the two experiments in spite of regional differences. To understand the behavior of the model versions, the total climate change is split into a direct C02 forcing and different feedback effects (water vapor. cloud, and surface albedo). The results show that, in the second model version, the cloud feedback decreases significantly, especially at high latitudes, due to an increase of low-level clouds in the 2×C02 simulation. The modification of the cloud scheme influences also the water vapor variation and the associated feedback is reduced, in particular, over the subtropical regions. The surface albedo feedback is increased. This is due to the fact that the cloudiness is smaller over high latitudes and the surface snow is more directly exposed to incoming radiation. Although the results are qualitatively similar to the results obtained with other models, the occurrence of such compensations between different feedback mechanisms leads to a different evaluation of the overall climate sensitivity.

Z. X. Li and D. Gambis. Relationship between the astrometric z-term, the Earth rotation and the southern oscillation index. Astronomy Astrophysics, 290:1001-1008, October 1994. [ bib | ADS link ]

The z-term series derived from astronomical observations is related to plumb-line deflection. Its analysis over the interval 1962-1982 reveals interannual variations with amplitudes of about 0.01”. These variations appear to be significantly correlated with both the length-of-day variations and the Southern Oscillation Index characterizing El Nino events, and this with a time lag of about 12 months. This z-term series appears to be of great interest in the study of relationships between Earth rotation and geophysical process like in particular oceano-atmospheric phenomena.

J. Polcher and K. Laval. A statistical study of the regional impact of deforestation on climate in the LMD GCM. Climate Dynamics, 10:205-219, September 1994. [ bib | DOI | ADS link ]

The present study uses the general circulation model of the Laboratoire de Météorologie Dynamique (LMD-GCM) coupled to the land-surface, vegetation model SECHIBA. The impact of deforestation on climate is discussed. Replacing tropical forests by degraded pastures changes albedo, the roughness length and the hydrological properties of the surface. The experiment was carried out over eleven years using the observed sea surface temperature from 1978 to 1988, which includes two major El Niño events. The discussion of the results in this study is limited to the regional impact of deforestation. The changes found for the surface fluxes in Amazonia, Africa and Indonesia are examined in detail and compared in order to understand the impact on temperature. Special attention is paid to feedback mechanisms which compensate for the surface changes and to the statistical significance of these results within the tropical variability of climate. It is shown that the relatively small regional impact of deforestation in this study is statistically significant and largely independent of the El Niño-Southern Oscillation phenomenon.

S. Bony and J.-P. Duvel. Influence of the vertical structure of the atmosphere on the seasonal variation of precipitable water and greenhouse effect. Journal of Geophysical Research, 99:12963, June 1994. [ bib | DOI | ADS link ]

By using satellite observations and European Centre for Medium Range Weather Forecasts analyses, we study the seasonal variations of the precipitable water and the greenhouse effect, defined as the normalized difference between the longwave flux emitted at the surface and that emergent at the top of the atmosphere. Results show a strong systematic influence of the vertical structure of the atmosphere on geographical and seasonal variations of both precipitable water and greenhouse effect. Over ocean, in middle and high latitudes, the seasonal variation of the mean temperature lapse rate in the troposphere leads to large seasonal phase lags between greenhouse effect and precipitable water. By contrast, the seasonal variation of the clear-sky greenhouse effect over tropical oceans is mainly driven by the total atmospheric transmittance and thus by precipitable water variations. Over land, the seasonal variation of the tropospheric lapse rate acts to amplify the radiative impact of water vapor changes, giving a strong seasonal variation of the greenhouse effect. Over tropical land regions, monsoon activity generates a seasonal phase lag between surface temperature and relative humidity variations that gives a seasonal lag of about 2 months between the surface temperature and the clear-sky greenhouse effect. Generally, the cloudiness amplifies clear-sky tendencies. Finally, as an illustration, obtained results are used to evaluate the general circulation model of the Laboratoire de Météorologie Dynamique.

P.-Y. Deschamps, F.-M. Breon, M. Leroy, A. Podaire, A. Bricaud, J.-C. Buriez, and G. Seze. The POLDER mission: instrument characteristics and scientific objectives. IEEE Transactions on Geoscience and Remote Sensing, 32:598-615, May 1994. [ bib | DOI | ADS link ]

A. Chedin, N. A. Scott, C. Claud, B. Bonnet, J. Escobar, S. Dardaillon, F. Cheruy, and N. Husson. Global scale observation of the earth for climate studies. Advances in Space Research, 14:155-159, January 1994. [ bib | DOI | ADS link ]

Developed since 1983 at LMD, the 3I (Improved Initialization Inversion) physical retrieval algorithm has been recently extended to the processing of NOAA (TIROS-N Operational Vertical Sounder) observations at global scale. Starting from the version implemented at ECMWF in Reading, this global scheme has been recently improved and installed on a CRAY-2. One month of observations (Feb. 1989) of NOAA-10 and 11 has recently been processed, at a spatial resolution of 100 × 100 km2. A two years period should now be processed, in conjunction with the PathFinder and GEWEX-GVaP programmes. Results expected are: weekly to monthly averages of quantities like temperature structure, cloud parameters or the vertical distribution and total content of water vapor analysed in relationship to pertinent meteorological or other parameters, especially with respect to quantifying the fundamental characteristics and origines of water vapor variability.

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