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1995 .

(3 publications)

F. Hourdin, O. Talagrand, R. Sadourny, R. Courtin, D. Gautier, and C. P. Mckay. Numerical simulation of the general circulation of the atmosphere of Titan. Icarus, 117:358-374, October 1995. [ bib | DOI | ADS link ]

The atmospheric circulation of Titan is investigated with a general circulation model. The representation of the large-scale dynamics is based on a grid point model developed and used at Laboratoire de Météorologie Dynamique for climate studies. The code also includes an accurate representation of radiative heating and cooling by molecular gases and haze as well as a parametrization of the vertical turbulent mixing of momentum and potential temperature. Long-term simulations of the atmospheric circulation are presented. Starting from a state of rest, the model spontaneously produces a strong superrotation with prograde equatorial winds (i.e., in the same sense as the assumed rotation of the solid body) increasing from the surface to reach 100 m sec -1 near the 1-mbar pressure level. Those equatorial winds are in very good agreement with some indirect observations, especially those of the 1989 occultation of Star 28-Sgr by Titan. On the other hand, the model simulates latitudinal temperature contrasts in the stratosphere that are significantly weaker than those observed by Voyager 1 which, we suggest, may be partly due to the nonrepresentation of the spatial and temporal variations of the abundances of molecular species and haze. We present diagnostics of the simulated atmospheric circulation underlying the importance of the seasonal cycle and a tentative explanation for the creation and maintenance of the atmospheric superrotation based on a careful angular momentum budget.

A. Harzallah and R. Sadourny. Internal Versus SST-Forced Atmospheric Variability as Simulated by an Atmospheric General Circulation Model. Journal of Climate, 8:474-495, March 1995. [ bib | DOI | ADS link ]

The variability of atmospheric flow is analyzed by separating it into an internal part due to atmospheric dynamics only and an external (or forced) part due to the variability of sea surface temperature forcing. The two modes of variability are identified by performing an ensemble of seven independent long-term simulations of the atmospheric response to observed SST (1970-1988) with the LMD atmospheric general circulation model. The forced variability is defined from the analysis of the ensemble mean and the internal variability from the analysis of deviations from the ensemble mean. Emphasis is put on interannual variability of sea level pressure and 5OO-hPa geopotential height for the Northern Hemisphere winter. In view of the large systematic errors related to the relatively small number of realizations, unbiased variance estimators have been developed. Although statistical significance is not reached in some extratropical regions, large significant extratropical responses are found at the North Pacific-Alaska sector for SLP and over western Canada and the Aleutians for 5OO-hPa geopotential height. The influence of SST variations on internal variability is also examined by using a 7-year simulation using the climatological SST seasonal cycle. It is found that interannual SST changes strongly influence the geographical distribution of internal variability; in particular, it tends to increase it over oceans. Patterns of internal and external variability of the 5OO-hPa geopotential height are further examined by using EOF decompositions showing that the model realistically simulates the leading observed variability modes. The geographical structure of internal variability patterns is found to be similar to that of total variability, although similar modes tend to evolve rather differently in time. The zonally symmetric seesaw dominates the internal variability for both observed and climatologically prescribed SST. The Pacific-North American (PNA) and Western Pacific (WP) patterns, on the other hand, are the dominant modes associated with patterns of SST variability: the latter is related to Atlantic anomalies, while the former responds to both El Niño events and extratropical forcing.

R. C. Raghava, K. Laval, R. Sadourny, and J. Polcher. Atmospheric response to tropical denuding of vegetation. Atmospheric Environment, 29:1963-2000, 1995. [ bib | DOI | ADS link ]

Two simulations of atmospheric circulations during June, July and August 1988 have been made with LMD Atmospheric General Circulation Model using a classified vegetation global cover with and without the tropical vegetation separately. The initial conditions prepared from ECMWF analysed data were used, while the Reynolds' monthly blended analysis, i.e., the blend of in situ, AVHRR satellite and ice data, were taken to prescibe the sea surface temperatures. The global charts of mean monthly precipitation and associated velocity potentials at 200 and 850 mb have been compared and analysed for June, July and August 1988. The temporal evolutions of precipitation averaged over a specific region of Indian summer monsoon during its regime from onset to retreat have also been discussed. Consequently, a pronounced impact of tropical vegetation on the precipitation has been observed so as to characterise a forest as one of the local rain inducing agents. Moreover, the tropical vegetation appears to modulate the Indian summer monsoon also for the contributive precipitation over India.

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