1999 .

(3 publications)

S. R. Lewis, M. Collins, P. L. Read, F. Forget, F. Hourdin, R. Fournier, C. Hourdin, O. Talagrand, and J.-P. Huot. A climate database for Mars. Journal of Geophysical Research, 104:24177-24194, 1999. [ bib | DOI | ADS link ]

A database of statistics which describe the climate and surface environment of Mars has been constructed directly on the basis of output from multiannual integrations of two general circulation models developed jointly at Laboratoire de Météorologie Dynamique du Center National de la Recherche Scientifique, France, and the University of Oxford, United Kingdom, with support from the European Space Agency. The models have been developed and validated to reproduce the main features of the meteorology of Mars, as observed by past spacecraft missions. As well as the more standard statistical measures for mission design studies, the Mars Climate Database includes a novel representation of large-scale variability, using empirical eigenfunctions derived from an analysis of the full simulations, and small-scale variability using parameterizations of processes such as gravity wave propagation. The database may be used as a tool for mission planning and also provides a valuable resource for scientific studies of the Martian atmosphere. The database is described and critically compared with a representative range of currently available observations.

F. Forget, F. Hourdin, R. Fournier, C. Hourdin, O. Talagrand, M. Collins, S. R. Lewis, P. L. Read, and J.-P. Huot. Improved general circulation models of the Martian atmosphere from the surface to above 80 km. Journal of Geophysical Research, 104:24155-24176, 1999. [ bib | DOI | ADS link ]

We describe a set of two “new generation” general circulation models of the Martian atmosphere derived from the models we originally developed in the early 1990s. The two new models share the same physical parameterizations but use two complementary numerical methods to solve the atmospheric dynamic equations. The vertical resolution near the surface has been refined, and the vertical domain has been extended to above 80 km. These changes are accompanied by the inclusion of state-of-the -art parameterizations to better simulate the dynamical and physical processes near the surface (boundary layer scheme, subgrid-scale topography parameterization, etc.) and at high altitude (gravity wave drag). In addition, radiative transfer calculations and the representation of polar processes have been significantly improved. We present some examples of zonal-mean fields from simulations using the model at several seasons. One relatively novel aspect, previously introduced by Wilson [1997], is that around northern winter solstice the strong pole to pole diabatic forcing creates a quasi-global, angular-momentum conserving Hadley cell which has no terrestrial equivalent. Within such a cell the Coriolis forces accelerate the winter meridional flow toward the pole and induce a strong warming of the middle polar atmosphere down to 25 km. This winter polar warming had been observed but not properly modeled until recently. In fact, thermal inversions are generally predicted above one, and often both, poles around 60-70 km. However, the Mars middle atmosphere above 40 km is found to be very model-sensitive and thus difficult to simulate accurately in the absence of observations.

S. Lebonnois and D. Toublanc. Actinic fluxes in Titan's atmosphere, from one to three dimensions: Application to high-latitude composition. Journal of Geophysical Research, 104:22025-22034, 1999. [ bib | DOI | ADS link ]

We present a study on diurnally and annually averaged values of the actinic fluxes used in one-dimensional (1-D) photochemical models, as well as a 3-D radiative transfer model, based on Monte Carlo calculations with application to the atmosphere of Titan. This study shows that the commonly used value =30deg for the mean incident angle at the equator in photochemical models of Titan is not the best choice, though changing the value has no dramatic effects on photochemistry. The results of the 3-D code give direct access to the photolysis rates at any point in the atmosphere. The necessity of 3-D values in a deep atmosphere such as Titan's is demonstrated particularly for high-latitude winter conditions. These 3-D photolysis rates are used to model the latitudinal variations of the chemical composition of Titan's atmosphere in a 1-D photochemical model adapted to different latitudes. This study shows that these kinds of simple photochemical models cannot reproduce the observed latitudinal behavior and that we need to develop real 2-D photochemical models of Titan's atmosphere.