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2006 .

(3 publications)

F. Hourdin, I. Musat, S. Bony, P. Braconnot, F. Codron, J.-L. Dufresne, L. Fairhead, M.-A. Filiberti, P. Friedlingstein, J.-Y. Grandpeix, G. Krinner, P. Levan, Z.-X. Li, and F. Lott. The LMDZ4 general circulation model: climate performance and sensitivity to parametrized physics with emphasis on tropical convection. Climate Dynamics, 27:787-813, December 2006. [ bib | DOI | ADS link ]

The LMDZ4 general circulation model is the atmospheric component of the IPSL CM4 coupled model which has been used to perform climate change simulations for the 4th IPCC assessment report. The main aspects of the model climatology (forced by observed sea surface temperature) are documented here, as well as the major improvements with respect to the previous versions, which mainly come form the parametrization of tropical convection. A methodology is proposed to help analyse the sensitivity of the tropical Hadley Walker circulation to the parametrization of cumulus convection and clouds. The tropical circulation is characterized using scalar potentials associated with the horizontal wind and horizontal transport of geopotential (the Laplacian of which is proportional to the total vertical momentum in the atmospheric column). The effect of parametrized physics is analysed in a regime sorted framework using the vertical velocity at 500 hPa as a proxy for large scale vertical motion. Compared to Tiedtkes convection scheme, used in previous versions, the Emanuels scheme improves the representation of the Hadley Walker circulation, with a relatively stronger and deeper large scale vertical ascent over tropical continents, and suppresses the marked patterns of concentrated rainfall over oceans. Thanks to the regime sorted analyses, these differences are attributed to intrinsic differences in the vertical distribution of convective heating, and to the lack of self-inhibition by precipitating downdraughts in Tiedtkes parametrization. Both the convection and cloud schemes are shown to control the relative importance of large scale convection over land and ocean, an important point for the behaviour of the coupled model.

S. K. Deb, H. C. Upadhyaya, J. Y. Grandpeix, and O. P. Sharma. On convective entrainment in a mass flux cumulus parameterization. Meteorology and Atmospheric Physics, 94:145-152, November 2006. [ bib | DOI | ADS link ]

A new entrainment/detrainment formulation in the Tiedtkes mass flux cumulus parameterization is discussed here. Apart from validating it with observations both in one and three dimensional cases, it is also evaluated in the light of the results from the original Tiedtke scheme and another mass flux scheme due to Emanuel. The proposed analytical profiles of entrainment and detrainment, easier to implement in any mass flux scheme, give reasonable results in GCM experiments.

S. Hallegatte, A. Lahellec, and J.-Y. Grandpeix. An Elicitation of the Dynamic Nature of Water Vapor Feedback in Climate Change Using a 1D Model. Journal of Atmospheric Sciences, 63:1878-1894, July 2006. [ bib | DOI | ADS link ]

The concept of feedback has been used by several authors in the field of climate science to describe the behavior of models and to assess the importance of the different mechanisms at stake. Here, a simple 1D model of climate has been built to analyze the water vapor feedback. Beyond a static quantification of the water feedback, a more general formal definition of feedback gain based on the tangent linear system is introduced. This definition reintroduces the dynamical aspect of the system response to perturbation from Bode's original concept.In the model here, it is found that, even though the water vapor static gain proves consistent with results from GCMs, it turns out to be negative for time scales below 4 yr and positive only for longer time scales. These results suggest two conclusions: (i) that the water vapor feedback may be fully active only in response to long-lived perturbations; and (ii) that the water vapor feedback could reduce the natural variability due to tropospheric temperature perturbations over short time scales, while enhancing it over longer time scales. This second conclusion would be consistent with studies investigating the influence of air sea coupling on variability on different time scales.<BR />HR ALIGN=”center” WIDTH=”30%”<BR />

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