Philippe Marbaix

A high-resolution simulation of a West African rainy season using a regional climate model

The regional climate model Modele Atmospherique Regional (MAR) is applied to West Africa and the year 1992 is simulated. MAR reproduces the observed intraseasonal variations of rainfall. It is suggested that such a phenomenon is associated with oscillations between a weak and a strong regime of the Hadley cell. The later is correlated with a stronger meridional gradient of moist static energy in the planetary boundary layer and is responsible for an enhanced convergence of this quantity and a subsequent increase of convection and rain. An enhanced consumption of moist static energy and finally a weakening of the meridional circulation result. The meridional gradient of the moist static energy is restored by surface processes. The model also simulates the observed abrupt northward shift of the rainband in the first half of July. The spatial variability of the simulated monthly mean rainfall is in good agreement with the observations, although the model overestimates rainfall in some places from the beginning of August. Time series of daily mean rainfall are averaged over two 2.5degrees x 2.5degrees grid meshes in the Niamey region and in the Oueme high valley. Maxima reaching up to 40 mm/day are found in both areas, as in the observations. Atmospheric variables such as temperature and wind are briefly compared with the European Center for Medium-Range Weather Forecasting reanalyses. The main (cold) biases are located where the hydrological cycle simulated by MAR is too strong.

Lateral boundary conditions in regional climate models: A detailed study of the relaxation procedure

In gridpoint regional climate models (RCMs), the lateral boundary conditions are usually provided by a procedure called relaxation. The technique was originally studied in the context of numerical weather forecasting. This paper complements the preceding theoretical studies in order to assess the practical choices of model relaxation coefficients. Several profiles of coefficients used in RCMs are then evaluated. The complexity of actual model numerics makes any definite choice of the coefficients out of reach of simple theoretical considerations, but these provide practical guidelines. The latter are confirmed by pragmatic considerations such as minimizing discontinuities and keeping relaxation rates in the range of the represented physical processes. The last part of the paper presents a sensitivity study with the Modele Atmospherique Regional (MAR).

The impact of the unilateral EU commitment on the stability of international climate agreements.

The negotiation strategy of the European Union was analysed with respect to the formation of an international climate agreement for the post-2012 era. Game theory was employed to explore the incentives for key players in the climate policy arena to join future climate agreements. A −20% unilateral commitment strategy by the EU was compared with a multilateral −30% emission reduction strategy for all Annex-B countries. Using a numerical integrated assessment climate—economy simulation model, we found that leakage, in the sense of strategic policy reactions on emissions, was negligible. The EU strategy to reduce emissions by 30% (compared with 1990 levels) by 2020, if other Annex-B countries follow suit, does not induce the participation of the USA with a comparable reduction commitment. However, we argue that the original EU proposal can be reshaped so as to stabilize a larger and more ambitious climate coalition than the Kyoto Protocol in its first commitment period.

High resolution simulations of precipitation over the Alps with the perspective of coupling to hydrological models

As the computer resources increases, regional climate simulations can be investigated at finer horizontal resolutions in order to improve the representation of surface characteristics and mesoscale circulation. This paper aims at examining the relevance of precipitation fields simulated by highresolution regional climate models for hydrological purposes.