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Accueil du site → Doctorat → Allemagne → Verbesserung der Simulation des westafrikanischen Klimas durch die Implementierung eines einfachen dynamischen Vegetationsmodells (SVege) in das Klimamodell ECHAM5 ; Improvement of West African climate simulations by coupling a dynamical, simple vegetation model (SVege) to the climate model ECHAM5

Universität zu Köln (2008)

Verbesserung der Simulation des westafrikanischen Klimas durch die Implementierung eines einfachen dynamischen Vegetationsmodells (SVege) in das Klimamodell ECHAM5 ; Improvement of West African climate simulations by coupling a dynamical, simple vegetation model (SVege) to the climate model ECHAM5

Brücher, Tim

Titre : Verbesserung der Simulation des westafrikanischen Klimas durch die Implementierung eines einfachen dynamischen Vegetationsmodells (SVege) in das Klimamodell ECHAM5

Improvement of West African climate simulations by coupling a dynamical, simple vegetation model (SVege) to the climate model ECHAM5

Auteur : Brücher, Tim

Grade : Doctor of Philosophy (PhD) 2008

Université de soutenance : Universität zu Köln

Résumé
Between 1970 and 2000 the climate of West Africa was affected by a severe drought. The reasons for the sudden change to the dry period are unclear. Therefore, investigations based on climate model simulations are undertaken to capture the observed decadal climate variability by including as many sub systems of the Earth Climate System as possible. Therefore, this work focuses the influence on simulating present day climate by including the biosphere into the general circulation model (GCM) ECHAM5 to capture the recent climate in a better way. It is well-known, that there is an amplification by vegetation on climate variability, in particular the decadal one. The Simple Vegetation (SVege) model is used for these coupled experiments. Furthermore, this optimised version is used to set up climate change simulations in an ensemble mode following the SRES (Special Report on Emission Scenarios ) greenhouse gas forcings A1B and B1 until the year 2100. It is assumed, that there will be a change within vegetation cover by a climate change, therefore the coupled version will be responsive to the climate impact. At least very promising results by doing this coupling with the former version No. 4 of ECHAM give the motivation for this work. To estimate the effect of using the vegetation model, simulations for present day climate (1960 to 1999) are investigated by using the biosphere-atmosphere model and the standalone version of the GCM both driven by an observed sea surface temperature (SST) data set (AMIP2). Both versions capture the observed climate of West Africa well and a small, insignificant shift to a better reproduction of the decadal variability in rainfall is noticed by including the vegetation model. There are no significant differences in simulating present day climate compared to the standard version. So, the coupled version using a dynamical vegetation model calculating time-dependent values for some surface parameters (e.g. albedo) gets the same results as using climatological mean numbers. It will be shown, that the coupled biosphere-atmosphere version captures the decadal variability better, but even the models are able to simulate the climatological mean state quite well, it has to be summarised, that both model versions fail to simulate the strength of the observed decadal variability of precipitation amounts in West Africa. Compared to similar studies using the previous version of the GCM (ECHAM4), the effect of the imbedded biosphere is low. Due to some changes in the surface parametrizations and formulating the surface scheme in a more sophisticated way, it can be assumed, that the used vegetation formulation is to simple for the new complex version of the GCM. The analysis of the 240 years (1860 to 2099) climate change simulations concentrates on possible shifts in precipitation intensities within a warmer world (SRES A1B and B1). Investigations on the yearly, quarterly, and daily precipitation sums and the analysis of the simulated tropical rain belt as well as the 2 m temperature are done. For the control climate (1960 to 1999) it is shown, that significant differences between the simulations based on observed and modelled SSTs exist. These differences are due to the two unequal SST data sets for the period 1960 to 1999 for computing (i) present day climate using observed values (AMIP2) and (ii) doing climate change experiments based on simulated (OM1, Ocean Model ) SST patterns. The trends indicate, that there is a possible shift of daily intensities to higher values. This shift is superimposed by increasing rainfall sums in summertime (June to August) at the Guinea Coast and less precipitation in the Sahel leading to a more prominent dipole between the wet Coast and dryer interior. With an increasing number of extreme precipitation events with respect to monthly rainfall sums (dry months in the Sahel and wet in the coastal region), the internal variability will increase and the scarce fresh water resources will become more precarious in a warmer climate. The changes due to the SRES scenarios are more pronounced in the stronger A1B scenario than in B1. In summary, the climate change results are similar to the ECHAM5-investigations pointed out by the fourth assessment report of the IPCC (Intergovernmental Panel on Climate Change ) without using a vegetation model.

Mots-clés : echam5, vegetation, west afrikan monsoon, ipcc, climate change ; earth sciences ; echam5, vegetation, westafrikanischer monsun, ipcc, climate change

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Page publiée le 24 mai 2011, mise à jour le 8 janvier 2019