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Accueil du site → Doctorat → Royaume-Uni → 1995 → Development of a land surface scheme and its application to the Sahel

University of Reading (1995)

Development of a land surface scheme and its application to the Sahel

Gedney, N.

Titre : Development of a land surface scheme and its application to the Sahel.

Auteur : Gedney, N.

Université de soutenance : University of Reading

Grade : Doctor of Philosophy (Ph.D.) : 1995

A land surface parameterisation scheme is developed and tested over several distinct climatic regions. The scheme used is a revised version of a surface model previously used by the ECMWF. It is validated and developed using a number of different methods. This includes comparing it with a large number of other schemes over temperature and tropical climates. The diurnal cycle of the model is investigated using observations from the Sahel region in North Africa. Climatologically-based forcing is applied to examine the effects of desertification on the seasonal and annual timescales.In common with many models, important fields such as albedo, vegetation fraction, leaf area index and surface roughness need to be specified. In this work, two commonly used global albedo fields are found to have some significant geographical discrepancies, particularly over semi-arid regions. The implied differences between the vegetation cover fields appear to be significant in relation to the UGAMP (UK Universities Global Atmospheric Modelling Program) model’s sensitivities in the Sahel.The comparison between land-surface models results in the development of new parameterisations for snow cover, near-surface mixing and soil thermal properties for the model. Sub-grid scale precipitation effects are also assessed. Further testing is carried out using observational data taken as part of the HAPEX-Sahel experiment. The work concentrates on the ability of the UGAMP thermally amalgamated vegetation-soil surface model to reproduce the observed surface state over an area of sparse vegetation cover.Finally an idealised study is carried out over the Sahel which investigates the significance of the surface properties. The results indicate that the proportion of bare soil to vegetation is significant over this region, both in the long term and in its seasonal development. It is also demonstrated that the time between consecutive rainfall events has a significant impact on the long term evaporation. Further investigation with a model which includes boundary layer feedbacks generally reduces the sensitivities seen in the simpler studies.

Annonce : EThOS (UK)

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