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Accueil du site → Doctorat → Afrique du Sud → 2007 → Simulating the effects of land-surface change on southern Africa’s climate

University of Cape Town (2007)

Simulating the effects of land-surface change on southern Africa’s climate

MacKellar, Neil Campbell

Titre : Simulating the effects of land-surface change on southern Africa’s climate

Auteur : MacKellar, Neil Campbell

Université de soutenance : University of Cape Town.

Grade : Doctor of Philosophy 2007

Regional Climate Model (RCM) experiments, using the MM5 model, are undertaken to investigate the potential climatic impacts of land-surface change in southern Africa. Simulations are fIrst run to assess the RCM’s sensitivity to perturbed initial soil moisture conditions. Following a disturbance in initial soil moisture, it is found that latent heat fluxes in the subsequent months are significantly affected in certain regions, but there is little effect on sensible heat flux and near-surface temperature. There is also no noticeable impact on regional circulation or precipitation. This implies that, for the magnitude of the initial soil moisture disturbance, soil moisture-rainfall coupling in the model is weak. Further MM5 integrations are performed to assess potential climatic impacts of altering the vegetation of southern Africa from an estimated natural state to present-day conditions. Contemporary vegetation cover is given by the United States Geological Survey land surface classification, and an estimated pristine state is simulated using the Sheffield Dynamic Global Vegetation Model. For three simulated periods : 1 Aug - 28 Feb 1988/89, 1991/92 and 1995/96, significant impacts on mean Sep-Nov (SON) and Dec-Feb (DJF) surface climate arise from the change in vegetation. The most notable result is cooling over large parts of the continent in SON, which gives rise to increased large-scale subsidence and decreased moisture convergence. Resultant decreases in rainfall cause a hydrological feedback through reduced latent heat flux which mitigates the initial cooling and weakens the subsidence anomaly in DJF. The subsidence anomalies extend as high 500 hPa, which has important implications for regional moisture transport over southern Africa. The role of synoptic forcing in modifying the atmosphere’s response to land surface change is explored using a self-organizing map (SOM). The SOM is used to identify archetypal patterns in the large-scale 850 and 500 hPa geopotential height and precipitable water fIelds in the reanalysis data that cover the same domain and force the RCM at its lateral boundaries. For each of the patterns (nodes) identified in these variables by the SOM, the mean RCM-simulated response to vegetation change is evaluated. Conditions characterized by strong sub-tropical anticyclones and low atmospheric moisture show the greatest temperature and geopotential height changes and are most sensitive to changes in radiative fluxes, whereas precipitation and surface hydrological processes are more sensitive under conditions of weak subsidence and high levels of atmospheric moisture.


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