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Title: East African rainfall : classification of rain producing systems : a modelling and observation study
Author: Pearce, Helen Elizabeth
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2013
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The study of anthropogenic climate change is a research area of vital importance for the coming decades, with rainfall change and variability expected to be keenly felt in vulnerable regions of the world, including Africa. The focus of this study is daily rainfall during the short rains season over East Africa from October to December, which has one of the most coherent increasing rainfall projections in the Coupled Model Intercomparison Project (CMIP) dataset. This thesis aims to examine the fidelity of coupled climate models over an East African domain, with the question approached through focus on the daily (rather than monthly) rainfall field. The self-organising map is used as a clustering tool to establish contemporary characteristics of daily rainfall events in reanalysis (ERA-40) and satellite (TRMM) rainfall datasets between 1971-2000 and 1998-2010 respectively for the East African short rains. Moisture flux divergence is found to be the circulation parameter that is most closely related to the presence of rainfall events or dry conditions over East Africa. Coupled climate models are poor at replicating the daily rainfall field over the domain. A key result of the analysis is the consistent overestimation of daily rainfall by climate models for days where dry conditions of suppressed convection should prevail. In contrast, the moisture flux divergence field maps well to dry nodes for days of the self-organising map array for the models. Dry days are associated with widespread anomalous moisture flux divergence and rainfall events with co-located anomalous moisture flux convergence. This is in agreement with the moisture flux divergence field in the ERA-40 dataset; it is the rainfall field where there is disagreement for days of suppressed convection. Twenty to thirty-five percent of the projected rainfall increase towards the end of the twenty-first century results from an increase in the proportion of days assigned to nodes of suppressed convection in six of the seven models and the ensemble mean. There is an accompanying projected rainfall increase associated with days assigned to these nodes. Such days in the 2090s are characterised by projected increased strength moisture flux divergence over East Africa. Given that the moisture flux field was more successfully simulated in the coupled models under contemporary conditions than the daily rainfall field, this suggests that rainfall projections under a high emissions scenario at the end of the twenty-first century are overestimated and that an important part of the key increase in the projected rainfall may not be real.
Supervisor: Washington, Richard Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Africa ; Climate systems and policy ; Environmental change ; climate ; climate change ; rainfall ; geography ; East Africa