Abstract/Summary

Anthropogenic emissions of greenhouse gases and aerosols have led to climate change including changes in surface temperature and precipitation. The surface temperature response is better understood than the precipitation response as a result of observed data availability and the complexity of the physics governing hydrological cycle changes. The complex general climate models (GeMs) are computationally demanding and include many physical processes that contribute to the changing water cycle. It remains necessary to understand the main drivers of this change. In this thesis, the main aim is to understand the water cycle changes by examining the degree to which simple models can simulate global-average results emerging from GeMs. For this purpose, a simple atmospheric energy budget model is used to calculate the global mean precipitation changes for the historical period and future scenarios. The results are then compared with GeMs to understand the physical processes affecting the global precipitation changes. The original form of the simple atmospheric energy budget model does not take into account many different factors included in GeMs, such as regional temperature and precipitation changes, fast surface sensible heat flux changes, fast precipitation response of volcanic aerosols and inter-annual variability. This work examines whether it is possible to extend the simple model to include some of these factors or compare the idealised experiments with the results of complex models (Wu et al. 2010). The simple model does well in producing the total global precipitation anomalies compared with GeMs multi-model mean consistent with earlier studies. The results of the simple model for individual GeMs are in less good agreement and different reasons for this disagreement have been investigated. Substituting the temperature changes from each GeM and also normalising the radiative forcings of simple model to the adjusted GeM RFs lead to an increase in compatibility between the simple model and GeMs, indicating that the main differences are related to the temperature equation and RFs. Adding the fast response of volcanic aerosols also increased the correlation between the simple model and GeMs particularly in volcanic years. Using new results from (Precipitation Driver Response Model Intercomparison Project) PDRMIP, the effect of fast surface sensible heat changes has been investigated which shows a considerable contribution to atmospheric energy budget changes particularly for aerosols. The simple model has been modified by adding the fast sensible heat changes which leads to a small improvement in the simple model; however it is not possible to be certain how robust this improvement is. More data and more work is still required but generally it is concluded that the simple model performs well compared with complex models.