Regional prediction of rainfall change - an energy budget approach

Changes in the hydrological cycle are one of the most important aspects of climate change prediction. Globally, precipitation change is well understood in terms of the atmospheric energy budget, whereby the latent heat released is balanced by net atmospheric cooling. As a result, forcing agents affect precipitation directly through forcing-dependent adjustments, as well as through temperature-driven feedbacks. However, the physical processes driving regional precipitation changes are less understood, particularly over land, and regional projections exhibit significant uncertainties. The global energetic perspective can be extended to regional scales through incorporating horizontal transport of dry static energy. Therefore, the aim of this thesis was to utilize analysis of the local atmospheric energy budget to improve understanding of how forcing agents affect regional precipitation patterns through both forcing-dependent adjustments and temperature-driven feedbacks. The precipitation response to a range of atmospheric forcing agents was analysed using the Met Office Hadley Centre climate model, HadGEM2, as well as output from a large number of the latest generation of global climate models. Land-mean precipitation was shown to have a weak sensitivity to global temperature change. Therefore, adjustment processes have a strong influence on land-mean precipitation trends. During the historical period temperature-driven intensification of land-mean precipitation has been entirely masked by negative adjustments in response to anthropogenic sulphate and volcanic forcing. However, as projected sulphate concentrations decline, temperature-driven changes will soon dominate. The rapid land surface response to forcing was found to play a key role in driving regional precipitation adjustment patterns. Adjustment processes were found to be particularly important for precipitation in the eastern Amazon. Projected drying of the eastern Amazon was shown to be dominated by the physiological effects of CO2 on plant stomata, through reducing evapotranspiration. These results highlight the importance of short-timescale adjustment processes in understanding historical and future precipitation changes over land.