Projected Responses of Extreme Precipitation and Atmospheric Radiative Energy (PREPARE)

Projected increases in rainfall and its intensity and the coverage of regions experiencing drought will lead to adverse impacts on societies, agriculture and health. An emerging body of evidence indicates that climate models may underestimate the current changes in the global water cycle. It is crucial and timely that the causes of discrepancies between simulated and observed responses of the atmospheric hydrological cycle to warming are identified and addressed. Bringing together important lines of research and collaborators with unique expertise the current proposal seeks to test the following hypotheses: (i) Are limitations of the satellite data hampering our ability to monitor changes in the global water cycle? (ii) Are present day changes in the Earth's energy balance relating to aerosol influencing trends in the hydrological cycle? (iii) What are the implications for projections of future changes in rainfall and its extremes? To answer these questions, an appreciation for the root causes of changes in rainfall is required. Evidence from sophisticated climate models and observations from a variety of sources point to robust increases in atmospheric moisture with warming at about the rate expected from basic physical considerations (around 7% per K warming). Rising moisture fuels intensification of the heaviest rainfall events. This is not the full story: global precipitation is intrinsically linked to Earth's atmospheric energy budget. The relatively slow rises in atmospheric radiative cooling, as the planet warms, can only support modest rises in latent heating through precipitation of around 1-3% per K, much slower than the rises in heavy rainfall. This leads to a reduction in rainfall away from convective regimes. While both climate models and satellite observations indicate that the dry regions are becoming drier and the wet regions wetter, further comparison suggests that the models underestimate this response. To understand the reasons for the important discrepancy between models and data, we bring together important areas of expertise in the Earth's energy balance and the Global water cycle. The three main approaches are to (i) monitor, (ii) inter-compare and (iii) understand and predict changes in the hydrological cycle. These can be achieved by employing a unique combination of satellite and surface-based measurements of precipitation, evaporation and the Earth's radiative energy balance. Inter-comparison of existing and new satellite datasets will allow improved monitoring of changes in key variables such as precipitation and evaporation. Combining the hydrological and radiative components of the energy and water balance in models and observational data will enable a better understanding of the physical processes involved and improve estimates of changes in the surface radiation budget. Carefully constructed model experiments will explore the impact of changes in aerosol on the hydrological cycle through radiative forcings. The results will be paramount in improving estimates of future impacts from changes in the hydrological cycle on societies and ecosystems.