The Organization of Tropical Rainfall

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Tropical cloud systems and rainfall help drive the global circulation of the atmosphere, transferring heat from near the Earth's surface upward for many kilometres. These convective systems can be found in groups of many different sizes, from isolated showers and thunderstorms to equatorial waves to tropical cyclones to the Madden-Julian Oscillation (MJO), an eastward-propagating weather system composed of superclusters of convection several thousand kilometres across which dominates tropical weather variability on weekly to monthly time scales. Global numerical weather forecast and climate models still do not adequately simulate these organized storm clusters and, as a result, have too little (or incorrect) variability of tropical rainfall. Improvement of the representation of organized tropical convection, and therefore the accuracy of weather forecasts, would greatly improve the lives of billions of people who rely on rainfall for agriculture in the tropics and subtropics; better forecasts of strong storms and flooding would also save countless lives and reduce property damage. Furthermore, these processes may change in the future as the climate changes due to human activities, so an improvement of the ability of global models to simulate organized convection will lead to better predictions of possible climate change scenarios over the whole globe. Global weather and climate models divide the Earth into grid boxes about 100 km across. These boxes are too large to directly simulate the motions responsible for small-scale rainstorms, instead estimating total rainfall based on average conditions in the box. This simplified rain estimation, necessary because of limited computer resources, ignores the interaction of isolated rain showers with each other and regional weather conditions. An exciting new research area is the study of organized convection in high-resolution idealized models. These models, with constant sea surface temperatures and constant sunlight, can now be run on domains several thousand kilometres across and with grid boxes of only a few kilometres long, allowing convection to be represented explicitly. These models are beginning to provide insight into processes that lead to spontaneous growth of convective clusters which can ultimately grow to a single large cluster accounting for all of the rainfall in the domain. These processes act over a wide range of spatial scales which are not fully resolved in global models. However, the processes which lead to organized convection in idealized models are still not well understood, and it is not known whether they are also important for organizing tropical convection in nature. This fellowship will exploit a large archive of high-resolution model runs, forecast analyses, and observations from satellites to make more direct comparisons between idealized cases and observed phenomena. Ultimately, this endeavour has the potential to lead to improvements in the way that global models, especially the UK Met Office Unified Model, simulate tropical rainfall and with it global weather and climate. This fellowship will benefit from being undertaken at the Department of Meteorology at the University of Reading, a world-leading centre for atmospheric science. Close ties with the Met Office ensure that this research will be interactive with ongoing efforts to improve the simulation of convection and tropical weather and climate in the Unified Model. In addition, there will be collaborations with other scientists approaching similar problems using different models, which will allow for exploration of possible model differences.