The Maritime Continent (MC) is the archipelago of tropical islands that lies between the Indian and Pacific Oceans, with a population of over 400 million. It comprises large (Sumatra, Java, Borneo, and New Guinea) and many smaller islands, with high mountains. High solar input warms the surrounding seas, which supply an abundance of moisture to the atmosphere, turning the whole region into an atmospheric "boiler box". Deep convective clouds rise up over the islands every day, leading to average rainfall rates in excess of 10 mm per day, approximately three times the rainfall rate over the UK. As well as supplying local agriculture, rain that falls over the MC has a far-reaching, global effect on weather and climate. Tremendous heat energy is released by condensation into the atmosphere in these convective clouds. This heat source drives giant, overturning circulations in the atmosphere: the Hadley and Walker cells, which feed into the jet streams and lead to weather and climate changes far downstream, even over the UK. For example, the origins of the infamous cold winter of 1962/63 and the recent very cold March of 2013 have been traced to atmospheric convection over the MC. For these reasons, the MC has been described as the engine room of the global climate system. Due to the complex nature of the distribution of the islands, and fundamental inadequacies in current models of the atmosphere (mainly related to their representation of convection), both climate predictions and weather forecasts show serious errors over the MC, particularly in their simulation of rainfall. Up until now, these errors have been extremely difficult to address, as there has been a lack of suitable observations over this region. Computing power, and the atmospheric modelling expertise to harness the advances in computing resources, has been inadequate to run computer models with sufficient detail to resolve the convective processes and their interactions, which are the building blocks of atmospheric circulation, for long enough to allow interactions with larger scales. However, we now stand on the cusp of transforming our understanding of atmospheric processes over the MC. Computer power and modelling expertise have progressed to the point where we have the capability to run simulations of the atmosphere at sufficient resolution to accurately capture the complex distribution of islands, and to accurately model the convective processes themselves. In response to this, the international Years of the Maritime Continent (YMC) field experiment (2017-2020) will make the measurements of the atmosphere and ocean at the very small scales that are needed to evaluate and understand the outputs of these new model simulations. Through TerraMaris the UK will take a leading role in YMC, by making observations of convective processes over the MC using the UK meteorological research aircraft, atmospheric radars, balloon and land-based measurements on the islands, and observing the surrounding seas using autonomous underwater and surface vehicles. This unprecedented suite of coordinated observations will complement measurements being taken by our international partners. The UK and the TerraMaris research team has led the way in developing high-resolution atmospheric modelling over recent years. We will apply the skills and knowledge learned to understand the complex mechanisms behind the multiple scales of convection and atmospheric circulations that have made the weather over the MC such a tough problem to crack. This knowledge will enable ground-breaking advances in atmospheric modelling, to improve weather forecasts and climate prediction over the MC region, with direct benefit to the substantial regional population. The downstream effects will see these benefits extend to the far corners of the globe, improving global and regional medium-range weather prediction, and climate projections.

Forecasting the weather from days to two weeks in advance has typically focused on the troposphere, the layer of the atmosphere closest to the ground. A typical weather forecast first attempts to estimate what the atmosphere is like now, and then extrapolates forward in time, using a complex model of the atmosphere based on the basic physical laws of motion. Over the last 15 years, evidence has been growing that different parts of the atmosphere and Earth system can also be exploited to improve weather forecasts. One of these regions is the stratosphere, the layer directly above the troposphere. Because, temperatures increase with height in the stratosphere, winds and weather systems are quite different, and a distinct community of scientific researchers who study the stratosphere exists around the world. Through the work of this community, many weather forecasting centres have been encouraged to look to the stratosphere to improve their weather forecasts and have been modifying their weather forecasting models accordingly. What has been missing, however, is a concerted effort to understand how best to make use of the stratosphere to improve weather forecasts and to determine how much weather forecasts might benefit. This proposal will fund a new international scientific network which will bring scientists from around the world together to study the stratosphere and how it might be used to improve weather forecasts. The network is made up of scientists from universities and weather forecasting centres around the world and is supported by two other international scientific research bodies. The network will allow scientists to come together to discuss current research in this area and to plan and carry out a new experiment which will compare the stratosphere and its impact on weather forecasts in their weather forecasting models. At the end of the research project, the network members will work together to produce a report which will provide guidance to all weather forecasting centres on the use of the stratosphere for weather forecasting.