Evaluation Of Soil Moisture Control On Surface Fluxes In Earth System Models (e-stress)

点击翻译按钮获取中文摘要

Soil water plays a key role in a range of processes which are important for weather and climate. During extended periods without rain, the soil can dry out due to the vegetation transpiring and evaporation of water direct from bare soil surfaces. At some point in this drying cycle, evaporation itself becomes limited by the lack of soil water. Under such water-stressed conditions, there is a change in the way that incoming radiation from the sun is partitioned at the land surface; less energy is required for evapotranspiration so more energy goes into heating up the ground and overlying air. As well as raising air temperatures, this change can have important effects on atmospheric circulations, clouds and rain. In addition to these physical effects, the drying out of soils also has important biogeochemical impacts. The seasonal evolution of crops and natural vegetation is often sensitive to drought, in turn affecting crop yields and natural habitats. In the longer-term, drying soils can trigger changes in the regional composition of vegetation, for example favouring shrubs over trees. Such changes in vegetation are expected to play an important future role in the global climate system; vegetation offsets much of the carbon dioxide which is emitted from man's activities, and loss of trees weakens this carbon sink. Soil water also affects a number of other important trace gases, such as ozone and volatile organic compounds. During heatwaves, soil water deficits contribute to high concentrations of such trace gases, as well as high temperatures, with impacts on human health.We rely on complex numerical codes run on powerful computers to make predictions of the atmosphere. For several decades, weather prediction models have incorporated simple descriptions of how soil water affects the atmosphere. Driven by a growing realisation of the importance of soil and vegetation processes for future climate, land surface models within so-called Earth System Models (ESMs) have become more complex, allowing us to simulate vegetation dynamics and trace gas responses to drought amongst other factors. These models rely on basic equations designed to capture the physical processes of e.g. evapotranspiration and soil drainage at a point in space. However, between locations there are huge and sometimes unknown differences in the nature of vegetation and soil which control these processes. All the same, the ESMs apply these equations over diverse areas of many thousands of square kilometres. Critically, there are no accurate in situ measurements at such large spatial scales which can be used to check how well the model simulates key land processes.This project will exploit the availability of images collected by satellites over recent years. These can provide both spatial detail (down to 1km) and global coverage of key land properties. We will look at how the temperature of the land surface rises as the soil dries, how long a dry spell is required for these temperatures to rise, and how they influence the occurrence of heat waves. We will look at these relationships at the same coarse spatial scale as the ESMs and identify which regions and vegetation types are more prone to drought stress. We will produce several measures which for the first time, will allow us to test how well the key processes are represented by the ESMs across the globe. We will identify specific weaknesses within the UK ESM, and also evaluate a number of other models used for the latest Intergovernmental Panel on Climate Change to make projections of future climate. We will make our new observational datasets available to climate and weather modelling groups around the world. This will allow the next generation of ESMs to benefit from our research, and in turn contribute to improved prediction on time scales from hours to decades.