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

土壤水分在对天气和气候有重要影响的一系列过程中起着关键作用。在长时间没有降雨的情况下，由于植被的蒸腾和裸露土壤表面的水分直接蒸发，土壤可能会干燥。在这个干燥循环的某个时刻，蒸发本身就会受到土壤水分缺乏的限制。在这种水分紧张的条件下，来自太阳的入射辐射在陆地表面的分配方式发生了变化；蒸发蒸腾所需的能量较少，因此更多的能量用于加热地面和上面的空气。除了提高气温，这种变化还可能对大气环流、云层和降雨产生重要影响。除了这些物理效应外，土壤的干燥也有重要的生物地球化学影响。作物和自然植被的季节性演变往往对干旱很敏感，进而影响作物产量和自然生境。从长远来看，干燥的土壤可能会引发植被区域组成的变化，例如，比起乔木，灌木更受青睐。植被的这种变化预计将在未来的全球气候系统中发挥重要作用；植被抵消了人类活动排放的大部分二氧化碳，而树木的丧失削弱了这种碳汇。土壤水分还影响其他一些重要痕量气体的浓度，如臭氧和挥发性有机化合物。在热浪期间，土壤水分不足导致此类微量气体的高浓度，以及高温，从而影响人类健康。我们依靠运行在强大计算机上的复杂数字代码来预测大气。几十年来，天气预报模型包含了土壤水分如何影响大气的简单描述。随着人们日益认识到土壤和植被过程对未来气候的重要性，所谓的地球系统模型(ESM)中的陆地表面模型变得更加复杂，使我们能够模拟植被动态和追踪气体对干旱的响应等因素。这些模型依赖于旨在捕捉空间某一点的蒸散和土壤排水等物理过程的基本方程。然而，在不同地点之间，控制这些过程的植被和土壤的性质存在巨大的差异，有时甚至是未知的。尽管如此，ESM将这些方程式应用于数千平方公里的不同地区。关键是，没有在如此大的空间尺度上进行准确的现场测量，可以用来检查该模型对关键陆地过程的模拟情况。这个项目将利用近年来卫星收集的图像。它们可以提供关键土地属性的空间细节(最小可达1公里)和全球覆盖范围。我们将观察随着土壤干燥，陆地表面的温度如何上升，这些温度上升需要多长时间，以及它们如何影响热浪的发生。我们将在与ESM相同的粗略空间尺度上研究这些关系，并确定哪些地区和植被类型更容易受到干旱胁迫。我们将制定几项措施，首次使我们能够测试全球ESM代表关键流程的情况。我们将找出英国ESM的具体弱点，并评估最新的政府间气候变化专门委员会使用的其他一些模型，以预测未来的气候。我们将向世界各地的气候和天气模型小组提供我们的新观测数据集。这将使下一代ESM从我们的研究中受益，并反过来有助于改进从几小时到几十年的时间尺度的预测。

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. As well as these physical effects, thr 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 the concentration of 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.