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

土壤水分在一系列对天气和气候至关重要的过程中起着关键作用。在长时间无雨的情况下，由于植被蒸腾和裸露土壤表面的水分蒸发，土壤可能会变干。在这个干燥循环的某个时刻，蒸发本身会受到土壤水分缺乏的限制。在这种水分紧张的条件下，来自太阳的辐射在陆地表面的分配方式发生了变化;蒸散所需的能量减少，因此更多的能量用于加热地面和上层空气。除了提高气温外，这种变化还会对大气环流、云和雨产生重要影响。除了这些物理影响外，土壤的干燥也具有重要的地球化学影响。作物和自然植被的季节性变化往往对干旱敏感，这反过来又影响到作物产量和自然生境。从长远来看，土壤干燥可能会引发植被区域组成的变化，例如有利于灌木而不是树木。预计植被的这种变化将在全球气候系统中发挥重要的未来作用;植被抵消了人类活动排放的大部分二氧化碳，而树木的损失削弱了这种碳汇。土壤水分还影响其他一些重要的痕量气体，如臭氧和挥发性有机化合物。在热浪期间，土壤水分不足会导致此类微量气体的高浓度以及高温，从而影响人类健康。我们依靠在功能强大的计算机上运行的复杂数字代码来对大气进行预测。几十年来，天气预测模型已经纳入了土壤水如何影响大气的简单描述。随着人们越来越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）中的陆面模型变得更加复杂，使我们能够模拟植被动态和痕量气体对干旱等因素的响应。这些模型依赖于基本方程，这些方程旨在捕捉空间某一点的蒸发蒸腾和土壤排水等物理过程。然而，在不同的地点之间，控制这些过程的植被和土壤的性质存在巨大的，有时是未知的差异。尽管如此，紧急状态机制仍将这些公式应用于数千平方公里的不同区域。关键的是，在如此大的空间尺度上没有准确的现场测量数据可用于检查模型模拟关键土地过程的效果，本项目将利用近年来卫星收集的图像。这些可以提供空间细节（低至1公里）和关键土地属性的全球覆盖。我们将研究土地表面的温度如何随着土壤干燥而上升，这些温度上升需要多长时间的干旱期，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的影响。我们将制定几项措施，这将首次使我们能够测试地球仪在多大程度上代表了关键进程。我们将确定英国ESM中的具体弱点，并评估最新政府间气候变化专门委员会用于预测未来气候的其他一些模型。我们将向世界各地的气候和天气建模小组提供我们新的观测数据集。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上的预测。

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. 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.