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

土壤水在一系列对天气和气候至关重要的过程中起着关键作用。在长时间不下雨的情况下，由于植被蒸腾和水分直接从裸露的土壤表面蒸发，土壤会变干。在这个干燥循环的某一点上，蒸发本身由于缺乏土壤水分而受到限制。在这种缺水的条件下，来自太阳的辐射在陆地表面被分割的方式发生了变化；蒸发蒸腾所需的能量更少，因此更多的能量用于加热地面和上面的空气。除了提高气温外，这种变化还会对大气环流、云和降雨产生重要影响。除了这些物理效应，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变化往往对干旱敏感，进而影响作物产量和自然生境。从长期来看，干燥的土壤会引发区域植被组成的变化，例如灌木比树木更受青睐。预计这种植被变化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，而树木的消失削弱了这种碳汇。土壤水分还影响其他一些重要微量气体的浓度，如臭氧和挥发性有机化合物。在热浪期间，土壤水分缺乏导致这些微量气体的高浓度以及高温，对人类健康产生影响。我们依靠在功能强大的计算机上运行的复杂数字代码来预测大气。几十年来，天气预报模型已经包含了对土壤水分如何影响大气的简单描述。由于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（ESMs）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中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着巨大的、有时是未知的差异。尽管如此，esm将这些方程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以用来检查模型模拟关键陆地过程的效果。该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范围。我们将研究随着土壤干燥，地表温度是如何上升的，这些温度上升需要多长时间的干旱期，以及它们是如何影响热浪的发生的。我们将在与esm相同的粗糙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。我们将提出一些措施，这将使我们第一次能够测试全球esm代表的关键过程的情况。我们将确定英国ESM的具体弱点，并评估最新的政府间气候变化专门委员会用于预测未来气候的一些其他模型。我们将把新的观测数据集提供给世界各地的气候和天气模拟小组。这将使下一代的esm从我们的研究中受益，并反过来有助于提高从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.