Continuum Absorption at Visible and Infrared Wavelengths and its Atmospheric Relevance (CAVIAR)

水蒸气是地球大气层中最重要的温室气体。由于其复杂的结构，它的不寻常之处在于它吸收了从紫外线到微波的各种波长的能量。水蒸气的红外吸收具有特别重要的意义。它导致了大部分的自然温室效应，使地球适合居住，从而影响了当今的气候。它在气候变化中也发挥着重要作用。如果地球变暖，例如由于二氧化碳的增加，水蒸气浓度增加;由于水蒸气本身是一种温室气体，这会导致正反馈，模型表明，这会使变暖大约增加一倍。不幸的是，目前对水蒸气的吸收特性的理解是不够的。水蒸气以两种方式吸收辐射。第一种是在窄波长区域（光谱线），对此理解相对较好。第二种是在宽光谱区域（连续光谱）上缓慢变化的吸收。对这种连续吸收的理解是本建议的主题。连续统的存在已经知道了几十年，但对它的原因和特征的理解是一个争议的来源。一种理论认为，这是由于数千条光谱线的累积小贡献;另一种理论，但不一定是唯一的，是由于弱结合水分子对（水二聚体）和相关物质的吸收。目前，大多数用于天气预报、气候预测和从卫星观测中检索数据的计算机模型都使用过去20年来发展起来的连续统的一种特定表示。这种代表性对社会有很好的服务。然而，它缺乏坚实的理论基础，仅通过对相当窄的波长范围和大气条件的观测加以验证;此外，这些观测是由不同的小组在不同的时间进行的，其可比性难以评估。这就限制了人们对其使用的信心，特别是在气候以及大气条件发生变化的情况下。连续吸收理论的发展，以及仪器的进步，意味着这是及时的协调努力，以提高我们的理解和连续特性。我们汇集了一个由8个领先的英国团体组成的财团，这些团体在水蒸气吸收理论，在实验室和大气中使用最先进的测量技术以及气候建模方面具有成熟的专业知识。研究方案包括几个组成部分。1对水二聚体的振动和旋转的高级计算，这将允许更好地预测其吸收特性及其对连续谱的贡献。2使用最先进的实验室仪器，能够在前所未有的广泛波长和条件范围内测量连续谱;将采用一种替代技术，能够以非常高的精度测量相对较弱的吸收，以便在较窄的波长区域进行详细研究。3.实地活动，将使用经过良好校准的地面和飞机上的仪器，并将在真实的大气条件下在宽波长范围内对连续光谱进行测量。我们提出了两个运动：一个在英格兰西南部和一个在欧洲的高山网站。这将允许在非常不同的大气条件下进行测量。4综合理论、实验室测量和实地活动的结果，将它们纳入一个共同的框架。5了解新结果对我们理解当今气候和气候变化的影响。6.以其他研究人员易于使用的形式开发连续统数据的表示。

Water vapour is the most important greenhouse gas in the Earth's atmosphere. Because of its complex structure, it is unusual in that it absorbs energy across a wide range of wavelengths from the ultra-violet, to the microwave. Infrared absorption by water vapour is of particular significance. It causes a large part of the natural greenhouse effect which makes the Earth habitable, hence impacting on the present day climate. It also plays an important role in climate change. If the Earth warms, for example due to increases in CO2, water vapour concentrations increase; since water vapour is itself a greenhouse gas, this leads to a positive feedback which, models indicate, approximately doubles the warming. Unfortunately, understanding of the absorbing properties of water vapour is currently inadequate. Water vapour absorbs radiation in two ways. The first is in narrow wavelength regions (spectral lines) for which understanding is relatively good. The second is slowly varying absorption over broad spectral regions (the continuum). It is the understanding of this continuum absorption which is the subject of this proposal. The existence of the continuum has been known for decades, but an understanding of its cause, and its characteristics, is a source of controversy. One theory is that it is due to cumulative small contributions from thousands of spectral lines; an alternative, but not necessarily exclusive, theory is that it is due to absorption by pairs of weakly bound water molecules (the water dimer) and related species. Currently, most computer models used in weather forecasting, climate prediction, and to retrieve data from satellite observations, use one particular representation of the continuum developed over the past twenty years. This representation has served the community well. However, it lacks a firm theoretical basis and has only been verified using observations for a quite narrow range of wavelengths and atmospheric conditions; additionally, these observations have been made by different groups at different times and their comparability is difficult to assess. This limits confidence in its use, particularly as climate, and hence atmospheric conditions, change. Developments in the theory of continuum absorption, as well as advances in instrumentation, mean that it is timely for a concerted effort to improve our understanding and characterisation of the continuum. We bring together a consortium of 8 leading UK groups with established expertise in the theory of water vapour absorption, in the use of state-of-the-art measurement techniques in both the laboratory and the atmosphere, and in climate modelling. The programme of research involves several components. 1 Advanced calculations of vibrations and rotations of the water dimer, which will allow a better prediction of its absorption properties and its contribution to the continuum. 2 The use of a state-of-the-art laboratory instrumentation enabling the measurement of the continuum over an unprecedentedly broad range of wavelengths and conditions; an alternative technique, capable of measuring relatively weak absorption at very high precision will be deployed for detailed studies in narrower wavelength regions. 3 Field campaigns, which will use a mixture of well-calibrated ground and aircraft based instruments, and will characterise the continuum over a broad range of wavelengths under real atmospheric conditions. We propose two campaigns: one in south-west England and one at a high mountain site in Europe. This will allow measurements to be made under very different atmospheric conditions. 4 Synthesis of the results from the theory, laboratory measurements and field campaigns, drawing them together into a common framework. 5 Understanding of the impact of the new results on our understanding of present-day climate and climate change. 6 Development of a representation of the continuum data in a form that can be readily used by other researchers.