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

水蒸气是地球大气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从紫外线到微波的各种波长的能量。水蒸气对红外线的吸收具有特别重要的意义。它造成了很大一部分自然温室效应，使地球适合居住，因此影响了当今的气候。它在气候变化中也起着重要作用。如果地球变暖，例如由于二氧化碳的增加，水蒸气的浓度就会增加；由于水蒸气本身就是一种温室气体，这导致了一个正反馈，模型表明，它使变暖大约加倍。不幸的是，目前对水蒸气吸收特性的了解还不够充分。水蒸气以两种方式吸收辐射。第一种是在较窄的波长区域（光谱线），对它的理解相对较好。第二种是在广谱区缓慢变化的吸收（连续体）。本建议的主题是对这种连续吸收的理解。连续体的存在几十年来一直为人所知，但对其原因及其特征的理解是争议的来源。一种理论认为，这是由于数千条光谱线累积的微小贡献；另一种理论，但不一定是唯一的，是由于对弱结合水分子（水二聚体）和相关物质的吸收。目前，用于天气预报、气候预测和从卫星观测中检索数据的大多数计算机模型都使用过去二十年发展起来的连续体的一种特定表示。这种代表对社区很有帮助。然而，它缺乏坚实的理论基础，只在相当窄的波长范围和大气条件下进行了观测验证；此外，这些观察结果是由不同的小组在不同的时间做出的，它们的可比性很难评估。这限制了对其使用的信心，特别是在气候和大气条件发生变化的情况下。连续统吸收理论的发展，以及仪器的进步，意味着现在是时候共同努力来提高我们对连续统的理解和表征。我们汇集了一个由8个领先的英国小组组成的联盟，这些小组在水蒸气吸收理论、在实验室和大气中使用最先进的测量技术以及在气候建模方面拥有成熟的专业知识。研究方案包括几个组成部分。1 .水二聚体的振动和旋转的先进计算，这将允许更好地预测其吸收特性及其对连续体的贡献。2 .使用最先进的实验室仪器，能够在前所未有的宽波长范围和条件下测量连续体；另一种技术，能够以非常高的精度测量相对较弱的吸收，将用于更窄波长区域的详细研究。实地活动，将使用校准良好的地面和飞机仪器的混合物，并将在实际大气条件下在广泛的波长范围内描述连续体的特征。我们提出了两个运动：一个在英格兰西南部，一个在欧洲的一个高山。这将允许在非常不同的大气条件下进行测量。综合理论、实验室测量和实地活动的结果，将它们结合在一起形成一个共同的框架。了解新结果对我们对当今气候和气候变化的认识的影响。以其他研究人员易于使用的形式发展连续体数据的表示法。

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.