A study of clear sky closure study using high resolution far-IR spectra from the high arctic

The Earth's energy balance is in equilibrium, that is to say, an equal amount of energy is absorbed and emitted, The Sun is the source of all energy for the Earth, heating the planet via sunlight, warming the surface and this energy is emitted back into space as heat. A simple calculation can be made to estimate the surface temperature needed to preserve this equilibrium, and is about 255K, or -18degC. The presence of gases in the atmosphere that can absorb heat (greenhouse gases, such as water vapour and carbon dioxide), affects this basic equilibrium, meaning that it isn't generally the surface, but a layer of the atmosphere some distance above our heads that releases the energy to space; keeping the surface warmer than we would initially expect, and habitable for us humans. Once we look into what actually happens in more detail, this picture becomes very complicated. For instance, whether the surface is covered by ocean, desert, forests or snow makes a big difference, as does the concentration of greenhouse gases through the depth of the atmosphere and the amount, height and type of any cloud. Generally speaking, in the tropics more energy is absorbed than is emitted back to space; the winds and oceans transport this excess heat north and south towards the poles. At the other extreme, the Arctic region absorbs very little sunlight, and emits more energy to space than it receives. There is highly reflective snow and ice in the Arctic region and because it is so cold, the air is relatively dry compared to lower latitudes. With water vapour being one of the major greenhouse gases, the Arctic doesn't trap the heat as well as in the tropics. In some places and meteorological situations, parts of the heat (infra-red) spectrum of the atmosphere become transparent, allowing the surface to cool directly to space, removing some of the warming effect of the atmosphere. Where this happens is very interesting for scientists who try to understand the subtle mechanisms that control the loss of heat from the atmosphere. Using the Imperial College high resolution far infrared spectrometer (TAFTS), together with the US spectrometer (AERI-ER) we have taken measurements of the whole infrared spectrum from a site in the high arctic, where these rare conditions occur. The site was the United States Department of Energy (DoE) Atmospheric Radiation Measurement (ARM) site in Barrow, Alaska, in the spring of 2007. These measurements allow us to look up through these transparent windows to study the interaction between the greenhouse gases and thermal radiation in these dry conditions. These measurements were funded by the US ARM program, with initial analysis having shown that the data is of good quality and of scientific use. We now wish to analyse this data to take the full scientific benefit and impact from it. In addition to the infrared measurements of spectral radiances, we have the auxiliary data, which defines the state of the atmosphere, both the temperature and water vapour profile throughout the atmosphere, evidence for the lack of clouds etc. Using these datasets we are aiming to be able to better quantify the strength of the water vapour absorption lines, and the underlying absorption by water vapour, known as the continuum absorption. We will also use our results and models to investigate the cooling to space in the far infrared. This work will improve our understanding of the atmosphere, with many applications in radiation components of global climate models, and is expected to give greater accuracy in atmospheric models.