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Distributions of Downwelling Radiance at 10 and 20 μm in the High Arctic

Abstract In the Arctic, most of the infrared (IR) energy emitted by the surface escapes to space in two atmospheric windows centred at 10 and 20 μm. As the Arctic warms and its water vapour burden increases, the 20 μm cooling-to-space window, in particular, is expected to become increasingly opaque (or “closed”), trapping more IR radiation, with implications for the Arctic’s radiative energy balance. Since 2006, the Canadian Network for the Detection of Atmospheric Change has measured downwelling IR radiation with Atmospheric Emitted Radiance Interferometers at the Polar Environment Atmospheric Research Laboratory at Eureka, Canada, providing measurements of the 10 and 20 μm windows in the High Arctic. In this work, measurements of the distribution of downwelling 10 and 20 µm brightness temperatures at Eureka are separated based on cloud cover, providing a comparison to an existing 10 µm climatology from the Southern Great Plains. The downwelling radiance at both 10 and 20 μm exhibits strong seasonal variability as a result of changes in cloud cover, temperature, and water vapour. Given the 20 µm window’s limited transparency, its ability to allow surface IR radiation to escape to space is found to be highly sensitive to changes in atmospheric water vapour and temperature. When separated by season, brightness temperatures in the 20 µm window are independent of cloud optical thickness in the summer, indicating that this window is opaque in the summer. This may have long-term consequences, particularly as warmer temperatures and increased water vapour “close” the 20 μm window for a prolonged period each year.