Abstract
Abstract. The first airborne measurements of the Far-InfraRed Radiometer (FIRR) were performed in April 2015 during the panarctic NETCARE campaign. Vertical profiles of spectral upwelling radiance in the range 8–50 µm were measured in clear and cloudy conditions from the surface up to 6 km. The clear sky profiles highlight the strong dependence of radiative fluxes to the temperature inversion typical of the Arctic. Measurements acquired for total column water vapour from 1.5 to 10.5 mm also underline the sensitivity of the far-infrared greenhouse effect to specific humidity. The cloudy cases show that optically thin ice clouds increase the cooling rate of the atmosphere, making them important pieces of the Arctic energy balance. One such cloud exhibited a very complex spatial structure, characterized by large horizontal heterogeneities at the kilometre scale. This emphasizes the difficulty of obtaining representative cloud observations with airborne measurements but also points out how challenging it is to model polar clouds radiative effects. These radiance measurements were successfully compared to simulations, suggesting that state-of-the-art radiative transfer models are suited to study the cold and dry Arctic atmosphere. Although FIRR in situ performances compare well to its laboratory performances, complementary simulations show that upgrading the FIRR radiometric resolution would greatly increase its sensitivity to atmospheric and cloud properties. Improved instrument temperature stability in flight and expected technological progress should help meet this objective. The campaign overall highlights the potential for airborne far-infrared radiometry and constitutes a relevant reference for future similar studies dedicated to the Arctic and for the development of spaceborne instruments.
Highlights
Since the early days of weather satellites, remote sensing in the infrared (IR) has been used to study the vertical structure of the Earth atmosphere (e.g. Conrath et al, 1970)
The FarInfraRed Radiometer (FIRR) radiometric performances are first analyzed based on experiments performed on the ground and during one flight
The five case studies are analyzed in detail and the vertical profiles of radiance acquired in clear sky and cloudy conditions are compared to radiative transfer simulations
Summary
Since the early days of weather satellites, remote sensing in the infrared (IR) has been used to study the vertical structure of the Earth atmosphere (e.g. Conrath et al, 1970). The FIR range includes the strongly absorbing pure rotation band of water vapour and coincides with a maximum in the water vapour continuum strength (Shine et al, 2012). As such, it is especially promising for remote sensing. The FIR signature of clouds carries much information about cloud phase, optical thickness, particle effective size and particle shape for ice clouds (Rathke, 2002; Yang et al, 2003; Baran, 2007). Observing long-term changes in the FIR emission of Earth could eventually provide valuable insight into the physical processes underlying climate change (Huang et al, 2010)
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