Extended Abstract (216K)8.3
Clear-sky radiance measurements from the far-IR TAFTS instrument during EMERALD 2001
Paul Green, Imperial College, London, United Kingdom; and J. Murray, J. Harries, A. Last, and J. Pickering
The influence of atmospheric humidity and clouds on the Earth's climate system is one of the major uncertainties in our present understanding of how the climate system works. Recent work has explained in detail how the infrared cooling to space of the Earth's surface and cloud-free atmosphere occurs, as a function of altitude and of spectral frequency. Clough et al, 1992, showed that a significant contribution to the outgoing longwave radiation (OLR) originated from the upper troposphere in the far IR between 250 and 500cm-1. Further theoretical work have looked into the proportion of the OLR that can be attributed to the pure rotation band, concluding that 30-70% of the OLR originates from this band, depending on latitude. Despite this progress, a number of difficult problems remain for the clear sky case, including the distribution of humidity in the troposphere, and its consequences for the global greenhouse effect; and the exact nature of continuum absorption at the low temperatures typical of the upper troposphere. This is because the current theory is an empirical one, relying on scant observations in the laboratory or the atmosphere to provide coefficients for the empirical description. High resolution aircraft data is necessary in the troposphere to verify the models. To meet the need for high resolution measurements in the upper troposphere, we at Imperial College have built an aircraft instrument. The Tropospheric Airborne Fourier Transform Spectrometer, TAFTS, is a Martin-Puplett polarising FTS, and is the first to operate in the spectral region of interest. TAFTS has a spectral resolution of 0.12cm-1 over a range from 80 - 750cm-1, although this range will be extended to 1000cm-1 in the near future. TAFTS views both in the nadir and the zenith alternately, by the use of steerable mirrors. The spectral range is divided into two spectral bands, with the Si:Sb detector covering the shortwave end between 320 - 750cm-1, whilst the Ge:Ga detector covers the longwave 80 - 320cm-1 region, both detectors are helium cooled. The EMERALD campaign was a collaboration of three UK universities, and a team from DLR, in Germany, and took place in September 2001. Its aim was to study the radiation and microphysical properties of cirrus cloud, using two aircraft run by Airborne Research Australia, in Adelaide, Australia. For this campaign TAFTS was installed upon ARA's Egrett. The Egrett is a slow flying high altitude aircraft that can reach heights of 15km. During the campaign we obtained both clear-sky and cirrus emission measurements, in the far-IR. This was the first measurement campaign that TAFTS has been involved in, the results from which will be presented at this meeting.
Supplementary URL: http://www.sp.ph.ic.ac.uk/tafts/
Session 8, High Spectral Resolution Measurements
Friday, 7 June 2002, 10:30 AM-12:00 PM
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