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Far-infrared Spectral Radiance Observations of the Arctic Atmosphere

Far-infrared Spectral Radiance Observations of the Arctic Atmosphere. Neil Humpage, Paul Green: Imperial College London Dave D. Turner: University of Wisconsin – Madison Eli Mlawer: AER Inc, Lexington, Massachusetts

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Far-infrared Spectral Radiance Observations of the Arctic Atmosphere

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  1. Far-infrared Spectral Radiance Observations of the Arctic Atmosphere Neil Humpage, Paul Green: Imperial College London Dave D. Turner: University of Wisconsin – Madison Eli Mlawer: AER Inc, Lexington, Massachusetts Ed Westwater: Cooperative Institute for Research in Environmental Sciences, Boulder, Colorado

  2. Overview • Why make far-infrared observations of the arctic atmosphere? • Description of RHUBC: • Spectrometers – TAFTS and AERI-ER • Other instruments • Line-by-line radiative transfer modelling using LBLRTM • Case study: 10th March 2007 • Sensitivity of modelled radiance spectra to column water vapour and foreign broadened H2O continuum strength • Comparison with TAFTS and AERI-ER observed spectra • Summary

  3. Why make far-infrared observations of the arctic atmosphere? • The Earth’s radiative emission spectrum peaks in the FIR • 27-35% of clear sky OLR is at far-infrared wavelengths (Sinha and Harries 1995) • Cooling of the upper troposphere to space is predominantly via the H2O rotation band (Clough et al 1992) • Strength and nature of the water vapour continuum remains uncertain – few atmospheric observations to date (Tobin et al 1999, Serio et al 2008) • Very dry arctic winter atmosphere enables ground based observations of water vapour absorption spectroscopy

  4. Radiative Heating in Under-explored Bands Campaign • RHUBC aims: • Water vapour spectroscopy through clear sky observations • Instrument cross-calibration and validation (TAFTS vs. AERI-ER) • Investigation of radiative properties of sub-arctic cirrus • February 22nd – March 14th 2007 • Typical temperatures of -30°C • Low PWV • February 2007 median: 1.76mm • March 2007 median: 1.47mm

  5. Spectrometers involved in RHUBC • TAFTS • 80 – 600 cm-1 spectral range • 0.12 cm-1 resolution • 2 second scan time • Higher resolution, range extends further into far-infrared • AERI-ER • 400 – 3300 cm-1 spectral range • 1.0 cm-1 resolution • 20 second scan time • Range includes mid-infrared window region and CO2 absorption band

  6. Auxiliary data available during RHUBC • Vaisala RS-92 sondes • Twice a day, up to once every 2 hours in low PWV conditions • Temperature and RH profiles • GSR • Ground-based Scanning Radiometer • Measures 183 GHz water vapour absorption line (PWV retrieval) • Retrieved PWV within 5% of sonde observations (Westwater et al 2008 ARM Science Meeting)

  7. Line-by-line radiative transfer modelling using LBLRTM Other absorbing gases T profile WV profile LBLRTM (S. Clough et al) Water vapour continuum model (MT_CKDv2.0) HITRAN 2004 database Clear sky radiances Instrument response function MODELLED RADIANCE SPECTRUM

  8. 10th March 2007 1401 UTC TAFTS vs. LBLRTM • Water vapour profile taken from RS-92 launched at 1333 UTC

  9. 10th March 2007 1401 UTC – PWV sensitivity

  10. 10th March 2007 1401 UTC – Cfgn sensitivity

  11. Sensitivity to PWV and Cfgn 0.75 0.95

  12. 10th March 2007 1401 UTC AERI-ER vs. LBLRTM

  13. Sensitivity to PWV and Cfgn 0.72 1.17

  14. TAFTS and AERI-ER vs. LBLRTM – PWV and Cfgn rescaled

  15. Summary • The far-infrared is a significant yet under-studied component of the Earth’s radiative energy budget • High resolution spectral measurements important for validating line-by-line radiative transfer models • In dry conditions, the far-infrared spectrum observed from the ground is highly sensitive to changes in PWV and foreign broadened continuum strength • Further observations of this kind required to validate (and if necessary revise) the foreign broadened continuum formulation at far-infrared wavelengths (e.g. Serio et al 2008)

  16. Acknowledgements • The RHUBC team: • Dave Turner (U. Wisconsin) • Eli Mlawer (AER Inc.) • Ed Westwater (CIRES) • Jeff Zirzow (Sandia Labs) • Mark Ivey (Sandia Labs) • Nico Cimini (CIRES) • … plus many more, including all the staff based at the ARM-NSA site • The TAFTS team (Imperial): • Paul Green • Caroline Cox • Juliet Pickering • Alan Last • John Harries Campaign website: www.science.arm.gov/rhubc Initial RHUBC results: http://www.arm.gov/publications/proceedings/conf18/index.php neil.humpage05@imperial.ac.uk

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