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: 10 th March 2007 Sensitivity of modelled radiance spectra to column water vapour and foreign broadened H 2 O continuum strength Comparison with TAFTS and AERI-ER observed spectra Summary

3. Why make far-infrared observations of the arctic atmosphere? The Earths 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 H 2 O 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 22 nd – March 14 th 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 CO 2 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 LBLRTM (S. Clough et al) WV profile T profile Other absorbing gases Clear sky radiances HITRAN 2004 database Water vapour continuum model (MT_CKDv2.0) Instrument response function MODELLED RADIANCE SPECTRUM

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

9. 10 th March 2007 1401 UTC – PWV sensitivity

10. 10 th March 2007 1401 UTC – C fgn sensitivity

11. Sensitivity to PWV and C fgn 0.75 0.95

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

13. Sensitivity to PWV and C fgn 0.72 1.17

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

15. Summary The far-infrared is a significant yet under-studied component of the Earths 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