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

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

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

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

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

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)

Line-by-line radiative transfer modelling using LBLRTM

LBLRTM(S. Clough et al)

WV profileT profile Other absorbing gases

Clear sky radiances

HITRAN 2004database

Water vapour continuum model (MT_CKDv2.0)

Instrument response function

MODELLED RADIANCESPECTRUM

10th March 2007 1401 UTC TAFTS vs. LBLRTM

• Water vapour profile taken from RS-92 launched at 1333 UTC

10th March 2007 1401 UTC – PWV sensitivity

10th March 2007 1401 UTC – Cfgn sensitivity

Sensitivity to PWV and Cfgn

0.75

0.95

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

Sensitivity to PWV and Cfgn

0.72

1.17

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

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)

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/rhubcInitial RHUBC results:

http://www.arm.gov/publications/proceedings/conf18/index.phpneil.humpage05@imperial.ac.uk