1. Far-infrared spectroscopy of atmospheric water vapour
Cathryn Fox

2. Why study the far-infrared? The TAFTS instrument
Overview
Why study the far-infrared?
The TAFTS instrument
The water vapour continuum
Modelling of the FIR spectrum with LBLRTM
The RHUBC campaign

3. The importance of the far-infrared
The Earth’s radiative emission spectrum peaks in the far infrared
FIR contributes 27-35% of clear sky out-going longwave radiation
Water vapour absorbs in a pure rotation band below 667 cm-1

4. Heating rate diagrams Tropical Standard Atmosphere
Sub-arctic winter Standard Atmosphere
Brindley & Harries (1998)

5. The TAFTS instrument
Tropospheric Airborne Fourier Transform Spectrometer
Dual-input Martin-Puplett (polarizing) FTS
Can measure both up- and down-welling radiation
4 detectors take measurements in range:
80-300cm-1 (2 x Ge:Ga)
cm-1 (2 x Si:Sb)
Integrated black body calibration
Resolution: 0.12cm-1
TAFTS has been employed in various data
campaigns, both ground-based and aboard
the NERC/UKMO FAAM BAe-146 research aircraft

6. The water vapour continuum
Water vapour is observed to absorb with two components:
monomer line absorption – around 50,000 spectral lines currently cataloged in databases such as HITRAN
the continuum - a slowly varying underlying absorption, contributes up to 40% of the total longwave cooling rate
necessary to reconcile observations with models
physical mechanism is not yet fully understood
competing theories exist - enhanced far wing absorption or dimers
Models currently use a semi-empirical formulation (MT-CKD) of the continuum which has not been tested at all wavelengths
LBLRTM simulations of downwelling radiances to demonstrate the water vapour continuum. The black spectrum includes the MT-CKD model, the blue spectrum does not. Taken from Beeby (2012)

7. LBLRTM
The Line-By-Line Radiative Transfer Model is used to create simulations of spectra based on input atmospheric profiles.
The water vapour continuum strength in these simulations can be fractionally adjusted
The observed spectra from TAFTS are interpolated onto the simulations to derive the most accurate continuum strength
N. Humpage (2010)

8. Recent measurements
R. Beeby (2012)

9. The RHUBC campaign Radiative Heating in Underexplored Bands Campaign
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
22 February to 14 March 2007
ARM NSA site in Barrow, AK
71° N ‘, 156° W ‘
Average temp -30C

10. Current findings
Observed (black) and simulated (blue) TAFTS clear sky spectra from 10th March The residual (simulation minus observation) is also shown.

11. Aims and summary
The far-infrared region plays an important role in the Earth’s radiative energy budget
Representation of water vapour in GCMs can be improved through spectral observations and LBLRTM modelling
Initial findings from RHUBC suggest a strengthening of the continuum is needed. Further calibration and simulations are currently in process.