Current status on the continuum derived from msf ral data
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Current status on the continuum derived from MSF RAL data. Igor V. Ptashnik Department of Meteorology, University of Reading Leicester, 24 September 2009. (JGR, in print). . .  +. 2 . CAVIAR measurements at MSF RAL: 1200 - 8000 cm -1.

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Current status on the continuum derived from MSF RAL data

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Current status on the continuum derived from MSF RAL data

Igor V. Ptashnik

Department of Meteorology, University of Reading

Leicester, 24 September 2009


(JGR, in print)

+

2

CAVIAR measurements at MSF RAL:1200 - 8000 cm-1


CAVIAR measurements at MSF RAL:1200-8000 cm-1

Sophisticated ab initio calculations of water dimer spectra

at close to room-temperatures are required.


2) Strong negative T-dependence:

Roberts, Selby, Biberman(Appl.Opt., 1976):

MT_CKD continuum model (1989):

Vigasin (JQSRT, 2000), water dimer model:

Under investigation…

CAVIAR measurements at MSF RAL

Retrieval of the in-band water vapour self-continuum

1) Quadratic PH2O-dependence:Cs (PH2O)2

Converting to the

"continuum" units

3) Well pronounced spectral features


Arguments for the dimers (in the band centres)

1) The spectral features of the derived continuum are:

  • too large to be explained by possible uncertainties/errors in water vapour line parameters in HITRAN or by any known deviation from Voigt profile (Dicke narrowing, Line mixing etc.);

  • coincide reasonably well with known ab initio predictions and low-T matrix measurements of water dimers fundamental and combinational bands; this often includes expected frequency shift towards smoothed water monomer spectral features;

  • not described by any conventional continuum model;

  • have expected from dimer theory P- and T-dependence.

    2) Partitioning of the pair states in phase space(Epifanov & Vigasin, 1997; Schenter et al., 2002): The role of free pair states is almost negligible at near room temperatures; metastable and true bound states should dominate instead.


Measurements at MSF RAL:Water continuum in band wings

First estimation of the continuum absorption was made in the far band wings. Nearly an order of magnitude larger absorption than predicts MT_CKD model!

However, so far uncertainty is too big and further work is required. RAL has been carrying out new high-pressure measurements…


Measurements at MSF RAL:Water continuum in band wings

First estimation of the continuum absorption was made in the far band wings. Nearly an order of magnitude larger absorption than predicts MT_CKD model!

However, so far uncertainty is too big and further work is required. RAL has been carrying out new high-pressure measurements…


Measurements at MSF RAL:Band wings (3.8 mcr)

2.1 mcr

3.8 mcr

Very good agreement with recent measurements by Baranov & Lafferty at NIST in 3.8 mcr window from 293 to 351K, and with Lorentzian-shape approximation for WD band wings (light blue line).


Measurements

RAL (MSF) FTS

Camb/Leics CDRS

NPL, FTS

MetOffice, ARIES

MetOffice, SWS

Imperial, TAFTS

visiblenear IRmid IRfar IR

CAVIAR measurements

  • Laboratory (reference!) and field high-resolution measurements all over the spectral region from visible to far infrared


Conclusions

  • RAL measurements gave us firm evidence of dominating water dimer contribution to the water vapour self-continuum within near-IR water vapour bands.

  • The continuum absorption in the middle- and near-IR windows is still uncertain and must be one of the main (and urgent!) priorities for the current CAVIAR investigation at RAL.

  • Sophisticated ab initio calculations of water dimer spectra at room-temperatures are necessary to compare with experimental continuum.

Thank you…


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