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
Current status on the continuum derived from MSF RAL data

Igor V. Ptashnik

Department of Meteorology, University of Reading

Leicester, 24 September 2009


Caviar measurements at msf ral 1200 8000 cm 1

(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).


Caviar measurements

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
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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