current status on the continuum derived from msf ral data

1

Current status on the continuum derived from MSF RAL data

Igor V. Ptashnik

Department of Meteorology, University of Reading

Leicester, 24 September 2009

2

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

(JGR, in print)

1000 2000 3000 4000 5000 6000 700010-5

10-4

10-3

10-2

10-1

100

101

102

mcr

cm-1

Dimers HWHM=25 cm -1, Keq=0.041 atm -1, Curtiss et al. (1979)/Path length = 10 m; Pure H2O (15 mbar), T=293K/

Op

tic

al

de

pth

HITRAN-2001 with CKD-2.4 Dimer: Schofield and Kjaergaard (2003) CKD-2.4 model

9 8 7 6 5 4 3 2

+2

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

Sophisticated ab initio calculations of water dimer spectra at close to room-temperatures are required.

1300 1400 1500 1600 1700 1800 19000.0

0.2

0.4

0.6

0.8

1.0

1.2Keq=0.04 atm-1, HWHM=30 cm-1

17.85 mb pure H2O

Threshold=3; Grad.=0.05

Cro

ss

-se

cti

on

(c

m2 m

ole

c-1a

tm-1

)

Wavenumber (cm-1)

MT_CKD model Dimers (S&K, 2003) Burch, 1981 /308K/ Tobin et al. 1996 /296 K/ RAL (2007) /295K, 128m/

3400 3500 3600 3700 3800 39000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Wavenumber, cm-1

/Res. 0.01 cm-1; 293K; 512.75m; 1.15mm apert., f=418.0mm/

Keq=0.04 atm-1, hwhm=30 cm-1

15.3 mb purer H2O

Cs,

10

-20 c

m2 *

mo

lec

-1at

m-1

CKD-2.4 MT_CKD WD (S&K-2003) WD (B&P <= Slipchenko) Burch low-res. (1985) MSF RAL hi-res. (2007)

5000 5100 5200 5300 5400 5500 56000.0

0.2

0.4

0.6

0.8

1.0

1.2


Keq=0.04 atm-1, hwhm=30 cm-115.3 mb pure H2O

opt.depth/273.15*293/2.69e19/0.0151^2/51275

Everything (excluding Burch data) is with the 'Base term' subtracted !!!

Wavenumber, cm-1


Cs,

1

0-21 c

m2 *m

ole

c-1a

tm-1

CKD 2.4, 293K MT_CKD 1.10, 293K Dimer theory (S&K-2003) Ptashnik et al. (2004) 299K MSF RAL (2007), 293K

6900 7000 7100 7200 7300 7400 75000.0

0.1

0.2

0.3

0.4

0.5

0.6/Res. 0.01 cm-1; 293K; 512.75m; 1.15mm apert., f=418.0mm/

Keq=0.04 atm-1, hwhm=30 cm-1

Wavenumber, cm-1

15.3 mb pure H2O

Cs,

10

-21 c

m2 m

ole

c-1

atm

-1

CKD-2.4 MT_CKD WD (S&K-2003) MSF RAL (2007), 293K

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CAVIAR measurements at MSF RALRetrieval of the in-band water vapour self-continuum

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

3) Well pronounced spectral features

Converting to the "continuum" units

TTT,CT,Cs ooo )(exp)()(

)/(1)/exp()/(1)/exp(

oeoe

eeoos kTDkTD

kTDkTDTT

CCk

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

2) Strong negative T-dependence:

MT_CKD continuum model (1989):

Vigasin (JQSRT, 2000), water dimer model:

Under investigation…

3400 3500 3600 3700 3800 39000.0

0.5

1.0

Thresh=4, Grad=1 Boxcar apodiz.!; 2mm, 418mm, res=0.03cm-1

Op

tical d

ep

th

30.3 mb 40.2 mb 60.3 mb

cm-1

317K

3400 3500 3600 3700 3800 39000

2

4

6

8

10Thresh=4, Grad=1 Boxcar apodiz.!; 2mm, 418mm, res=0.03cm-1

Cs , 10-2

1 cm

2m

ole

c-1atm

-1

30.3 mb 40.2 mb 60.3 mb

cm-1

317K

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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.

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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

2000 3000 4000 5000 6000 7000

10-25

10-24

10-23

10-22

All continuums (except Burch data) are with the 'Base term' subtracted !!!

Wavenumber, cm-1

Cs,

cm

2 *mo

lec-1

atm

-1

CKD-2.4, 351K MT_CKD, 351K Watkins et al. (1979) / 5.5 Burch (1981) 350K MSF RAL (2007), 351K

? ??

?2.1 mcr

1.6 mcr

3.8 mcr

2000 3000 4000 5000 6000 7000

10-25

10-24

10-23

10-22


Wavenumber, cm-1

Cs,

cm

2 *mo

lec-1

atm

-1

CKD-2.4, 351K MT_CKD, 351K Watkins et al. (1979) / 5.5 Burch (1981) 350K MSF RAL (2007), 351K

? ??

?2.1 mcr

1.6 mcr

3.8 mcr? ?

??

2.1 mcr1.6 mcr

3.8 mcr

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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

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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 at MSF RAL: Band wings (3.8 mcr)

1.E-25

1.E-24

1.E-23

1.E-22

1.E-21

1.E-20

1.E-19

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

Wavenumber, cm-1

Cs,

cm

-1(a

tm*m

ole

c/cm

3 )-1

NIST310.8325.8351.9

CKD310.8325.8351.9

Watkins, 1979298K


WD Lorentzian wings Burch (1981) 350K MSF RAL (2007), 293 K MSF RAL (2007), 351K

2.1 mcr3.8 mcr

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CAVIAR measurements

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

visible near IR mid IR far IR

MeasurementsRAL (MSF) FTS

Camb/Leics CDRS

NPL, FTS

MetOffice, ARIES

MetOffice, SWS

Imperial, TAFTS

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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…

current status on the continuum derived from msf ral data

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