David Paynter, Igor Ptashnik, Keith Shine Department of Meteorology

University of Reading

Kevin Smith (RAL)

June 2006

Pure water vapor continuum measurements between 3100 and 4400 cm-1 :

Evidence for water dimer absorption in near atmospheric conditions

Key Facts:

•Used a FT-IR spectrometer at MSF RAL to measure the absorption of a known amount of pure water vapour through a multi-pass cell.

•Measured spectrum between 1200 cm-1 and 8000 cm-1 – Near infrared

• Resolution of 0.03cm-1 -No Apodization

Temperature 296-351K Pressure 20mb -350mb

Deriving the Continuum

)()()( vvv Tmc Theory calculated Using Lorentzian lines out to 25cm-1. Line data from HITRAN 2004

Conclusions

•Evidence for water dimer absorption between 3000 and 4200 (1/cm )

•Temperature dependence of features consistent with that of the S&K dimer.

•In other regions the continuum absorption agrees ‘well’ with the MT_CKD continuum

•Considerable uncertainty of continuum absorption in the wings and of Continuum absorption under 300 K.