David Paynter, Igor Ptashnik, Keith Shine Department of Meteorology University of Reading Kevin Smith (RAL) June 2006 Pure water vapor continuum measurements.

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.