Infrared continuum water vapor absorption coefficients derived from satellite data

Area of Science:

• Atmospheric Science
• Radiative Transfer
• Spectroscopy

Background:

• Water vapor continuum absorption is crucial for atmospheric radiative transfer models.
• Previous studies and atmospheric codes (HITRAN, LOWTRAN-7, LOWTRAN-6) show variations in water vapor absorption coefficients.
• Discrepancies exist between different experimental measurement techniques.

Purpose of the Study:

• To critically review and compare experimental measurements of water vapor continuum absorption coefficients in the 8-13 micrometer spectral region.
• To investigate the temperature dependence of these absorption coefficients.
• To reconcile differences between various experimental techniques and atmospheric models.

Main Methods:

• Review of existing literature on ground-based techniques for measuring water vapor absorption coefficients.
• Description of a novel experimental method using satellite-measured infrared radiances and sea surface temperature data.
• Comparison of experimental results with absorption coefficients used in atmospheric transmission codes (HITRAN, LOWTRAN-7, LOWTRAN-6).

Main Results:

• Grant's review indicated HITRAN and LOWTRAN-7 coefficients align with measurements, approximately 20% lower than LOWTRAN-6.
• A new method suggests coefficients are 20-40% larger than LOWTRAN-6.
• Reconciliation requires a stronger negative temperature dependence, increasing coefficients significantly below 270 K.

Conclusions:

• Discrepancies among measurement techniques highlight the need for accurate water vapor continuum absorption coefficients.
• A stronger negative temperature dependence is essential for agreement between different experimental methods.
• The findings impact the accuracy of atmospheric radiative transfer models, particularly in cold conditions.