Relationship between asymmetry parameter and hemispheric backscatter ratio: implications for climate forcing by...

Area of Science:

• Atmospheric science
• Climate science
• Radiative transfer

Background:

• Direct climate forcing by anthropogenic aerosols relies on radiative transfer parameters like asymmetry parameter (g).
• The asymmetry parameter is often derived from measured backscatter ratio (b).
• Current conversion methods may introduce inaccuracies.

Purpose of the Study:

• To compare two methods for converting backscatter ratio to asymmetry parameter: Mie calculations and the Henyey-Greenstein (HG) phase function.
• To assess the impact of asymmetry parameter overestimation on aerosol climate forcing.
• To develop an improved empirical relationship for converting backscatter ratio to asymmetry parameter.

Main Methods:

• Mie calculations and Henyey-Greenstein (HG) phase function were used to derive conversion methods.
• Delta-Eddington radiative transfer calculations assessed the impact of asymmetry parameter errors on climate forcing.
• Empirical relationships were derived using Mie computations for log-normal accumulation-mode aerosols.

Main Results:

• The HG method systematically overestimates asymmetry parameter (g) for typical accumulation-mode aerosols.
• A 10% overestimation in g can reduce aerosol climate forcing by 12% or more.
• An empirical relationship derived from Mie computations allows accurate conversion of backscatter ratio to asymmetry parameter, dependent on size distribution breadth (σ(g)).

Conclusions:

• The Henyey-Greenstein method is unreliable for converting backscatter ratio to asymmetry parameter for aerosols.
• Accurate asymmetry parameter determination is crucial for precise climate forcing estimations.
• The newly derived empirical relationship offers a more reliable method for calculating aerosol climate forcing.