Calculation of Optical Depth

The aerosol optical thickness (for a given wavelength) is a dimensionless quantity defined as the extinction integrated along the optical path, with respect to a reference altitude level.

Optical Depth

Many authors [Brogniez and Lenoble, 1987; Brogniez and Lenoble, 1991; Trepte et al., 1994] take this level to be 2 km above the local tropopause level zT. This choice is the result of two considerations. First, it is supposed to reasonably prevent the optical thickness to be affected by possible clouds. Secondly, the altitude surfaces of equal aerosol extinction roughly follow the global tropopause surface.

Extinction coefficients ß(λ,z) used in the integral are interpolated as described in the spectral interpolation section. The local tropopause level for given latitude, year and month is obtained from Cospar international reference atmosphere [D. Rees et al. 1986]. For location were no measures are available at zT, the first altitude above zT for which interpolated extinction coefficients are available is used as reference altitude.




References:

  • Brogniez, C. and Lenoble J., "Modelling of stratospheric background aerosols from zonally averaged SAGE profiles", J. Geophys. Res., 92, 3051-3060, 1987.
  • Brogniez, C. and Lenoble J., "Analysis of 5-year aerosol data from the Stratospheric Aerosol Gas Experiment II", J.
  • Geophys. Res., 96, 15, 479-15,497, 1991.
  • Trepte, C. R., Thomason, L. W., and Kent, G. S., "Banded structures in stratospheric aerosols distributions", Geophys. Res. Lett., 21, 2397-2400, 1994.
  • Rees, D., Barnett, J. J., and Labitzke, K., "Cospar international reference atmosphere:1986, Part II: middle atmosphere models", ed.(Pergamon Press, Oxford, 1986), pp (12)430-(12)441.