Spectral discrimination of coarse and fine mode optical depth

  • O'Neill N
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The recognition that the aerosol particle size distribution (PSD) is effectively bimodal permits the extraction of the fine and coarse mode optical depths ($τ$f and $τ$c) from the spectral shape of the total aerosol optical depth ($τ$a = $τ$f + $τ$c). This purely optical technique avoids intermediate computations of the PSD and yields a direct optical output that is commensurate in complexity with the spectral information content of $τ$a. The separation into $τ$f and $τ$c is a robust process and yields aerosol optical statistics, which are more intrinsic than those, obtained from a generic analysis of $τ$a. Partial (optical) validation is provided by (1) demonstrating the physical coherence of the simple model employed, (2) demonstrating that $τ$c variation is coherent with photographic evidence of thin cloud events and that $τ$f variation is coherent with photographic evidence of clear sky and haze events, and (3) showing that the retrieved values of $τ$f and $τ$c are well-correlated, if weakly biased, relative to formal inversions of combined solar extinction and sky radiance data. The spectral inversion technique permitted a closer scrutiny of a standard (temporally based) cloud-screening algorithm. Perturbations of monthly or longer-term statistics associated with passive or active shortcomings of operational cloud screening were inferred to be small to occasionally moderate over a sampling of cases. Diurnal illustrations were given where it was clear that such shortcomings can have a significant impact on the interpretation of specific events; (1) commission errors in $τ$f due to the exclusion of excessively high-frequency fine mode events and (2) omission errors in $τ$c due to the inclusion of insufficiently high-frequency thin homogeneous cloud events.

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  • N. T. O'Neill

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