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David Painemal MPO531

Determination of the optical thickness and effective radius from reflected solar radiation measurements. David Painemal MPO531. Outline. Theory Applications Results Conclusions. Theory.

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David Painemal MPO531

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  1. Determination of the optical thickness and effective radius from reflected solar radiation measurements David Painemal MPO531

  2. Outline • Theory • Applications • Results • Conclusions

  3. Theory • The asymptotic theory: The reflection (and transmission) properties of thick layer depend essentially on three parameters of the atmosphere, the scaled optical thickness (t’), the similarity parameter (s) and reflectivity of the underlying surface. S= 0: conservative scattering • S=0 for 1m.  • 1.65m2.16m, S sensitive to re. re S as a function of wavelength for selected values of the effective radius

  4. Theory • We chose the wavelengths: 0.75m and 2.5m • Outside of water vapor and oxygen absorption. • Reflection at 2.16m: sensitive to re • Reflection at 0.75m: sensitive to   We can estimate re andseparately Theoretical relationships between the reflection function at 0.75m and 2.16 m for various values of  and r

  5. Theory • Optical thickness at 0.75m does not depend strongly on re. • Reflection function at 2.16m is independent of the optical thickness. Reflection function as a function of r for different values of , and azimuth angle.

  6. Determination of  and re • We assume that measurements have a relative precision • Minimizing , we obtain  and r.

  7. Determination of  and re Measurements of reflectance albedo,  Minimum  , re

  8. Determination of  and re at 0.75 and 2.16m at 0.75, 1.65 and 2.16m at 0.75, 1.65 2.16 and 3.7m at 0.75 and 3.7 m

  9. Determination of  and re • Two minima regardless of the number of channels. • The introduction of a third channel at 1.65m does not improve the retrieval for liquid water clouds.

  10. Stratocumulus observations: • Why? • Uniform layer • Dark ocean surface, reducing problems associated with surface reflection. • Liquid water droplets: Mie scattering by spherical particles is applicable

  11. Marine Sc observations • Comprehensive measurements off the coast of California 29 June-18 July • The intent of this work is to provide comparisons of remote sensing and in situ estimates of cloud properties.

  12. Instrumentation • Aircraft measurements (7, 10, 13 and 16 July) • ER-2 aircraft: 18 km of altitude.Spectral scanning radiometer, seven channel narrowband solar flux radiometer, spectral scanning radiometer. • C-131A aircraft: within the clouds. Measurements of cloud microphysics. • Satellite measurements on 2 days (7 and 16 July.

  13. Effective radius • Good spatial correlation • Overestimation • Remote sensing based on reflectivity is insensitive to drizzle.

  14. Optical thickness • Geometric thickness assumed constant

  15. Including additional absorption • Overestimation of effective radius  It is necessary to introduce additional absorption for water vapor. • We can adopt a gaseous volume extinction=0.6km-1.

  16. Conclusions • A statistical technique has been described for inferring optimum values of r and . • Reflection function at: • 2.16 m is primarily sensitive to re. • 0.75 m is primarily sensitive to . • Comparisons between in situ and remote sensing estimates of: • Effective radius • Correlated variables • Remote sensing overestimates the radius of cloud droplets. • Optical thickness: • Close agreement. • Problems with Johnson-Williams hot wire probe? • The discrepancy between in-situ and remote sensing estimates of re can be explained by additional absorption by water vapor at 2.16m.

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