Effects of nonsphericity on Saharan dust optical properties

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Pergamon

po: 50011-8501(96)0035501

EFFECTS OF NONSPHERICITY ON

K. WENZEL, W.

J. A",,.,,I &/.• Vol. 27. Suppl. I. pp. SS6S-S566. 1996 Copyrishl C 1996 Eloeviet Science LId Printed In 0 ..... 8ritain. All rish'" raenecI

0021-8S02/96 SI$.OO + 0.00

SAHARAN DUST OPTICAL PROPERTIES

VON HOYNINGEN-HUENE, S. SCHIENBEIN

Institut fUr Meteorologie, Universitiit Leipzig, Stephanstr.3, D-04103 Leipzig, Germany Email: [email protected]@rzaix340.rz.uni-Ieipzig.de

KEYWORDS Saharan Dust; Optical Properties; Nonsphericity; Saharan dust particles typically are non-spherical (Schutz et a1. 1990). Particle nonsphericity increases the side scattering and leads to a decrease in the asymmetry parameter. But the up to now used aerosol models (d' Almeida et a1. 1991, SchultIKoepke 1995) assume spherical particle shape in every case for the evaluation of the radiative influence and therefore underestimate the aerosol contribution to the radiative forcing. With Sun-and-Sky-Radiometers the spectral optical thickness in the solar range (0.35 - 1.2 um) and the skybrightness in the almucantar (0.550 and 0.850 urn) is measured. Combined with a Coupled Inversion Radiation Transfer Program (CmATRA, Wendischlvon Hoyningen-Huene 1994) the main climate-relevant radiative aerosol parameters (aerosol size distribution, real part of the refractive index, phasefunction, asymmetry parameter) are determined considering also particle nonsphericity with the help of the semi-empirical theory of PollacklCuzzi. The figures on the following side show the results of measurement and computation for May 28th 1995, from a campaign near Dakar (Senegal). The relative optical thickness already is corrected for Rayleigh-scattering and shows the typical absorption bands of ozone and water vapour. The ozone content can be estimated by a fit curve for the absorption peak. The size distribution is inverted from the spectral aerosol optical thickness and the aureole data of the skybrightness measurement. The comparison between the phasefunction of the sky as derived from the skybrightness and the computed phasefunctions from Mie-Theorie and Pollack/Cuzzi shows an enhanced side scattering as it is typical for nonspherical particles. The results of the interpretation of the complete measuring campaign data shall yield a more detailed aerosol model for radiative calculations, that takes into concern the enhanced sidescattering of the nonspherical particles as they dominate in the regime of the Saharan dust plume over the Atlantic and shall help to quantify the effect of nonsphericity for climate forcing.

ACKNOWLEDGEMENTS We thank the Deutsche Forschungsgemeinschaft (DFG) for supporting this work.

REFERENCES d'Almeida et al. (1991), Atmospheric aerosols: global climatology and radiative characteristics, A. Deepak Publishing L. W. Schutz et a1. (1990), in: A. H. Knap (ed.), The Long-Rang Atmospheric Transport of Natural and Contaminant Substances, Kluwer Academic Publishers

I. Schult I P. Koepke (1991), European Geophys. Soc.• 20th General Assembly, Hamburg 3.-7.4. 1995, Annales Geophysicae, Supp1. to Vol. 13, p. C389 M. Wendischl W. von Hoyningen-Huene (1994), Atm. Env, 28, 785-792 5565

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Some Measurements and Results for Saharan Dust Properties (Senegal, 28th May 1995)