Solar radiative effects of a Saharan dust plume observed during SAMUM assuming spheroidal model particles

Abstract

Citing articles: 6 View citing articles Tellus (2009), 61B, 270-296 C were explored based on measured size-number distributions and chemical composition. The size-resolved complex refractive index of the dust was derived with real parts of 1.51-1.55 and imaginary parts of 0.0008-0.006 at 550 nm wavelength. At this spectral range a single scattering albedo ω o and an asymmetry parameter g of about 0.8 were derived. These values were largely determined by the presence of coarse particles. Backscatter coefficients and lidar ratios calculated with Mie theory (spherical particles) were not found to be in agreement with independently measured lidar data. Obviously the measured Saharan mineral dust particles were of non-spherical shape. With the help of these lidar and sun photometer measurements the particle shape as well as the spherical equivalence were estimated. It turned out that volume equivalent oblate spheroids with an effective axis ratio of 1:1.6 matched these data best. This aspect ratio was also confirmed by independent single particle analyses using a scanning electron microscope. In order to perform the non-spherical computations, a database of single particle optical properties was assembled for oblate and prolate spheroidal particles. These data were also the basis for simulating the non-sphericity effects on the dust optical properties: ω o is influenced by up to a magnitude of only 1% and g is diminished by up to 4% assuming volume equivalent oblate spheroids with an axis ratio of 1:1.6 instead of spheres. Changes in the extinction optical depth are within 3.5%. Non-spherical particles affect the downwelling radiative transfer close to the bottom of the atmosphere, however, they significantly enhance the backscattering towards the top of the atmosphere: Compared to Mie theory the particle non-sphericity leads to forced cooling of the Earth-atmosphere system in the solar spectral range for both dust over ocean and desert.

Physico‐chemical and optical properties of Sahelian and Saharan mineral dust: in situ measurements during the GERBILS campaign

“…Increasing the percentage fraction of oblate or prolate spheroid with AR > 3 did not modify the results. These results are in agreement with the work of Otto et al (2009) showing that the spherical/non-spherical differences influence ω 0 only within 1%.…”

“…The lower boundaries correspond to the GK aggregate and the higher boundaries to the IH aggregate. These values are in good agreement with recent estimates of the complex refractive index for African mineral dust Kandler et al, 2008;Müller et al, 2009;Otto et al, 2009;Petzold et al, 2009;McConnell et al, 2010).…”

“…Recent studies tend to show that the particle asphericity only induces differences in the order of few percent on ω 0 and g values (Mishchenko et al, 1997;Yang et al, 2007;Otto et al, 2009). Nonetheless, these weak differences can lead to additional cooling of the earth-atmosphere system in the solar spectrum compared to the case where particles are spherical, because of an enhancement of backscattering by the non-spherical mineral dust (Kahnert and Kylling, 2004;Kahnert et al, 2005;Otto et al, 2009).…”

On realistic size equivalence and shape of spheroidal Saharan mineral dust particles applied in solar and thermal radiative transfer calculations

“…As recently discussed, this particle fraction significantly increases the absorption of solar radiation, represented by a decreased single scattering albedo (SSA) in the visible spectral range (Otto et al, 2007;Otto et al, 2009;McConnell et al, 2010), which affects the radiative budget. The realistic determination of large particles and the absorbing properties of the dust is, hence, very important, since uncertainties in the latter may have big effects when regionally modelling scenarios of dust outbreaks over Africa: Solmon et al (2008) have shown that different cases of solar SSA result in different forcings and heating effects and, thus, cause different dynamical feedbacks, which influence regional precipitation in Northwest Africa.…”

Impact of dust aerosols on the radiative budget, surface heat fluxes, heating rate profiles and convective activity over West Africa during March 2006

“…Such studies (Tegen and Lancis, 1996;Myhre and Stordahl, 2001;Lelieveld et al, 2002;Huebert et al, 2003;Tanré et al, 2003;Abel et al, 2005;Ramanathan et al, 2007;Haywood et al, 2008;Heinold et al, 2009;Bierwirth et al, 2009;Otto et al, 2009) underline that fine aerosols (urban/industrial and/or smoke particles) decrease significantly surface incoming shortwave radiations and generally increase (or rarely decrease in some specific cases; Haywood and Shine, 1995) outgoing shortwave fluxes reflected back to space. Except in cases of pure scattering particles, the net effect for the atmosphere is positive revealing a gain of solar energy within the aerosol layer.…”