Atmospheric radiative effects of an in situ measured Saharan dust plume and the role of large particles

Abstract

Abstract. This work will present aerosol size distributions measured in a Saharan dust plume between 0.9 and 12 km altitude during the ACE-2 campaign 1997. The distributions contain a significant fraction of large particles of diameters from 4 to 30 µm. Radiative transfer calculations have been performed using these data as input. Shortwave, longwave as well as total atmospheric radiative effects (AREs) of the dust plume are investigated over ocean and desert within the scope of sensitivity studies considering varied input parameters like solar zenith angle, scaled total dust optical depth, tropospheric standard aerosol profiles and particle complex refractive index. The results indicate that the large particle fraction has a predominant impact on the optical properties of the dust. A single scattering albedo of ω o =0.75−0.96 at 550 nm was simulated in the entire dust column as well as 0.76 within the Saharan dust layer at ∼4 km altitude indicating enhanced absorption. The measured dust leads to cooling over the ocean but warming over the desert due to differences in their spectral surface albedo and surface temperature. The large particles absorb strongly and they contribute at least 20% to the ARE in the dusty atmosphere.From the measured size distributions modal parameters of a bimodal lognormal column volume size distribution were deduced, resulting in a coarse median diameter of ∼9 µm and a column single scattering albedo of 0.78 at 550 nm. A sensitivity study demonstrates that variabilities in the modal parameters can cause completely different AREs and emphasises the warming effect of the large mineral dust particles.

Direct radiative effects during intense Mediterranean desert dust outbreaks

“…8b). Along the dust plume, with AODs reaching up to 1.40, moderate concentrations (up to 0.5 × 10 −6 kg m −3 ) are simulated between 15 and 20 • N, being low (less than 0.2 × 10 −6 kg m −3 ) between 20 and 25 • N while the maximum ones (higher than 2 × 10 −6 kg m −3 ) are recorded between 25 and 35 • N. Due to the emission of LW radiation by mineral particles, dust aerosols cool the atmospheric layers (Otto et al, 2007) in which they reside, by up to 0.8 K, and increase the temperature, by up to 0.4 K, just above the dust layer. Between the bottom of the dust layer and surface, positive RADON-RADOFF temperature differences (i.e.…”

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.…”

“…Its basic features are described in previous papers (Otto et al, 2007). The following issues were revised or extended:…”

“…The simulations are performed separately for solar, thermal and total spectral range. Otto et al (2007) computed the spectral single scattering albedo of a dust plume over the Canary Islands based on literature data of the spectral complex refractive index of dust. Large particles (D e >∼ 3 µm), which were observed at lower altitudes, led to smaller values of at visible wavelengths (Solmon et al, 2008) compared to higher altitudes where no large particles were found.…”

“…It turned out that SSA affects surface forcing stronger than reasonable changes in the asymmetry parameter (ASP) and surface albedo. In combination with ASP these authors found that, keeping in mind that large particles enhance ASP and diminish SSA in the solar spectral range (Otto et al, 2007), a decrease/increase (and vice versa) in SSA/ASP results in radiative effects at the bottom of the atmosphere (BOA) counteracting each other. However, at the top of the atmosphere (TOA) the same changes in both SSA and ASP lead to radiative effects having the same sign.…”