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University of Reading (2009)

Aircraft measurements of Saharan mineral dust

Mcconnell, Claire Louise

Titre : Aircraft measurements of Saharan mineral dust

Auteur : Mcconnell, Claire Louise

Université de soutenance : University of Reading

Grade : Doctor of Philosophy (Ph.D.) 2009

Mineral dust is an important component of the Earth’s atmosphere, affecting climate through the direct radiative effect and through the deposition of dust to the ocean. Understanding of these processes is limited by a lack of in-situ observations of dust which results in a large uncertainty in the microphysical, optical, chemical and radiative proper ties of dust. This thesis presents an analysis of dust measurements obtained during the DODO (Dust Outflow and Deposition to the Ocean) aircraft campaigns which took place in February and August 2006 over West Africa and the tropical East Atlantic Ocean. In-situ and radiative measurements are used to examine the variations in optical, microphysical and radiative properties of the dust in the two seasons. Specific limitations relating to the nephelometer and upper pyranometer are overcome through aircraft, ground-based and model comparisons and appropriate correctional procedures have been applied. Vertical profiles of dust extinction and size distributions differed between land and ocean areas, and between the dry and wet seasons. Dry season dust was found at low altitudes, whereas wet season dust extended up to 6km, reflecting the seasonal meteorology. Measurements of the single scattering albedo for the accumulation mode at 550nm are found to range from 0.93 to 0.99. This variation is related to differences in chemical composition and dust sources, but not to changes in the measured size distributions. Optical properties are found to be sensitive to inclusion of coarse mode size distribution measurements. The range of optical properties observed are modelled in a radiative transfer code in order to investigate their impact on the dust shortwave direct radiative effect. The results are compared to irradiance measurements from the aircraft pyranometers, which have undergone a detailed quality assessment. The range of optical properties cause a difference of up to a factor of five in atmospheric heating, and can alter the sign of the top of atmosphere radiative effect. The variations in the optical properties measured during the campaigns are therefore significant and have implications for accurate dust simulations in models and for satellite retrievals.

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