Abstract
Abstract. The evolution of the vertical distribution and optical properties of aerosols in the free troposphere, above stratocumulus, is characterized for the first time over the Namibian coast, a region where uncertainties on aerosol–cloud coupling in climate simulations are significant. We show the high variability of atmospheric aerosol composition in the lower and middle troposphere during the Aerosols, Radiation and Clouds in southern Africa (AEROCLO-sA) field campaign (22 August–12 September 2017) around the Henties Bay supersite using a combination of ground-based, airborne and space-borne lidar measurements. Three distinct periods of 4 to 7 d are observed, associated with increasing aerosol loads (aerosol optical thickness at 550 nm ranging from ∼0.2 to ∼0.7), as well as increasing lofted aerosol layer depth and top altitude. Aerosols are observed up to 6 km above mean sea level during the later period. Aerosols transported within the free troposphere are mainly polluted dust (predominantly dust mixed with smoke from fires) for the first two periods (22 August–1 September 2017) and smoke for the last part (3–9 September) of the field campaign. As shown by Lagrangian back-trajectory analyses, the main contribution to the aerosol optical thickness over Henties Bay is shown to be due to biomass burning over Angola. Nevertheless, in early September, the highest aerosol layers (between 5 and 6 km above mean sea level) seem to come from South America (southern Brazil, Argentina and Uruguay) and reach Henties Bay after 3 to 6 d. Aerosols appear to be transported eastward by the midlatitude westerlies and towards southern Africa by the equatorward moving cut-off low originating from within the westerlies. All the observations show a very complex mixture of aerosols over the coastal regions of Namibia that must be taken into account when investigating aerosol radiative effects above stratocumulus clouds in the southeast Atlantic Ocean.
Highlights
The western coast of southern Africa is a complex area in terms of both atmospheric composition, circulation and climate, with aerosol–radiation–cloud interactions playing a significant role
The available lidar observations performed over the coastal site of Henties Bay allowed to highlight three contrasted periods of biomass burning aerosol transport (P1, P2 and P3)
The inversion of the ground-based lidar profiles was carried out using the constraints provided by the aerosol types of the CALIOP and CloudAerosol Transport System (CATS) space-borne instruments and the photometric measurements from Aerosol Robotic Network (AERONET) network
Summary
The western coast of southern Africa is a complex area in terms of both atmospheric composition, circulation and climate, with aerosol–radiation–cloud interactions playing a significant role. State-of-the-art climate models diverge by several W m−2 when attempting to calculate the regional direct radiative effect over coastal southern region (Myhre et al, 2013; Stier et al, 2013) ranging from negative (−3 W m−2) to strong positive forcing (+5 W m−2) for mean seasonal averages These model shortcomings, that can affect the simulation of climate features in distant areas (e.g. rainfall anomalies in Brazil, the position of the Intertropical Convergence Zone; Jones et al, 2009; Jones and Haywood, 2012), are mainly due to a limited knowledge of the aerosol properties, the vertical position of aerosol and cloud layers, and the distribution of cloud properties with and without aerosol present (Zuidema et al, 2016). The description of the ground-based lidar is given in Appendix A, together with the calibration and data inversion processes
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