The impact of an eastward traveling cut-off low on tropospheric composition and the formation of a river of smoke: a case from AEROCLO-sA

We investigate the impact of an eastward moving cut-off low (CoL) on the formation of a river of smoke as well as on the vertical distribution of biomass burning aerosols (BBAs) in the troposphere during the Aerosols, Radiation and Clouds in southern Africa (AEROCLO-sA) campaign in September 2017. The CoL developed in the westerlies over the Southeast Atlantic Ocean and advected over southern Africa between 1 and 6 September 2017. Northern Namibia, were most of the AEROCLO-sA related operations took place, was under the influence of a well formed, stationary, isolated CoL on 3 and 4 September. Subsequently, the fast evolving CoL travelled south-eastward over South Africa between 5 and 6 September before merging back with the main westerly flow. Based on the use of a tailored complementary suite of global and mesoscale numerical simulations as well as ground-based, airborne and space-borne observations of the atmospheric dynamics, thermodynamics and composition, the picture emerges that the characteristics of the river of smoke (timing, vertical extent of the BBA layer) are very much tied to the later (fast evolving) stage of the evolution of the CoL than the earlier (stationary) stage.  The mechanisms by which the CoL observed over southern Africa influences the vertical structure of the BBA layer is essentially through the ascending (descending) motion above the BBA layer to the northeast (southwest) of the CoL center. In the presence of the CoL, the top of the BBA layer over northern Namibia was found to reach attitudes in excess of 8 km AMSL. This is much higher that the height of the top of the BBA layer over the regions where the smoke originated from (Angola, Zambia, Zimbabwe, Mozambic) which was between 4 and 5 km AMSL. Also, the CoL favored the formation of mid-level clouds near the top of the BBA layer. Mid-level clouds were embedded in the river of smoke that were related to the circulation and ascending motions in the lee of the CoL.