25 Nov 2021

25 Nov 2021

Review status: this preprint is currently under review for the journal ACP.

Smoke in the river: an AEROCLO-sA case study

Cyrille Flamant1, Marco Gaetani1,2,3, Jean-Pierre Chaboureau4, Patrick Chazette5, Juan Cuesta2, Stuart J. Piketh6, and Paola Formenti2 Cyrille Flamant et al.
  • 1Laboratoire Atmosphère Milieux Observations Spatiales (LATMOS)/IPSL, UMR CNRS 8190, Sorbonne Université, UVSQ, Paris, France
  • 2Université de Paris and Univ Paris Est Creteil, CNRS, LISA, F-75013 Paris, France
  • 3Scuola Universitaria Superiore IUSS, Pavia, Italy
  • 4Laboratoire d’Aérologie (LAERO), UMR CNRS 5560, Université de Toulouse, Toulouse, France
  • 5Laboratoire des Sciences du Climat et de l’Environnement (LSCE)/IPSL, UMR CNRS 1572, CEA, UVSQ, Gif-sur-Yvette, France
  • 6School of Geo- and Spatial Science, North-West University, Potchefstroom, South Africa

Abstract. The formation of a river of smoke crossing southern Africa is investigated during the Aerosols, Radiation and Clouds in southern Africa (AEROCLO-sA) campaign in September 2017. A complementary set of global and mesoscale numerical simulations as well as ground-based, airborne and space-borne observations of the dynamics, thermodynamics and composition of the atmosphere are used to characterize the river of smoke in terms of timing and vertical extent of the biomass burning aerosol (BBA) layer.

The study area was under the synoptic influence of a coastal low rooted in a tropical easterly wave, a high-pressure system over the continent and westerly waves in mid-latitudes, one of which had an embedded cut-off low (CoL). The coastal low interacted with the second of two approaching westerly waves and ultimately formed a mid-level temperate tropical trough (TTT). The TTT created the fast moving air mass transported to the southwestern Indian Ocean as a river of smoke. The CoL, which developed and intensified in the upper levels associated with the first (easternmost) westerly wave, remained stationary above northern Namibia prior to the formation of the TTT and was responsible for the thickening of the BBA layer.

This shows that the evolution of the river of smoke is very much tied to the evolution of the TTT while its vertical extent is related to the presence of the CoL. The mechanisms by which the CoL, observed over Namibia in the entrance region of the river of smoke, influences the vertical structure of the BBA layer is mainly associated with the ascending motion above the BBA layer. In the presence of the CoL, the top of the BBA layer over northern Namibia reaches altitudes above 8 km. This is much higher than the average height of the top of the BBA layer over the regions where the smoke comes from (Angola, Zambia, Zimbabwe, Mozambique) which is 5 to 6 km.

The results suggest that the interaction between the TTTs and the CoLs which form during the winter may have a role in promoting the transport of BBA from fire-prone regions in the tropical band to the temperate mid-latitudes and southwestern Indian Ocean.

Cyrille Flamant et al.

Status: open (until 06 Jan 2022)

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Cyrille Flamant et al.

Cyrille Flamant et al.


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Short summary
Rivers of smoke extends from tropical southern Africa towards the Indian Ocean during the winter fire season, controlled by the interaction of tropical easterly waves, and westerly waves in mid-latitudes. During the AEROCLO-sA field campaign in 2017, a river of smoke was directly observed over Namibia. In this paper, the evolution and atmospheric drivers of the river of smoke are described, and the role of a mid-latitude cut-off low in lifting the smoke to the upper troposphere is highlighted.