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.

中文翻译：

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河中冒烟：AEROCLO-sA 案例研究

摘要。在 2017 年 9 月的南部非洲气溶胶、辐射和云 (AEROCLO-sA) 活动期间，对穿越南部非洲的烟雾河的形成进行了调查。 一套互补的全球和中尺度数值模拟以及地面、空中和对大气动力学、热力学和组成的星载观测被用来根据生物质燃烧气溶胶 (BBA) 层的时间和垂直范围来表征烟河。研究区受基于热带东风波的沿海低压、大陆上的高压系统和中纬度西风波的天气影响，其中一个具有嵌入式截止低压 (CoL)。沿海低气压与两个接近的西风波中的第二个相互作用，最终形成了中层温带热带低压槽（TTT）。TTT 创造了快速移动的气团，以烟雾的形式输送到印度洋西南部。CoL 在与第一个（最东端）西风波相关的上层发展和加强，在 TTT 形成之前在纳米比亚北部上空保持静止，并导致 BBA 层增厚。这表明烟河的演变与 TTT 的演变密切相关，而其垂直范围与 CoL 的存在有关。CoL 在纳米比亚烟河入口区域观察到的机制，影响 BBA 层垂直结构的主要与 BBA 层上方的上升运动有关。在存在 CoL 的情况下，纳米比亚北部的 BBA 层顶部达到 8 公里以上的高度。这远高于烟雾来源地区（安哥拉、赞比亚、津巴布韦、莫桑比克）的 BBA 层顶部的平均高度，即 5 至 6 公里。结果表明，冬季形成的 TTTs 和 CoLs 之间的相互作用可能在促进 BBA 从热带带火灾多发地区向温带中纬度和印度洋西南部的运输中发挥作用。这远高于烟雾来源地区（安哥拉、赞比亚、津巴布韦、莫桑比克）的 BBA 层顶部的平均高度，即 5 至 6 公里。结果表明，冬季形成的 TTTs 和 CoLs 之间的相互作用可能在促进 BBA 从热带带火灾多发地区向温带中纬度和印度洋西南部的运输中发挥作用。这远高于烟雾来源地区（安哥拉、赞比亚、津巴布韦、莫桑比克）的 BBA 层顶部的平均高度，即 5 至 6 公里。结果表明，冬季形成的 TTTs 和 CoLs 之间的相互作用可能在促进 BBA 从热带带火灾多发地区向温带中纬度和印度洋西南部的运输中发挥作用。