Evidence of small-scale quasi-isentropic mixing in ridges of extratropical baroclinic waves
Evidence of small-scale quasi-isentropic mixing in ridges of extratropical baroclinic waves
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DOI：
10.5194/acp-19-12607-2019
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发表时间：
2019-10
影响因子：
6.3
通讯作者：
D. Kunkel;P. Hoor;T. Kaluza;J. Ungermann;Björn Kluschat;A. Giez;Hans-Christoph Lachnitt;M. Kaufmann;M. Riese
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文献类型：
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作者：
D. Kunkel;P. Hoor;T. Kaluza;J. Ungermann;Björn Kluschat;A. Giez;Hans-Christoph Lachnitt;M. Kaufmann;M. Riese
摘要。温带气旋内的平流层-对流层交换为人为和自然地表排放迅速到达平流层以及平流层的臭氧深入对流层，甚至深入边界层提供了可能性。这一过程的效率直接影响到地表气候、平流层的化学成分、温带过渡层的化学成分和地表污染水平。在这里，我们提出了温带气旋内的混合过程的证据，该过程迄今为止只得到很少的关注，并促进对流层气团在斜压波脊中向平流层输送。我们分析了2017年秋季在北大西洋上空进行的WISE（波浪驱动等熵交换）研究飞行的机载测量数据，并辅以数值天气预报模型和轨迹计算的预测。从理想斜压生命周期的实验中获得了进一步详细的过程理解。这一分析的主要结果是，气团在对流层顶区域混合，并可能在急流反气旋一侧的斜压波脊中进入平流层，而不会剧烈改变其潜在温度。这种准等熵交换发生在温暖传送带流出上方，在平流层下层表现出增强的静态稳定性和对流层顶的开尔文-亥姆霍兹不稳定性的区域。静态稳定性的增强与对流层顶以下的辐射冷却和小尺度波的存在有关。开尔文-亥姆霍兹不稳定性与水平风的垂直切变有关，并与急流上边缘的小波有关。这种不稳定性导致了湍流的发生，并导致了对流层顶区域微量气体的混合。虽然这一过程的总体相关性还有待评估，但它有可能显著改变平流层最底层的温带过渡层的化学成分，这些区域以前在斜压波混合方面得到了很少的关注。
Abstract. Stratosphere–troposphere exchange within extratropical cyclones provides the potential for anthropogenic and natural surface emissions to rapidly reach the stratosphere as well as for ozone from the stratosphere to penetrate deep into the troposphere, even down into the boundary layer. The efficiency of this process directly influences the surface climate, the chemistry in the stratosphere, the chemical composition of the extratropical transition layer, and surface pollution levels. Here, we present evidence for a mixing process within extratropical cyclones which has gained only a small amount of attention so far and which fosters the transport of tropospheric air masses into the stratosphere in ridges of baroclinic waves. We analyzed airborne measurement data from a research flight of the WISE (Wave-driven ISentropic Exchange) campaign over the North Atlantic in autumn 2017, supported by forecasts from a numerical weather prediction model and trajectory calculations. Further detailed process understanding is obtained from experiments of idealized baroclinic life cycles. The major outcome of this analysis is that air masses mix in the region of the tropopause and potentially enter the stratosphere in ridges of baroclinic waves at the anticyclonic side of the jet without changing their potential temperature drastically. This quasi-isentropic exchange occurs above the outflow of warm conveyor belts, in regions which exhibit enhanced static stability in the lower stratosphere and a Kelvin–Helmholtz instability across the tropopause. The enhanced static stability is related to radiative cooling below the tropopause and the presence of small-scale waves. The Kelvin–Helmholtz instability is related to vertical shear of the horizontal wind associated with small-scale waves at the upper edge of the jet stream. The instability leads to the occurrence of turbulence and consequent mixing of trace gases in the tropopause region. While the overall relevance of this process has yet to be assessed, it has the potential to significantly modify the chemical composition of the extratropical transition layer in the lowermost stratosphere in regions which have previously gained a small amount of attention in terms of mixing in baroclinic waves.