Strong day-to-day variability of the Asian Tropopause Aerosol Layer (ATAL) in August 2016 at the Himalayan foothills
Strong day-to-day variability of the Asian Tropopause Aerosol Layer (ATAL) in August 2016 at the Himalayan foothills
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DOI：
10.5194/acp-20-14273-2020
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发表时间：
2020-11-24
影响因子：
6.3
通讯作者：
Peter, Thomas
中科院分区：
文献类型：
--
作者：
Hanumanthu, Sreeharsha;Vogel, Baerbel;Peter, Thomas
南亚夏季风与对流层上层和平流层下层（UTLS）的大规模反气旋环流有关，该环流限制了内部的气团。在北方夏季，该气团的限制导致气溶胶在大约13至18公里（360至440 K的势温）之间积累；这种气溶胶的积累构成了亚洲对流层顶气溶胶层（ATAL）。本文介绍了2016年8月在印度北部Nainital使用紧凑型光学后向散射气溶胶探测器（COBALD）对ATAL进行的气球载气溶胶后向散射测量，并将其与2016年11月季风后时间的COBALD测量结果进行了比较。测量结果表明，阿塔尔的高度、垂直范围、气溶胶后向散射强度和卷云发生频率具有很强的变异性。这种变率不能从ATAL的气候资料中推断出来，因为它们是从卫星测量得来的。为了解释这种观测到的变化，我们使用平流层化学拉格朗日模型（CLaMS）进行了拉格朗日反轨迹分析。我们确定了2016年8月Nainital上空的运输路径以及航空包裹的来源地区。我们的分析揭示了导致观测到的阿塔尔日变化的多种因素：大陆对流、热带气旋（海洋对流）、反气旋动力学和平流层入侵。因此，ATAL中的空气是来自不同大气高度层的气团的混合物。此外，模式边界层的贡献来自不同的地理源区。最强上升气流沿后向轨迹的位置揭示了喜马拉雅山麓南缘有一团强大的上升气流。在UTLS高度南亚夏季风反气旋气流的非绝热加热驱动下，从对流流出层顶部（约13 km； 360 K），气流包在大尺度上升螺旋中缓慢上升至ATAL高度。较弱的阿塔尔案例表明来自海洋边界层的贡献较大，通常与热带气旋有关，表明清洁的海洋空气和受污染的大陆空气混合在一起。一方面，由于亚洲经济的强劲增长，预计未来人为排放将增加；另一方面，新的排放控制措施的实施（特别是在中国）大大减少了一些造成大气污染指数的污染物的人为排放。未来需要监测ATAL的厚度和强度是否会进一步增加，这可能会影响地表气候。
The South Asian summer monsoon is associated with a large-scale anticyclonic circulation in the upper troposphere and lower stratosphere (UTLS), which confines the air mass inside. During boreal summer, the confinement of this air mass leads to an accumulation of aerosol between about 13 and 18 km (360 and 440 K potential temperature); this accumulation of aerosol constitutes the Asian Tropopause Aerosol Layer (ATAL). We present balloon-borne aerosol backscatter measurements of the ATAL performed by the Compact Optical Backscatter Aerosol Detector (COBALD) instrument in Nainital in northern India in August 2016, and compare these with COBALD measurements in the post-monsoon time in November 2016. The measurements demonstrate a strong variability of the ATAL's altitude, vertical extent, aerosol backscatter intensity and cirrus cloud occurrence frequency. Such a variability cannot be deduced from climatological means of the ATAL as they are derived from satellite measurements. To explain this observed variability we performed a Lagrangian back-trajectory analysis using the Chemical Lagrangian Model of the Stratosphere (CLaMS). We identify the transport pathways as well as the source regions of air parcels contributing to the ATAL over Nainital in August 2016. Our analysis reveals a variety of factors contributing to the observed day-to-day variability of the ATAL: continental convection, tropical cyclones (maritime convection), dynamics of the anticyclone and strato-spheric intrusions. Thus, the air in the ATAL is a mixture of air masses coming from different atmospheric altitude layers. In addition, contributions from the model boundary layer originate in different geographic source regions. The location of the strongest updraft along the backward trajectories reveals a cluster of strong upward transport at the southern edge of the Himalayan foothills. From the top of the convective outflow level (about 13 km; 360 K) the air parcels ascend slowly to ATAL altitudes within a large-scale upward spiral driven by the diabatic heating in the anticyclonic flow of the South Asian summer monsoon at UTLS altitudes. Cases with a strong ATAL typically show boundary layer contributions from the Tibetan Plateau, the foothills of the Himalayas and other continental regions below the Asian monsoon. Weaker ATAL cases show higher contributions from the maritime boundary layer, often related to tropical cyclones, indicating a mixing of clean maritime and polluted continental air. On the one hand increasing anthropogenic emissions in the future are expected due to the strong growth of Asian economies; on the other hand the implementation of new emission control measures (in particular in China) has reduced the anthropogenic emissions of some pollutants contributing to the ATAL substantially. It needs to be monitored in the future whether the thickness and intensity of the ATAL will further increase, which will likely impact the surface climate.