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Preprints
https://doi.org/10.5194/acp-2020-552
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-552
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  22 Jun 2020

22 Jun 2020

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A revised version of this preprint is currently under review for the journal ACP.

Strong variability of the Asian Tropopause Aerosol Layer (ATAL) in August 2016 at the Himalayan foothills

Sreeharsha Hanumanthu1, Bärbel Vogel1, Rolf Müller1, Simone Brunamonti2, Suvarna Fadnavis3, Dan Li1,4, Peter Ölsner5, Manish Naja6, Bhupendra Bahadur Singh3, K. Ravi Kumar7, Sunil Sonbawne3, Hannu Jauhiainen8, Holger Vömel9, Beiping Luo2, Teresa Jorge2, Frank G. Wienhold2, Ruud Dirkson5, and Thomas Peter2 Sreeharsha Hanumanthu et al.
  • 1Institute of Energy and Climate Research (IEK-7), Forschungszentrum Jülich, Jülich, Germany
  • 2Institute for Atmospheric and Climate Science (IAC), Swiss Federal Institute of Technology (ETH), Zürich, Switzerland
  • 3Indian Institute of Tropical Meteorology (IITM), Pune, India
  • 4Key Laboratory of Middle Atmosphere and Global Environment Observation (LAGEO), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China
  • 5Deutscher Wetterdienst (DWD)/GCOS Reference Upper Air Network (GRUAN) Lead Center, Lindenberg, Germany
  • 6Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital, India
  • 7Centre for Atmospheric Sciences, Indian Institute of Technology (IIT), Delhi, India
  • 8Vaisala Oyj, Vantaa, Finland
  • 9Earth Observing Laboratory, National Center for Atmospheric Research, Boulder, CO, USA

Abstract. 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 km and 18 km (360 K 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 of air parcels contributing to the ATAL over Nainital in August 2016, as well as the source regions of the air masses contributing to the composition of the ATAL. 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 stratospheric intrusions. Thus, the ATAL is a mixture of air masses coming from different atmospheric height layers. In addition, contributions from the model boundary layer originate in different geographic source regions. The location of strongest updraft along the backward trajectories reveal 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 unpolluted and polluted air masses. Because of the strong growth of Asian economies, increasing anthropogenic emissions in the future are expected to enhance the thickness and intensity of the ATAL, thereby also enhancing the global stratospheric aerosol loading, which likely impacts surface climate.

Sreeharsha Hanumanthu et al.

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Sreeharsha Hanumanthu et al.

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Short summary
During boreal summer, anthropogenic sources yield Asian Tropopause Aerosol Layer (ATAL) found in Asia between about 13–18 km altitude. Balloon-borne measurements of the ATAL conducted in Northern India in 2016 demonstrate a strong variability of the ATAL. To explain its observed variability model simulations are performed to deduce the origin of air masses on the Earth's surface which is important to develop recommendations for for regulations of anthropogenic surface emissions of ATAL.
During boreal summer, anthropogenic sources yield Asian Tropopause Aerosol Layer (ATAL) found in...
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