Identification of source regions of the Asian Tropopause Aerosol Layer on the Indian subcontinent in August 2016
Abstract. The Asian tropopause aerosol layer (ATAL) is a distinct feature during the Asian summer monsoon season with an impact on the regional radiative balance of the Earth's atmosphere. However, the source regions and the detailed transport pathways of ATAL particles are still uncertain. In this study, we investigate transport pathways from different regions at the model boundary (MB) to the ATAL using the two Lagrangian transport models CLaMS (Chemical Lagrangian Model of the Stratosphere) and MPTRAC (Massive-Parallel Trajectory Calculations), two reanalyses (ERA5 and ERA-Interim), and balloon-borne measurements of the ATAL performed by the Compact Optical Backscatter Aerosol Detector (COBALD) above Nainital (India) in August 2016. Trajectories are initialized at the location of the ATAL, as measured by COBALD in the Himalayas, and calculated 90 days backward in time to investigate the relation between the measured, daily averaged, aerosol backscatter ratio and different source regions at the MB. Nine source regions at the MB are distinguished, marking continental and maritime sources in the region of the Asian monsoon. Different simulation scenarios are performed, to find systematic differences as well as robust patterns, when the reanalysis data, the trajectory model, the vertical coordinate (kinematic and diabatic approach) or the convective parameterisation are varied.
While there are many robust features, the simulation scenarios also show some considerable differences between the air mass contributions of different source regions at the MB in the region of the Asian monsoon. The contribution to all air parcels from the MB varied between 5 % and 40 % for the Indo-Gangetic plain, the contribution from the Tibetan Plateau varied between 30 % and 40 % and contributions from oceans varied between 14 % and 43 % for different scenarios. However, the robust finding among all scenarios is that the largest continental air mass contributions originate from the Tibetan plateau and the India subcontinent (mostly the Indo-Gangetic plain), and largest maritime air mass contributions in Asia come from the Western Pacific (e. g. related to tropical cyclones such as typhoons). Additionally, all simulation scenarios indicate that transport of maritime air from the Tropical Western Pacific to the region of the ATAL lowers the backscatter ratio (BSR) of the ATAL, while most scenarios indicate that transport of polluted air from the Indo-Gangetic plain increases the BSR. Therefore, while the results corroborate key findings from previous ERA-Interim based studies, they highlight the variability of the contributions of different MB regions to the ATAL depending on the meteorological input data, vertical velocities and in particular on the treatment of convection within the model calculations.
Jan Clemens et al.
Status: final response (author comments only)
- RC1: 'Comment on egusphere-2022-1462', Anonymous Referee #2, 15 May 2023
- RC2: 'Comment on egusphere-2022-1462', Anonymous Referee #1, 26 May 2023
Jan Clemens et al.
Jan Clemens et al.
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The paper addresses the interesting topic of the Asian tropopause aerosol layer (ATAL). The presented results provide interesting information regarding the source regions and the transport pathways of the ATAL based on balloon-borne measurements and two Lagrangian transport models driven by different reanalysis data (ERA5, ERA-interim) and model parameterisations/setups. The investigation of the ATAL is a topic of many studies during the last years. The added value of the present paper is the thorough intercomparison between the results obtained with different simulation scenarios (with different Langrangian model employed, the use of different reanalysis data -ERA5 vs ERA-interim-, vertical coordinate -kinematic vs diabatic approach- and convective parametrisation).
In my opinion, since the publication focuses mainly on the comparison between the different simulation scenarios, this should be reflected in the title. Overall the manuscript is well written and I recommend its publication in ACP after providing some clarifications and addressing some minor issues.
I think that it is better to use the term "model boundary layer" instead of the term "model boundary". (at least when it is mentioned for the first time).
The Asian highlands is not a continuous region. It would be interesting to mention the percentage of air masses with origin only from the Thibetan Plateau.
l45: "up to 360K". It would be useful to clarify here (for readers less familiar with the subject) that you are referring to potential temperature.
ll120-122 "The hybrid coordinate ζ ... above around 300 hPa and 380 K, respectively. "Please rephrase to "The hybrid coordinate ζ is near the surface an orography-following sigma coordinate and transforms continuously into potential temperature at higher altitudes above around 300 hPa (or 380 K).
l229 "transport pathways from two regions with two very different land-cover properties converge, the Tibetan Plateau or the Indo-Gangetic plain." please correct to "Tibetan Plateau and the Indo-Gangetic plain"
l240 and Fig. 1d: please correct the typo "Arabian See" -> "Arabian Sea"
l245 "from the Bay of Bengal more air parcels can enter the ASMA directly from the maritime boundary than from the Arabian Sea". Can you elaborate on this and provide an explanation? Also why the mean transport time from the MB into the UTLS is higher compared to the AS?
ll253-254: "Wide agreement can be found with ERA5 even when models, integration step-sizes, and vertical velocities are varied." and l268: "Model scenarios driven with ERA5 show very similar results." The scenario with extreme convection parameterisation (E5-kin-M-ECP) is also driven by ERA5 reanalysis data. It's better to mention this here (e.g. Model scenarios driven with ERA5 show very similar results except when employing the extreme convection parameterisation)
According to Figs 3b and 4c under the scenario with ECP the contributions from SE Asia increase. Please elaborate more on this in the text.
ll300-301: " This leads to... Indo-Gangetic plain." Please elaborate more on this.
ll313-314: Please rephrase (begin the sentence with "For ERA-Interim...")
l322: " However, our approach of the ECP has to be considered as an upper limit for convective transport." Can you elaborate more on this? In the Appendix F You mention that "the parametrization can be improved to avoid spurious parameterized convection events over the Persian gulf and the Red Sea". It was not possible to further imporove the parametrization? I mention this because I am concerned about the large differences between the scenario with ECP and the other scenarios.
Appendix F: Please provide a table with a short description of the scenarios shown in Fig F1. (similar to Table 2)
parametrization vs parameterisation: Please choose one spelling for consistency.