Identification of source regions of the Asian Tropopause Aerosol Layer on the Indian subcontinent in August 2

Abstract

<p>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 our 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.</p> <p>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. </p> <p> </p>

Reconsidering the Existence of a Trend in the Asian Tropopause Aerosol Layer (ATAL) From 1979 to 2017

“…The corresponding trends in anthropogenic emissions between 1980 and 2015 for India are shown in Figure 7. Main source regions responsible for the ATAL and its composition have been found to be located in India, including North India, China and South of the Tibetan Plateau (e.g., Clemens et al., 2023; Legras & Bucci, 2020; Tissier & Legras, 2016). Simulated mineral dust components, which are not directly influenced by human activities, do not show an increasing trend from 1979 to the 2010 decade.…”

Lagrangian transport simulations using the extreme convection parameterization: an assessment for the ECMWF reanalyses

“…Considering the proposed modification of the ECP method, we conclude that introducing the threshold CIN 0 , which prevents the occurrence of parameterized convective updrafts for warm, stable layers, leads to local improvements in areas with climatically large CIN, e.g., over North Africa and the Arabian Peninsula. This aspect is further elaborated on in a recent study by Clemens et al (2023), which used Lagrangian transport simulations to identify the source regions of the Asian tropopause aerosol layer over the Indian subcontinent in August 2016. The study of Clemens et al (2023) found that using a CIN threshold of 50 J kg −1 helps to remove spurious parameterized convection events over the Persian Gulf in the case of ECP simulations with MPTRAC.…”

“…For a CIN threshold of 200 J kg −1 , the spatial patterns of the convective events mostly resemble the unfiltered case, except for specific regions (i.e., North American Great Plains, Mediterranean Sea, Central Africa and West Africa, Arabian Sea), where CIN shows local maxima and the CIN filter therefore still has strong effects. A recent study of Clemens et al (2023) shows how the additional CIN threshold in the ECP can be used to remove unrealistic parameterized convection events over the Persian Gulf in an August 2016 case study investigating source regions of the Asian tropopause aerosol layer (ATAL) on the Indian subcontinent.…”

Evaluating the Model Representation of Asian Summer Monsoon Upper Troposphere and Lower Stratosphere Transport and Composition Using Airborne In Situ Observations

“…This dynamical core gives MUSICA the capability for user‐customized horizontal grid refinement to improve sampling over a region of interest. For the purposes of this study, we select a ∼30 km grid spacing region encompassing the southern flank of the Tibetan Plateau to the western north Pacific (Figure S1a in Supporting Information ) to cover the StratoClim airborne sampling (Figure 1) and the areas which dominate the convective contribution to the ASM UTLS anticyclone (e.g., Bergman et al., 2013; Clemens et al., 2023; Honomichl & Pan, 2020). The remainder of the globe is covered by ∼1° spacing, similar to WACCM.…”

“…Short‐lived halogenated species transported to the UTLS by this mechanism may delay the recovery of stratospheric ozone (Bednarz et al., 2022), with the modeled impact depending on the complexity of the chemical mechanism or treatment considered (Fernandez et al., 2021). The application of chemical and transport modeling techniques to predict ASM impacts on global atmospheric composition remains an active research area (e.g., Ploeger et al., 2017; Vogel et al., 2019; Yan et al., 2019; Pan et al., 2016, 2022; Clemens et al., 2023).…”