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

Abstract

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.

The ATAL within the 2017 Asian Monsoon Anticyclone: Microphysical aerosol properties derived from aircraft-borne in situ measurements

“…The scattering of the size distribution based SR values shows the highly variable nature of the ATAL in time and space. This high variability of the ATAL, even on a day by day basis, was also reported byHanumanthu et al (2020) from balloon-borne backscatter measurements conducted during the ASM season 2016. In Fig.…”

In situ observations of CH<sub>2</sub>Cl<sub>2</sub> and CHCl<sub>3</sub> show efficient transport pathways for very short-lived species into the lower stratosphere via the Asian and North American summer monsoons

“…To support the interpretation of airborne measurements we use global three-dimensional simulations of the Chemical Lagrangian Model of the Stratosphere (CLaMS; McKenna, 2002a,b;Pommrich et al, 2014) as well as pure CLaMS backtrajectory calculations. Pure CLaMS trajectory calculations consider only the advective (reversible) transport, neglecting (irreversible) mixing processes entirely (e.g., Vogel et al, 2019;Hanumanthu et al, 2020). However, in this study back-trajectories are useful to trace back the detailed transport pathway and transport time of an air parcel in the UTLS to possible source regions in the boundary layer and therefore provide added value compared to three-dimensional CLaMS simulations.…”

Lower-stratospheric aerosol measurements in eastward shedding vortices over Japan from the Asian summer monsoon anticyclone during the summer of 2018

“…The enhanced aerosol particle signature in the ASM anticyclone at 14-18 km altitude is known as the Asian Tropopause Aerosol Layer (ATAL), which was believed to consist of carbonaceous and sulphate materials, mineral dust, and nitrate particles (Vernier et al, 2015(Vernier et al, , 2018Brunamonti et al, 2018;Bossolasco et al, 2020;Hanumanthu et al, 2020). Through analysis of satellite and high-altitude aircraft observations and laboratory experiments, Höpfner et al (2019) provided evidence that a considerable part of the ATAL may contain solid ammonium nitrate (NH4NO3) particles.…”