title: Trajectory Analysis on the Asian Tropopause Aerosol Layer (ATAL) based on Balloon Measurements at the Foothills of the Himalayas
Creators: name: Hanumanthu, Sreeharsha
Publisher: name: Bergische Universität Wuppertal
Year: 2022

description: The South Asian summer monsoon is associated with a large-scale anticyclonic circulation in the Upper Troposphere and Lower Stratosphere, 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). In this thesis balloon-borne aerosol back-scatter measurements of the ATAL are presented by the Compact Optical Backscatter Aerosol Detector (COBALD) instrument in Nainital in Northern India in August 2016, and in the post-monsoon time in November 2016. The presence of an ATAL is then detected by the enhancement of the August measurements compared to the November measurements. 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 a Lagrangian back-trajectory analysis was performed 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. The 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 clean maritime and polluted continental air. On the one hand increasing anthropogenic emissions in the future are expected because of the strong growth of Asian economies, on the other hand implementation of new emission control measures ( in particular in China ) have reduced substantially the anthropogenic emissions of some pollutants contributing to the ATAL. It needs to be monitored in the future, whether the thickness and intensity of the ATAL will further increase, which likely impacts surface climate.