Strong day-to-day variability of the Asian Tropopause Aerosol Layer (ATAL) in August 2016 at the Himalayan foothills
Authors: Hanumanthu Sreeharsha , Vogel Bärbel , Müller Rolf , Brunamonti Simone , Fadnavis Suvarna , Li Dan , Ölsner Peter , Naja Manish , Singh Bhupendra Bahadur , Kumar Kunchala Ravi , Sonbawne Sunil , Jauhiainen Hannu , Vömel Holger , Luo Beiping , Jorge Teresa , Wienhold Frank G. , Dirkson Ruud , Peter Thomas 
Published: November 2020

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 and 18 km (360
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
as well as the source regions of air parcels contributing to the ATAL
over Nainital in August 2016. 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 air
in the ATAL is a mixture of air masses coming from different atmospheric
altitude layers. In addition, contributions from the model boundary
layer originate in different geographic source regions. The location
of the strongest updraft along the backward trajectories reveals 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 due to the strong
growth of Asian economies; on the other hand the implementation of new
emission control measures (in particular in China) has reduced
the anthropogenic emissions of some pollutants
contributing to the ATAL substantially. It needs to be monitored in the future whether the thickness and intensity of the ATAL will further
increase, which will likely impact the surface climate.