Role of Dynamics and Meteorology in the formation and maintenance of elevated aerosol layers over Indian region

Atmospheric aerosol and clouds play important role in our weather and climate. A recent discovery of high altitude (~16km) aerosol layer occurring during monsoon in the south Asian region using CALIPSO observations has started puzzling the atmospheric scientists. Very little is known on the composition and the formation mechanisms of this intense aerosol layer. This layer is of concern since it could play an important role on the climate and weather. Intense in situ campaigns called as BATAL are conducted jointly by ISRO and NASA during 2015-2019. Besides, long-term remote sensing data is used to understand these layers. Existence of elevated aerosol layer (above ABL) during monsoon season in the troposphere is noticed. Long-range transport through LLJ is found as a source for the observed elevated aerosol layer. Rain water samples collected over Gadanki conforms the Arabian sea influence. Deep convection persisting overhead Bay of Bengal can influence the transport of troposphere pollutants to the UTLS. However, Aerosols are likely to undergo rainout or washout during the monsoon season and no major source exists. Existence of strong shears in the horizontal wind during monsoon season can also restrict the vertical transport. However, long-term Mie lidar observations confirm the presence of aerosols in the UTLS region similar to CALIPSO. Long-range transport from north part of India through TEJ as a main source for the observed UTLS region aerosols. Regional simulation of ATAL using coupled chemistry-meteorology model WRF-Chem also confirms this feature. Existence of Anti-cyclonic flow in the UTLS region makes confinement of aerosols and trace gases and re-distribution to other regions through BD and Walker circulations. Dynamics play a major role in the formation and maintenance of the elevated aerosol layers. Gas to aerosol formation can be considered as a most possible source for the UTLS region aerosols other than dynamics. At present it is not clear on the relative roles of convection verses long-range transport and natural verses anthropogenic sources on the formation of ATAL. In future, quantification of role of convection in transporting the boundary layer aerosols to UTLS region, relative roles of convection and long-range transport need to be investigated.