Scientific rationale:

The composition of the upper troposphere and lower stratosphere (UTLS) with radiatively active trace gas species is a key factor for Earth‘s climate, as relatively small
changes in this composition cause large changes in surface temperatures. A particularly large radiative effect results for water vapour and aerosol (e.g., Solomon et al., 2010). Despite the urgency to correctly predict water vapour in numerical weather forecast and climate models, the distribution of water vapour across the tropopause and in the lower stratosphere (LS) is not well quantified and poorly constrained in numerical models.

In summer and early autumn East and South-east Asia is an important region for water vapour and other trace species to enter the lower stratosphere. Several dynamical features cause transport of trace species with near surface sources first into the UT and potentially further into the LS. This leads initially to an alteration of the composition of the UTLS over this region. A good example is the Asian Tropopause Aerosol Layer (ATAL) which is thought to be a direct consequence of the upward motions within the Asian summer monsoon (ASM). It has recently been shown that the ATAL consists to a large part of ammonium nitrate particles and affects the regional climate.

The ASM circulation during Northern summer is also thought to be the major pathway for tropospheric air masses, rich in water vapour, aerosol precursors and pollutants, into the UTLS. During summer (July-August), the large-scalenanticyclonic circulation in the UTLS traps lifted polluted air over Asia. Subsequently, when this circulation becomes weaker and more unstable, external forces disturb the temporally isolated system. Ultimately, frequent but irregular events of eddy shedding occur which lead to transport of polluted air from within the anti-cyclone into the extratropical LS. Observations and model simulations suggest a strong moistening effect of the eddy shedding transport on the Northern hemisphere UTLS, but with the strength of this effect being highly uncertain due to a lack of available observational data with sufficient precision and coverage. Very recent observations show evidence for a strong contribution of ammonium nitrate transport by the ASM to the UTLS aerosol budget and the ATAL, likely relevant for cirrus cloud formation.

Furthermore, the UTLS trace gas composition over the Eastern Pacific is affected by additional pathways in late summer and autumn: i) quasi-horizontal transport across the subtropical jet, exchanging air between the tropical UT and the extratropical LS, ii) vertical convective transport by tropical typhoons, which eventually move towards the extratropics, and iii) transport within extratropical weather systems, in particular, within warm conveyor belts and convection. All these features alter the composition of the UTLS over the eastern Pacific, thereby setting the initial chemical conditions before the air masses are transported further to middle and high latitudes and mix with LS background air. Initially, the respective chemical anomalies are relatively isolated within the shed eddy. Processes related to Rossby and gravity wave dynamics as well as diabatic forcings due to clouds in the UT and/or radiation lead to a mixing of these isolated anomalies into the background stratosphere. However, the time scales and the relative contribution of the underlying processes and their impact at different altitudes in the extratropical UTLS is largely unknown. Thus the temporal variability of the composition and in particular the gradients of radiatively active trace species are largely uncertain. This, in turn, is of importance for climate.
To investigate the relative importance of these processes contributing to the chemical composition of the UTLS we plan to characterize the evolution of the chemical composition of filaments during the full life cycle of eddy shedding process from the monsoon anticyclone and their effect particularly on the gradients of radiatively active species in the UTLS.