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Abstract
Abstract. This paper presents measurements of ozone, water vapour and nitric acid (HNO3) in the upper troposphere/lower stratosphere (UTLS) over North Atlantic and Europe. The measurements were acquired with the Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) during the Wave Driven Isentropic Exchange (WISE) campaign in October 2017. GLORIA is an airborne limb imager capable of acquiring both 2-D data sets (curtains along the flight path) and, when the carrier aircraft is flying around the observed air mass, spatially highly resolved 3-D tomographic data. Here, we present a case study of a Rossby wave (RW) breaking event observed during two subsequent flights 2 d apart. RW breaking is known to steepen tracer gradients and facilitate stratosphere–troposphere exchange (STE). Our measurements reveal complex spatial structures in stratospheric tracers (ozone and nitric acid) with multiple vertically stacked filaments. Backward-trajectory analysis is used to demonstrate that these features are related to several previous Rossby wave breaking events and that the small-scale structure of the UTLS in the Rossby wave breaking region, which is otherwise very hard to observe, can be understood as stirring and mixing of air masses of tropospheric and stratospheric origin. It is also shown that a strong nitric acid enhancement observed just above the tropopause is likely a result of NOx production by lightning activity. The measurements showed signatures of enhanced mixing between stratospheric and tropospheric air near the polar jet with some transport of water vapour into the stratosphere. Some of the air masses seen in 3-D data were encountered again 2 d later, stretched to very thin filament (horizontal thickness down to 30 km at some altitudes) rich in stratospheric tracers. This repeated measurement allowed us to directly observe and analyse the progress of mixing processes in a thin filament over 2 d. Our results provide direct insight into small-scale dynamics of the UTLS in the Rossby wave breaking region, which is of great importance to understanding STE and poleward transport in the UTLS.
Highlights
The upper troposphere and lower stratosphere (UTLS) is the atmospheric region heavily influenced by both stratospheric and tropospheric air masses and is characterised by sharp gradients of many tracer concentrations in both vertical and horizontal directions
The results presented are based on the GLORIA 2-D data products
The UTLS in the Rossby wave surf zone is a highly interesting region of the atmosphere characterised by sharp gradients and fine structure of the tracer distributions that are very challenging to observe directly
Summary
The upper troposphere and lower stratosphere (UTLS) is the atmospheric region heavily influenced by both stratospheric and tropospheric air masses and is characterised by sharp gradients of many tracer concentrations in both vertical and horizontal directions. The main focus of this case study is to determine to what extent the small-scale tracer structure of UTLS around the polar jet can be understood as a consequence of Rossby wave related stratosphere–troposphere exchange and how this structure is affected by mixing. The outcome of this investigation could further be used to constrain mixing in models as well as to gain insights into STE transport pathways. The appendices provide technical information about data retrieval methods, errors and validation
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