Abstract
Abstract. Stratosphere–troposphere exchange within extratropical cyclones provides the potential for anthropogenic and natural surface emissions to rapidly reach the stratosphere as well as for ozone from the stratosphere to penetrate deep into the troposphere, even down into the boundary layer. The efficiency of this process directly influences the surface climate, the chemistry in the stratosphere, the chemical composition of the extratropical transition layer, and surface pollution levels. Here, we present evidence for a mixing process within extratropical cyclones which has gained only a small amount of attention so far and which fosters the transport of tropospheric air masses into the stratosphere in ridges of baroclinic waves. We analyzed airborne measurement data from a research flight of the WISE (Wave-driven ISentropic Exchange) campaign over the North Atlantic in autumn 2017, supported by forecasts from a numerical weather prediction model and trajectory calculations. Further detailed process understanding is obtained from experiments of idealized baroclinic life cycles. The major outcome of this analysis is that air masses mix in the region of the tropopause and potentially enter the stratosphere in ridges of baroclinic waves at the anticyclonic side of the jet without changing their potential temperature drastically. This quasi-isentropic exchange occurs above the outflow of warm conveyor belts, in regions which exhibit enhanced static stability in the lower stratosphere and a Kelvin–Helmholtz instability across the tropopause. The enhanced static stability is related to radiative cooling below the tropopause and the presence of small-scale waves. The Kelvin–Helmholtz instability is related to vertical shear of the horizontal wind associated with small-scale waves at the upper edge of the jet stream. The instability leads to the occurrence of turbulence and consequent mixing of trace gases in the tropopause region. While the overall relevance of this process has yet to be assessed, it has the potential to significantly modify the chemical composition of the extratropical transition layer in the lowermost stratosphere in regions which have previously gained a small amount of attention in terms of mixing in baroclinic waves.
Highlights
The extratropical transition layer (ExTL) as a unique feature of the extratropical upper troposphere and lower stratosphere (UTLS) is a direct consequence of the exchange and consequent mixing between air masses from the stratosphere and troposphere (e.g., Danielsen, 1968; Gettelman et al, 2011)
This coincidence of turbulence in a region of the so-called tropopause inversion layer (TIL; Birner et al, 2002) addresses an open question with regard to the extratropical UTLS which is relevant for the ExTL: does stratosphere– troposphere exchange (STE) occur and does it affect the formation of the ExTL in the ridge of baroclinic waves where the static stability is usually strong in the extratropical lowermost stratosphere?
HALO was mostly flying in the UTLS up to ceiling altitudes of about 15 km, which corresponds to maximum potential temperature values of about 405 K and minimum pressure values of 130 hPa
Summary
The extratropical transition layer (ExTL) as a unique feature of the extratropical upper troposphere and lower stratosphere (UTLS) is a direct consequence of the exchange and consequent mixing between air masses from the stratosphere and troposphere (e.g., Danielsen, 1968; Gettelman et al, 2011). Kunkel et al (2016) showed that turbulent motions can occur on top of warm conveyor belt outflows at the altitude of the tropopause This coincidence of turbulence in a region of the so-called tropopause inversion layer (TIL; Birner et al, 2002) addresses an open question with regard to the extratropical UTLS which is relevant for the ExTL: does STE occur and does it affect the formation of the ExTL in the ridge of baroclinic waves where the static stability is usually strong in the extratropical lowermost stratosphere?. In this study we aim to analyze whether those turbulent signatures lead to mixing and potential exchange of tropospheric and stratospheric air masses in the ridge of baroclinic waves For this we use complementary data of airborne measurements, numerical weather forecast data, trajectory calculations, and idealized baroclinic life cycle experiments which are introduced in Sect.
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