Evidence of small-scale quasi-isentropic mixing in ridges of extratropical baroclinic waves

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.

SOUTHTRAC-GW: An Airborne Field Campaign to Explore Gravity Wave Dynamics at the World’s Strongest Hotspot

“…GLORIA was applied under different measurement strategies to the observation of GW (Krisch et al 2017(Krisch et al , 2020. GW effects on the distribution of trace gases were investigated by Woiwode et al (2018) and Kunkel et al (2019).…”

POLSTRACC: Airborne Experiment for Studying the Polar Stratosphere in a Changing Climate with the High Altitude and Long Range Research Aircraft (HALO)

“…This dedicated UTLS payload weighs about 3 tons. A similar payload had already been used before during the TACTS/ESMVAL mission in 2012 (Müller et al 2016) and was later on used during the WISE mission in 2017 (www.wise2017 .de/; Kunkel et al 2019). Further deployment of this payload with minor modifications is used for studying the Southern Hemisphere mid-to high-latitude UTLS during the SOUTHTRAC mission recently operated in September and November 2019.…”

Mixing at the extratropical tropopause as characterized by collocated airborne H2O and O3 lidar observations

“…These mixing processes strongly depend on the dynamical situation. For a better understanding of the role of multi-scale dynamical processes on the composition of the ExTL in the mid-latitudes the Wave-driven Isentropic Exchange (WISE) field campaign (Kunkel et al, 2019) was conducted over the North Atlantic Ocean in autumn 2017. The HALO (High Altitude LOng range) research aircraft performed in situ and remote sensing measurements of various trace gases in the ExTL from turbulence to synoptic scale in a variety of synoptic situations.…”

“…typical phenomena related to breaking Rossby waves. A case study on in situ observations above a WCB outflow is discussed inKunkel et al (2019).…”