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.
<p><strong>Abstract.</strong> The variability and similarities in the evolution of the tropopause inversion (TIL) layer during cyclongenesis in the North Atlantic storm track are investigated using operational meteorological analysis data (Integrated Forecast System from the European Centre for Medium-Range Weather Forecasts). For this a total amount of 130 cyclones have been analysed which evolved during the months August through October between 2010&#8211;2014 over the North Atlantic. Their paths of migration along with associated flow features in the upper troposphere/lower stratosphere (UTLS) have been tracked using the mean sea level pressure. Subsets of the 130 cyclones have been used for composite analysis using minimum sea level pressure to filter the cyclones based on their strength.</p> <p>The composite structure of the TIL strength distribution in connection with the overall UTLS flow strongly resembles the structure of the individual cyclones. Key results are that a strong dipole in tropopause inversion layer strength forms in regions of cyclonic wrap-up of UTLS air masses of different origin and isentropic potential vorticity. These air masses are associated with the cyclonic rotation of the underlaying cyclones. The maximum values of enhanced static stability above the tropopause occur north and northeast of the cyclone centre, vertically aligned with outflow regions of strong updraft and cloud formation up to the tropopause, which are situated in anticyclonic flow patterns in the upper troposphere. These regions are colocated with a maximum of vertical shear of the horizontal wind. The strong wind shear within the TIL results in a local minimum of Richardson numbers, representing the possibility for turbulent instability and potential mixing (or air mass exchange) within regions of enhanced static stability in the lowermost stratosphere.</p>
Abstract. The evolution of the tropopause inversion layer (TIL) during cyclogenesis in the North Atlantic storm track is investigated using operational meteorological analysis data (Integrated Forecast System from the European Centre for Medium-Range Weather Forecasts). For this a total of 130 cyclones have been analysed during the months August through October between 2010 and 2014 over the North Atlantic. Their paths of migration along with associated flow features in the upper troposphere and lower stratosphere (UTLS) have been tracked based on the mean sea level pressure field. Subsets of the 130 cyclones have been used for composite analysis using minimum sea level pressure to filter the cyclones based on their strength. The composite structure of the TIL strength distribution in connection with the overall UTLS flow strongly resembles the structure of the individual cyclones. Key results are that a strong dipole in TIL strength forms in regions of cyclonic wrap-up of UTLS air masses of different origin and isentropic potential vorticity. These air masses are associated with the cyclonic rotation of the underlying cyclones. The maximum values of enhanced static stability above the tropopause occur north and northeast of the cyclone centre, vertically aligned with outflow regions of strong updraft and cloud formation up to the tropopause, which are situated in anticyclonic flow patterns in the upper troposphere. These regions are co-located with a maximum of vertical shear of the horizontal wind. The strong wind shear within the TIL results in a local minimum of Richardson numbers, representing the possibility for turbulent instability and potential mixing (or air mass exchange) within regions of enhanced static stability in the lowermost stratosphere.
Abstract. A climatology of the occurrence of strong wind shear in the upper troposphere–lower stratosphere (UTLS) is presented, which gives rise to defining a tropopause shear layer (TSL). Strong wind shear in the tropopause region is of interest because it can generate turbulence, which can lead to cross-tropopause mixing. The analysis is based on 10 years of daily northern hemispheric ECMWF ERA5 reanalysis data. The vertical extent of the region analyzed is limited to the altitudes from 1.5 km above the surface up to 25 km, to exclude the planetary boundary layer as well as strong wind shear in higher atmospheric layers like the mesosphere–lower thermosphere. A threshold value of St2=4×10-4s-2 of the squared vertical shear of the horizontal wind is applied, which marks the top end of the distribution of atmospheric wind shear to focus on situations which cannot be sustained by the mean static stability in the troposphere according to linear theory. This subset of the vertical wind shear spectrum is analyzed for its vertical, geographical, and seasonal occurrence frequency distribution. A set of metrics is defined to narrow down the relation to planetary circulation features, as well as indicators for momentum-gradient-sharpening mechanisms. The vertical distribution reveals that strong vertical wind shear above the threshold occurs almost exclusively at tropopause altitudes, within a vertically confined layer of about 1–2 km in extent directly above the local lapse rate tropopause. The TSL emerges as a distinct feature in the tropopause-based 10-year temporal and zonal mean climatology, spanning from the tropics to latitudes around 70∘ N, with average occurrence frequencies on the order of 1 %–10 %. The horizontal distribution of the strong vertical wind shear near the tropopause exhibits distinctly separated regions of occurrence, which are generally associated with jet streams and their seasonality. At midlatitudes, strong wind shear values occur most frequently in regions with an elevated tropopause and at latitudes around 50∘ N, associated with jet streaks within northward-reaching ridges of baroclinic waves. At lower latitudes in the region of the subtropical jet stream, which is mainly apparent over the east Asian continent, the occurrence frequency of strong wind shear near the tropopause reaches maximum values of about 30 % during winter and is tightly linked to the jet stream seasonality. The interannual variability of the occurrence frequency for strong wind shear might furthermore be linked to the variability of the zonal location and strength of the jet. The east-equatorial region features a bi-annual seasonality in the occurrence frequencies of strong vertical wind shear near the tropopause. During the summer months, large areas of the tropopause region over the Indian Ocean are up to 70 % of the time exposed to strong wind shear, which can be attributed to the emergence of the tropical easterly jet. During winter, this occurrence frequency maximum shifts eastward over the maritime continent, where it is exceptionally pronounced during the DJF 2010/11 La Niña phase, as well as quite weak during the El Niño phases of 2009/10, 2014/15, and 2015/16. This agrees with the atmospheric response of the Pacific Walker circulation cell in the El Niño–Southern Oscillation (ENSO) ocean–atmosphere coupling.
Abstract. Orographic gravity waves (i.e., mountain waves) can potentially lead to cross-isentropic fluxes of trace gases via the generation of turbulence. During the DEEPWAVE (Deep Propagating Gravity Wave Experiment) campaign in July 2014, we performed tracer measurements of carbon monoxide (CO) and nitrous oxide (N2O) above the Southern Alps during periods of gravity wave activity. The measurements were taken along two stacked levels at 7.9 km in the troposphere and 10.9 km in the stratosphere. A detailed analysis of the observed wind components shows that both flight legs were affected by vertically propagating gravity waves with momentum deposition and energy dissipation between the two legs. Corresponding tracer measurements indicate turbulent mixing in the region of gravity wave occurrence. For the stratospheric data, we identified mixing leading to a change of the cross-isentropic tracer gradient of N2O from the upstream to the downstream region of the Southern Alps. Based on the quasi-inert tracer N2O, we identified two distinct layers in the stratosphere with different chemical composition on different isentropes as given by constant potential temperature Θ. The CO–N2O relationship clearly indicates that irreversible mixing between these two layers occurred. Further, we found a significant change of the vertical profiles of N2O with respect to Θ from the upstream to the downstream side above the Southern Alps just above the tropopause. A scale-dependent gradient analysis reveals that this cross-isentropic gradient change of N2O is triggered in the region of gravity wave occurrence. The power spectra of the in situ measured vertical wind, Θ, and N2O indicate the occurrence of turbulence above the mountains associated with the gravity waves in the stratosphere. The estimated eddy dissipation rate (EDR) based on the measured three-dimensional wind indicates a weak intensity of turbulence in the stratosphere above the mountain ridge. The Θ–N2O relation downwind of the Alps modified by the gravity wave activity provides clear evidence that trace gas fluxes, which were deduced from wavelet co-spectra of vertical wind and N2O, are at least in part cross-isentropic. Our findings thus indicate that orographic waves led to turbulent mixing on both flight legs in the troposphere and in the stratosphere. Despite only weak turbulence during the stratospheric leg, the cross-isentropic gradient and the related composition change on isentropic surfaces from upstream to downstream of the mountain unambiguously conserves the effect of turbulent mixing by gravity wave activity on the trace gas distribution prior to the measurements. This finally leads to irreversible trace gas fluxes across isentropes and thus has a persistent effect on the upper troposphere and lower stratosphere (UTLS) trace gas composition.
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