Global large-scale stratosphere–troposphere exchange in modern reanalyses

Boothe, Alexander C.; Homeyer, Cameron R.

doi:10.5194/acp-17-5537-2017

Cited by 52 publications

(44 citation statements)

References 55 publications

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“…Similar to ERA-Interim, JRA-55 has 750-1250 m vertical grid spacing in the UTLS. MERRA-2 is available from 1979 to the present on a 0.5 • × 0.625 • longitude-latitude grid and at ∼ 1100 m vertical resolution throughout the UTLS (Bosilovich, 2015). CFSR is available from 1979 to 2010 on a 0.5 • × 0.5 • longitude-latitude grid and at 700-900 m vertical resolution in the UTLS.…”

Section: Reanalysis Outputmentioning

confidence: 99%

Global tropopause altitudes in radiosondes and reanalyses

Xian

Homeyer

2019

Atmos. Chem. Phys.

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Abstract. Accurate depictions of the tropopause and its changes are important for studies on stratosphere–troposphere exchange and climate change. Here, the fidelity of primary lapse-rate tropopause altitudes and double tropopause frequencies in four modern reanalyses (ERA-Interim, JRA-55, MERRA-2, and CFSR) is examined using global radiosonde observations. In addition, long-term trends (1981–2015) in these tropopause properties are diagnosed in both the reanalyses and radiosondes. It is found that reanalyses reproduce observed tropopause altitudes with little bias (typically less than ±150 m) and error comparable to the model vertical resolution. All reanalyses underestimate the double tropopause frequency (up to 30 % lower than observed), with the largest biases found in JRA-55 and the smallest in CFSR. The underestimates in double tropopause frequency are primarily attributable to the coarse vertical resolution of the reanalyses. Significant increasing trends in both tropopause altitude (40–120 m per decade) and double tropopause frequency (≥3 % per decade) were found in both the radiosonde observations and the reanalyses over the 35-year analysis period (1981–2015). ERA-Interim, JRA-55, and MERRA-2 broadly reproduce the patterns and signs of observed significant trends, while CFSR is inconsistent with the remaining datasets. Trends were diagnosed in both the native Eulerian coordinate system of the reanalyses (fixed longitude and latitude) and in a coordinate system where latitude is defined relative to the mean latitude of the tropopause break (the discontinuity in tropopause altitude between the tropics and extratropics) in each hemisphere. The coordinate relative to the tropopause break facilitates the evaluation of tropopause behavior within the tropical and extratropical reservoirs and revealed significant differences in trend estimates compared to the traditional Eulerian analysis. Notably, increasing tropopause altitude trends were found to be of greater magnitude in coordinates relative to the tropopause break, and increasing double tropopause frequency trends were found to occur primarily poleward of the tropopause break in each hemisphere.

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Section: Reanalysis Outputmentioning

confidence: 99%

Global tropopause altitudes in radiosondes and reanalyses

Xian

Homeyer

2019

Atmos. Chem. Phys.

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“…However, they are also highly dependent on the details of the underlying general circulation models and assimilation systems as well as on the input datasets and processing. Several previous studies have shown differences in upper tropospheric jet and/or tropopause information from multiple reanalyses (e.g., Archer and Caldeira, 2008;Peña-Ortiz et al, 2013;Boothe and Homeyer, 2017). Studies of tropical width using metrics related to zonal mean upper tropospheric jets and/or the tropopause have shown inconsistent results between models and reanalyses as well as among reanalyses (e.g., Davis and Rosenlof, 2012;Davis and Birner, 2017).…”

Section: Introductionmentioning

confidence: 99%

Reanalysis comparisons of upper tropospheric–lower stratospheric jets and multiple tropopauses

et al. 2017

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Abstract. The representation of upper tropospheric–lower stratospheric (UTLS) jet and tropopause characteristics is compared in five modern high-resolution reanalyses for 1980 through 2014. Climatologies of upper tropospheric jet, subvortex jet (the lowermost part of the stratospheric vortex), and multiple tropopause frequency distributions in MERRA (Modern-Era Retrospective analysis for Research and Applications), ERA-I (ERA-Interim; the European Centre for Medium-Range Weather Forecasts, ECMWF, interim reanalysis), JRA-55 (the Japanese 55-year Reanalysis), and CFSR (the Climate Forecast System Reanalysis) are compared with those in MERRA-2. Differences between alternate products from individual reanalysis systems are assessed; in particular, a comparison of CFSR data on model and pressure levels highlights the importance of vertical grid spacing. Most of the differences in distributions of UTLS jets and multiple tropopauses are consistent with the differences in assimilation model grids and resolution – for example, ERA-I (with coarsest native horizontal resolution) typically shows a significant low bias in upper tropospheric jets with respect to MERRA-2, and JRA-55 (the Japanese 55-year Reanalysis) a more modest one, while CFSR (with finest native horizontal resolution) shows a high bias with respect to MERRA-2 in both upper tropospheric jets and multiple tropopauses. Vertical temperature structure and grid spacing are especially important for multiple tropopause characterizations. Substantial differences between MERRA and MERRA-2 are seen in mid- to high-latitude Southern Hemisphere (SH) winter upper tropospheric jets and multiple tropopauses as well as in the upper tropospheric jets associated with tropical circulations during the solstice seasons; some of the largest differences from the other reanalyses are seen in the same times and places. Very good qualitative agreement among the reanalyses is seen between the large-scale climatological features in UTLS jet and multiple tropopause distributions. Quantitative differences may, however, have important consequences for transport and variability studies. Our results highlight the importance of considering reanalyses differences in UTLS studies, especially in relation to resolution and model grids; this is particularly critical when using high-resolution reanalyses as an observational reference for evaluating global chemistry–climate models.

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“…One reason why the process described in this study has not gained much attention is that numerical weather prediction models and in particular reanalysis products as well as climate models do not resolve the UTLS sufficiently, thus potentially miss or misrepresent the relevant processes. However, especially reanalysis data sets build the basis for almost all recent 25 climatological studies of STE (e.g., Škerlak et al, 2014;Boothe and Homeyer, 2017). Figure 15 shows the same cross section for different ECMWF products.…”

Section: Discussionmentioning

confidence: 99%

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

Kunkel¹,

Hoor²,

Kaluza³

et al. 2019

Preprint

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<p><strong>Abstract.</strong> Stratosphere&#8211;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 little 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 anti-cyclonic side of 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&#8211;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&#8211;Helmholtz instability is related to vertical shear of the horizontal wind associated to 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 only little attention in terms of mixing in baroclinic waves.</p>

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Global large-scale stratosphere–troposphere exchange in modern reanalyses

Cited by 52 publications

References 55 publications

Global tropopause altitudes in radiosondes and reanalyses

Global tropopause altitudes in radiosondes and reanalyses

Reanalysis comparisons of upper tropospheric–lower stratospheric jets and multiple tropopauses

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

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