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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 04 Jul 2019

Research article | 04 Jul 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

The tropical tropopause layer in reanalysis data sets

Susann Tegtmeier1, James Anstey2, Sean Davis3, Rossana Dragani4, Yayoi Harada5, Ioana Ivanciu1, Robin Pilch Kedzierski1, Kirstin Krüger6, Bernard Legras7, Craig Long8, James S. Wang9, Krzysztof Wargan10,11, and Jonathon S. Wright12 Susann Tegtmeier et al.
  • 1GEOMAR Helmholtz Centre for Ocean Research Kiel, 24105 Kiel, Germany
  • 2Canadian Centre for Climate Modelling and Analysis, ECCC, Victoria, Canada
  • 3Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO 80305, USA
  • 4European Centre for Medium-Range Weather Forecasts, Reading, RG2 9AX, UK
  • 5Japan Meteorological Agency, Tokyo, 100-8122, Japan
  • 6Section for Meteorology and Oceanography, Department of Geosciences, University of Oslo, 0315 Oslo, Norway
  • 7Laboratoire de Météorologie Dynamique, CNRS/PSL-ENS, Sorbonne University Ecole Polytechnique, France
  • 8Climate Prediction Center, National Centers for Environmental Prediction, National Oceanic and Atmospheric Administration, College Park, MD 20740, USA
  • 9Institute for Advanced Sustainability Studies, Potsdam, Germany
  • 10Science Systems and Applications, Inc., Lanham, MD 20706, USA
  • 11Global Modelingand Assimilation Office, Code 610.1, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
  • 12Department of Earth System Science, Tsinghua University, Beijing, 100084, China

Abstract. The tropical tropopause layer (TTL) is the transition region between the well mixed, convective troposphere and the radiatively controlled stratosphere with air masses showing chemical and dynamical properties of both regions. The representation of the TTL in meteorological reanalysis data sets is important for studying the complex interactions of circulation, convection, trace gases, clouds and radiation. In this paper, we present the evaluation of TTL characteristics in reanalysis data sets that has been performed as part of the SPARC (Stratosphere– troposphere Processes and their Role in Climate) Reanalysis Intercomparison Project (S-RIP).

The most recent atmospheric reanalysis data sets all provide realistic representations of the major characteristics of the temperature structure within the TTL. There is good agreement between reanalysis estimates of tropical mean temperatures and radio occultation data, with relatively small cold biases for most data sets. Temperatures at the cold point and lapse rate tropopause levels, on the other hand, show warm biases in reanalyses when compared to observations. This tropopause-level warm bias is related to the vertical resolution of the reanalysis data, with the smallest bias found for data sets with the highest vertical resolution around the tropopause. Differences of the cold point temperature maximise over equatorial Africa, related to Kelvin wave activity and associated disturbances in TTL temperatures. Model simulations of air mass transport into the stratosphere driven by reanalyses with a warm cold point bias can be expected to have too little dehydration.

Interannual variability in reanalysis temperatures is best constrained in the upper TTL, with larger differences at levels below the cold point. The reanalyses reproduce the temperature responses to major dynamical and radiative signals such as volcanic eruptions and the QBO. Long-term reanalysis trends in temperature in the upper TTL show good agreement with trends derived from adjusted radiosonde data sets indicating significant stratospheric cooling of around −0.5 to −1 K/decade. At 100 hPa and the cold point, most of the reanalyses suggest small but significant cooling trends of −0.3 to −0.6 K/decade that are statistically consistent with trends based on the adjusted radiosonde data sets.

Advances of the reanalysis and observational systems over the last decades have led to a clear improvement of the TTL reanalyses products over time. Biases of the temperature profiles and differences in interannual variability clearly decreased in 2006, when densely sampled radio occultation data started being assimilated by the reanalyses. While there is an overall good agreement, different reanalyses offer different advantages in the TTL such as realistic profile and cold point temperature, continuous time series or a realistic representation of signals of interannual variability. Their use in model simulations and in comparisons with climate model output should be tailored to their specific strengths and weaknesses.

Susann Tegtmeier et al.
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Susann Tegtmeier et al.
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Short summary
The tropical tropopause layer is an important atmospheric region right in between the troposphere and the stratosphere. We evaluate the representation of this layer in reanalyses data sets, which create a complete picture of the state of Earth's atmosphere using atmospheric modelling and available observations. The recent reanalyses show realistic temperatures in the tropical tropopause layer. However, where the temperature is lowest, the so-called cold point, the reanalyses are too cold.
The tropical tropopause layer is an important atmospheric region right in between the...