Abstract
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 climatological and long-term TTL temperature and tropopause characteristics in the reanalysis data sets ERA-Interim, ERA5, JRA-25, JRA-55, MERRA, MERRA-2, NCEP-NCAR (R1), and CFSR. The evaluation 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 (ERA-Interim, ERA5, JRA-55, MERRA-2, and CFSR) 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 in the cold point temperature maximize over equatorial Africa, related to Kelvin wave activity and associated disturbances in TTL temperatures. 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 quasi-biennial oscillation (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 per 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 per 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 in the TTL reanalysis 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.
Highlights
The tropical tropopause layer (TTL) is the transition region between the well-mixed convective troposphere and the radiatively controlled stratosphere
The trend of cold point temperature cannot be derived from the unadjusted IGRA data set due to inhomogeneities or time-varying biases caused by changes in instruments and measurement practices
We focus on the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Reanalysis (ERAInterim; Dee et al, 2011), the forthcoming reanalysis developed by ECMWF (ERA5; Hersbach et al, 2018), the Japanese 25-year Reanalysis (JRA-25; Onogi et al, 2007), the Japanese 55-year Reanalysis (JRA-55; Kobayashi et al, 2015), the Modern Era Retrospective-Analysis for Research and Applications (MERRA; Rienecker et al, 2011), the MERRA-2 (Gelaro et al, 2017), the National Centers for Environmental Prediction (NCEP) – National Center for Atmospheric Research (NCAR) Reanalysis 1 (NCEP-NCAR Reanalysis 1; Kistler et al, 2001; referred to hereafter as R1), and the NCEP Climate Forecast System Reanalysis (CFSR; Saha et al, 2010)
Summary
The tropical tropopause layer (TTL) is the transition region between the well-mixed convective troposphere and the radiatively controlled stratosphere. Meteorological reanalysis data sets are widely used in scientific studies of atmospheric processes and variability, either as initial conditions for historical model runs or in comparisons with climate model output. Often, they are utilized as “stand-ins” for observations, when the available measurements lack the spatial or temporal coverage needed. Many offline chemical transport models or Lagrangian particle dispersion models are driven by reanalysis data sets (e.g., Chipperfield, 1999; Krüger et al, 2009; Schoeberl et al, 2012; Tao et al, 2019) Their representation of the cold point determines how realistically such models simulate dehydration and stratospheric entrainment processes.
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