Temperature and tropopause characteristics from reanalyses data in the tropical tropopause layer
Authors: Tegtmeier Susann , Anstey James , Davis Sean , Dragani Rossana , Harada Yayoi , Ivanciu Ioana , Pilch Kedzierski Robin , Krüger Kirstin , Legras Bernard , Long Craig , Wang James S. , Wargan Krzysztof , Wright Jonathon S. 
Published: January 2020

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.