The tropical tropopause layer in reanalysis data sets

Abstract

<p><strong>Abstract.</strong> 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).</p> <p>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.</p> <p>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.</p> <p>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.</p>

Differences in tropical high clouds among reanalyses: origins and radiative impacts

“…For ERA-Interim, weak long-term trends thus reflect relatively large excursions in these variables in the early 1980s acting to offset relatively large changes in the same direction after the turn of the century. These early-1980s excursions have also been reported for TTL temperatures based on ERA-Interim (Tegtmeier et al, 2019). : 2001ISCCP/SRB: 1984CFSR: 1980CFSR: -2009all other reanalyses: 1980.…”

“…Most of the reanalyses also suggest decreasing trends in all-sky OLR over this period, although the signs (over 1980-2014) and magnitudes (over 2001-2014) of these trends are not supported by observations. Associated declines in upwelling LW radiation at the tropopause may help to explain long-term decreases in tropical cold point temperatures based on JRA-55, MERRA-2, and CFSR (Tegtmeier et al, 2019). With the exception of CFSR/CFSv2 (affected by discontinuities around the CFSR-CFSv2 transition as discussed above) and ERA5 (for which trends are small), relatively large decreasing trends in all-sky OLR among reanalyses reflect relatively large increases in LWCRE, which are linked in turn to increases in high cloud fraction.…”

Confinement of air in the Asian monsoon anticyclone and pathways of convective air to the stratosphere during summer season

“…The ERA5 was shown in several recent studies (Hoffmann et al, 2019;Tegtmeier et al, 2019a;Bucci et al, 2019) to provide a better representation of atmospheric properties, including transport. The ERA5 is, however, singular in favouring very high https://doi.org /10.5194/acp-2019-1075 Preprint.…”

Variability and long-term changes of brominated VSLS at the tropical tropopause

“…Most interesting for our analysis is the correlation between the SST trends and the long-term changes of stratospheric into the atmosphere and at the same time cause enhanced convection, 1 transporting surface air masses into the TTL (Tegtmeier et al, 2015). As the cold point 2 tropopause altitude shows no significant trend in radiosondes or ERA-Interim data over the 3 1980-2013 time period (Tegtmeier et al, 2019), CHBr3 changes at 17 km corresond directly to 4 changes of stratospheric CHBr3 entrainment. Future SST changes can be expected to drive a 5 continued postive trend of stratospheric CHBr3 entrainment (Hossaini et al, 2012a).…”