ALBERT

Description: 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.

Description: In August 2018, European Space Agency launched Aeolus satellite with the first spaceborne Doppler Wind Lidar onboard that provided global coverage of the horizontal line-of-sight (HLOS) wind profiles. Despite their lower accuracy compared to conventional wind observations, Aeolus data improved analyses and forecasts in all operational numerical weather prediction (NWP) models that assimilated the HLOS winds The largest improvements by Aeolus are seen in the tropical upper troposphere and lower stratosphere (UTLS) with Aeolus correcting both systematic and random errors. The UTLS is the region with the strongest equatorial wave activity, especially Kelvin waves. It is also the region with the largest amplitudes of analysis and short-term forecast uncertainties in global NWP models. We shall present impact of Aeolus wind profiles on the ECMWF analyses and forecasts in the UTLS focusing on the Kelvin wave, the most prominent large-scale wave in the tropical atmosphere. The analysis is based on the comparison of the observing system experiments (OSE) that included Aeolus winds on top of all other observations and the referent experiment with all (but Aeolus) observations. The OSE was performed for the period from July 2019 to September 2020 using the second reprocessing of the Aeolus measurement from 2022. The work focuses on regions of strong shear of the horizontal wind and on coupling the effects on forecasts with the phase of the quasi-biennial oscillation (QBO) including its major disruption during the studied period.

Description: Biases in mid- to high-latitude Southern Hemisphere ocean and atmosphere temperatures, winds, currents, and other properties are a common issue in climate models. FOCI is a fully coupled climate model employing a 1/2° NEMO3.6 ocean and a T63 ECHAM6.3 atmosphere as default, including modules representing sea ice (LIM2) and land surface (JSBACH) processes. Similar to some CMIP models, FOCI has a warm bias in the surface and intermediate ocean across the Southern Ocean, and jet stream winds in the Southern Hemisphere are displaced equatorward. This wind bias is theorized to be partly due to biases in sea surface properties. The Antarctic Circumpolar Current (ACC) is weak in FOCI compared to observations, which is common in models of intermediate resolution. In this study, we test approaches of improving the above-mentioned biases. Using AGRIF nesting, we run additional simulations where ocean resolution south of 28°S is increased from 1/2˚ to 1/10°, which yields a stronger ACC transport, as discussed by Martin et al. in this session. Shortening the ocean-atmosphere coupling time step, from 3-hourly to hourly, clearly reduces the warm bias, and in consequence, improves the representation of sea ice and water mass properties at depth. This links to a weaker Weddell Gyre. The change in coupling frequency is effective at default as well as at nest resolution. Further improvements in simulated surface temperature and sea ice are found with reduced iso-neutral diffusion. The jet stream position remains unchanged, suggesting that it is insensitive to the surface temperature and sea ice biases.

Description: Stratospheric warming events have been extensively studied on the Northern Hemisphere. The interest in stratospheric warming events on the Southern Hemisphere (SH) increased recently after a minor stratospheric warming (MSW) in 2019 had significant surface impacts. These were especially large over Australia: the strong negative phase of the Southern Annular Mode (SAM) favored extreme hot and dry conditions and thereby the occurrence of severe wildfires over the eastern part of the continent. Here, we use a large ensemble of AMIP-like simulations with ECHAM6 as the atmosphere component to investigate the effect of different global warming levels relative to pre-industrial conditions (+1.5K, +2K, +3K and +4K) on the frequency and surface impact of SH MSWs. We find that increasing global warming levels leads to a decrease in the number of MSWs that have an impact on the tropospheric SAM, consistent with previous studies. The coupling between stratosphere and troposphere starts to decrease at +2K warming. Nevertheless, also under +4K conditions, MSWs with a significant surface impact can still occur. The modeled surface response is close to that observed in 2019 with warmer and dryer conditions over eastern Australia for most of the warming levels. Furthermore, we evaluate the surface impact of MSWs on South Africa, South America and New Zealand. At these regions, we find different responses under increasing global warming conditions. Over South Africa, for example, the largest surface response, also characterized by dry and warm conditions, is found for +2K warming.