An assessment of tropopause characteristics of the ERA5 and ERA-Interim meteorological reanalyses

Abstract

Abstract. The tropopause layer plays a key role in manifold processes in atmospheric chemistry and physics. Here we compare the representation and characteristics of the lapse rate tropopause according to the definition of the World Meteorological Organization (WMO) as estimated from European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis data. Our study is based on ten-year records (2009 to 2018) of ECMWF's state-of-the-art reanalysis ERA5 and its predecessor ERA-Interim. The intercomparison reveals notable differences between ERA5 and ERA-Interim tropopause data, in particular on small spatiotemporal scales. The monthly mean differences of ERA5 minus ERA-Interim tropopause heights vary between −300 m at the transition from the tropics to the extratropics (near 30° S and 30° N) to 150 m around the equator. Mean tropopause temperatures are mostly lower in ERA5 than in ERA-Interim, with a maximum difference of up to −1.5 K in the tropics. Monthly standard deviations of tropopause heights of ERA5 are up to 350 m or 60 % larger than for ERA-Interim. Monthly standard deviations of tropopause temperatures of ERA5 exceed those of ERA-Interim by up to 1.5 K or 30 %. The occurrence frequencies of double tropopause events in ERA5 exceed those of ERA-Interim by up to 25 percentage points at mid latitudes. We attribute the differences between the ERA5 and ERA-Interim tropopause data and the larger, more realistic variability of ERA5 to improved spatiotemporal resolution and better representation of geophysical processes in the forecast model as well as improvements in the data assimilation scheme and the utilization of additional observations in ERA5. The improved spatiotemporal resolution of ERA5 allows for a better representation of mesoscale features, in particular of gravity waves, which affect the temperature profiles in the upper troposphere and lower stratosphere and thus the tropopause height estimates. We evaluated the quality of the ERA5 and ERA-Interim reanalysis tropopause data by comparisons with COSMIC and MetOp Global Positioning System (GPS) satellite observations as well as high-resolution radiosonde profiles. The comparison indicates an uncertainty of the first tropopause for ERA5 (ERA-Interim) of about ±150 m to ±200 m (±250 m) based on radiosonde data and ±120 m to ±150 m (±170 to ±200 m) based on the coarser resolution GPS data at different latitudes. Consequently, ERA5 will provide more accurate information than ERA-Interim for future tropopause-related studies.

A global view on stratospheric ice clouds: assessment of processes related to their occurrence based on satellite observations

“…1) compared to Zou et al (2020, Fig.3) using ERA-interim tropopauses. The statistical analysis of ERA-interim and ERA5 tropopause heights shows that depending on season and hemisphere, the mean midlatitude tropopause in ERA5 is between 100 m lower to 80 m higher than the ERA-interim tropopause (Hoffmann and Spang, 2021, Fig6a, at 45 • ). Hence, the ERA5 tropopause at midlatitudes remains the approximately same, lowering of the threshold distance to the tropopause results in more cloud detections, as one would expect.…”

Latent Heating Is Required for Firestorm Plumes to Reach the Stratosphere

“…We illustrate the tropopause layer with the top and bottom contours given by the cold point definition and World Meteorological Organization (WMO) lapse‐rate definition, respectively. The WMO tropopause is the lowest level where the absolute value of the temperature lapse rate decreases to 2K/km, or less, with the average lapse rate between this level and all higher levels within 2 km not exceeding 2K/km (Hoffmann & Spang, 2021). The “dynamic” tropopause, a potential vorticity contour of +3.5 PVU, is also shown.…”

“…The “dynamic” tropopause, a potential vorticity contour of +3.5 PVU, is also shown. The zonal and time average tropopause contours are computed from the ERA5 tropopause data set (Hoffmann & Spang, 2021).…”

“…Stronger mixing, with ℓ = 5 km corresponding to a plume with R = 1 km (right), raises the required temperature anomaly by multiples ranging from a factor of two (poles) to six (tropics). The tropopause is shown as defined by the cold point, dynamic definition, and the WMO lapse‐rate definition (Hoffmann & Spang, 2021). Vertical axis is log‐pressure Z = − H log( p / p r ) with scale height H = 8 km and reference pressure p r = 1,000 mb.…”

Higher-resolution tropopause folding accounts for more stratospheric ozone intrusions

“…However, because folding is a meso-to synoptic-scale process, capturing fold morphology and fold-related turbulent STT processes requires resolutions <50 km (Knowland et al, 2017;Buker et al, 2005;Spreitzer et al, 2019). Consequently, the frequency of "double-tropopause" structures (of which folding is one type) is significantly higher in high-resolution ERA5 versus ERA-Interim (Hoffmann & Spang, 2022). High-resolution observational evidence, although sparse and localized, has suggested that atmospheric transport structures are horizontally and vertically filamentary, characterized by thin, diffusion-resistant layers (Danielsen, 1959;Appenzeller & Davies, 1992;Appenzeller et al, 1996;Newell et al, 1996Newell et al, , 1999Trickl et al, 2010Trickl et al, , 2020.…”