Global tropopause altitudes in radiosondes and reanalyses

Abstract

Abstract. Accurate depictions of the tropopause and its changes are important for studies on stratosphere–troposphere exchange and climate change. Here, the fidelity of primary lapse-rate tropopause altitudes and double tropopause frequencies in four modern reanalyses (ERA-Interim, JRA-55, MERRA-2, and CFSR) is examined using global radiosonde observations. In addition, long-term trends (1981–2015) in these tropopause properties are diagnosed in both the reanalyses and radiosondes. It is found that reanalyses reproduce observed tropopause altitudes with little bias (typically less than ±150 m) and error comparable to the model vertical resolution. All reanalyses underestimate the double tropopause frequency (up to 30 % lower than observed), with the largest biases found in JRA-55 and the smallest in CFSR. The underestimates in double tropopause frequency are primarily attributable to the coarse vertical resolution of the reanalyses. Significant increasing trends in both tropopause altitude (40–120 m per decade) and double tropopause frequency (≥3 % per decade) were found in both the radiosonde observations and the reanalyses over the 35-year analysis period (1981–2015). ERA-Interim, JRA-55, and MERRA-2 broadly reproduce the patterns and signs of observed significant trends, while CFSR is inconsistent with the remaining datasets. Trends were diagnosed in both the native Eulerian coordinate system of the reanalyses (fixed longitude and latitude) and in a coordinate system where latitude is defined relative to the mean latitude of the tropopause break (the discontinuity in tropopause altitude between the tropics and extratropics) in each hemisphere. The coordinate relative to the tropopause break facilitates the evaluation of tropopause behavior within the tropical and extratropical reservoirs and revealed significant differences in trend estimates compared to the traditional Eulerian analysis. Notably, increasing tropopause altitude trends were found to be of greater magnitude in coordinates relative to the tropopause break, and increasing double tropopause frequency trends were found to occur primarily poleward of the tropopause break in each hemisphere.

Revisiting global satellite observations of stratospheric cirrus clouds

“…The lapse rate tropopause (LRT) is defined as the lowest level at which the lapse rate decreases to 2 • C/km or less, provided the average lapse rate between this level and all higher levels within 2 km does not exceed 2 • C/km (WMO, 1957). Due to the close relations to temperature and relative humidity, the LRT shows good agreement with sharp stability and chemical transitions between the troposphere and stratosphere, globally (Pan and Munchak, 2011;Spang et al, 2015;Xian and Homeyer, 2019).…”

Effects of the prewhitening method, the time granularity and the time segmentation on the Mann-Kendall trend detection and the associated Sen's slope

“…The negative aerosol slopes are related to the decreasing aerosol load in Western Europe and North America (Collaud Coen et al, 2020, Yoon et al, 2016). The increasing 260 tropopause level trend is related to global warming (Xian and Homeyer, 2019). The results of the trends will not be further described and discussed, since this study is only focused on the methodology of the trend analysis.…”

Double Tropopauses and the Tropical Belt Connected to ENSO

“…In both hemispheres, an annual pattern of peaks in DT numbers moves equatorward during winter, stopping at around ±30° latitude and withdrawing poleward in summer. The DT rates in the NH are more pronounced, last longer, and cover a greater area than in the SH, due to the different land‐sea distribution and thus different Rossby wave patterns (e.g., Schmidt et al., 2006; Xian et al., 2019).…”