| dc.description.abstract | The tropical tropopause layer TTL acts as the gateway for water vapor and other chemical tracers to the stratosphere, and is important for the chemical composition and radiative balance. In this thesis, two reanalyses from the ECMWF, ERA5 and ERA-Interim, are evaluated and compared to GPS Radio Occultation (GPS-RO), as an extension and deepening of the SPARC Reanalysis Intercomparison Project (S-RIP) report, Chapter 8 (under revision) and Tegtmeier et al. 2020. The temperature and height of two tropical tropopause definitions are studied from 2007-2018, the cold point tropopause (CPT) and the lapse rate tropopause (LRT). The aim of the thesis is to compare the newly published ERA5 to its predecessor, ERA-Interim, by using satellite data as a reference point, and evaluate if seasonal and interannual variations are well represented in reanalyses when compared to GPS-RO. The updates in ERA5 includes highly improved temporal and spatial resolution, new model cycle and assimilation of more observations. The climatological tropical annual mean temperature bias is positive and the height bias is negative for both reanalyses relative to GPS-RO. ERA5 has the smallest differences for both tropopause definitions, 0.2 K for CPT and 0.1 K for LRT, likely due to high vertical resolution, representing small changes in the temperature profiles accurately. The larger differences in ERA-Interim, 0.4 K for CPT and 0.7 K for LRT, maximises over central Africa and east Pacific. \newline The seasonal cycle is well represented in ERA5, while the tropopause height difference varies considerably throughout the year in ERA-Interim. Areas with increased CPT temperature biases are central Africa, the maritime continent and the east Pacific, which maximise during March and April for ERA5 (+0.4 K) and during January and February for ERA-Interim (+0.8 K). The effect of the ENSO is studied and the difference between ERA5 and GPS-RO is not enhanced during ENSO events, while the LRT temperature bias in ERA-Interim shows a small increase over the maritime continent during La Niña events. The shift in location of the lowest temperatures and highest altitudes during boreal summer to the area of the Asian summer monsoon region is captured by both reanalyses. ERA5 and ERA-Interim have enhanced CPT temperature biases south of the Indian peninsula, and an additional LRT temperature and height maximum is found over the Arabian peninsula in ERA-Interim. ERA5 produces tropical tropopause temperatures and heights in close agreement to GPS-RO, and improves the representation of the tropopause compared to its predecessor, ERA-Interim. | eng |