Relative humidity over ice as a key variable for Northern Hemisphere midlatitude tropopause inversion layers

Abstract. The tropopause inversion layer (TIL) is a prominent feature of the midlatitude tropopause region, constituting a transport barrier. Adiabatic and diabatic processes might contribute to the formation and sharpening of the inversion. For both types of processes, relative humidity over ice is ideal for attribution; from theory and former model case studies, we expect enhanced relative humidity values with a sharp TIL. We use high-resolution radiosonde and ERA5 reanalysis data to show very good qualitative and quantitative agreement in terms of TIL features; thus, coarser ERA5 data can be used for further investigations. Next, we investigate the connection between TIL features and relative humidity measures in both radiosonde and ERA5 data, revealing a clear relationship. Moister profiles, on average, exhibit significantly higher maximum values of the Brunt–Väisälä frequency (N2), indicating a more stable stratification of the tropopause in these cases. This result holds true in both radiosonde measurements and ERA5 data. For TIL thickness, an inverse pattern emerges: moister, more stable TILs exhibit lower thickness. Because of the good agreement between radiosonde and ERA5 data, we use ERA5 data for seasonal and regional investigations. These analyses reveal consistent TIL properties in various midlatitude regions of the Northern Hemisphere under different meteorological conditions. However, differences in the strength of the dependence of TIL properties on relative humidity over ice are evident between the different regions.