On the temperature profile of the lower stratosphere and the sharpness of the tropopause in the extratropics

Resumo

This dissertation aims to explain why the Earth´s tropopause is persistently sharp in the extratropics, displaying the tropopause inversion layer (TIL) – a phenomenon that has received much scientific attention, but is not yet well understood. Since the TIL forms part of the lower stratosphere, the study attempts to model the vertical temperature profile of the extratropical lower stratosphere. An analytical model is formulated, assuming grey absorption by H2O and CO2 in the far-infrared and radiative-dynamical equilibrium on the seasonal mean. The model indicates that the time-averaged TIL is due to the interaction of thermal radiation with the sudden decrease of water vapour in air in the transition layer troposphere–stratosphere. Nevertheless, it would be impossible under radiative equilibrium, requiring dynamical warming. A model testing against over 10 years of stratospheric balloon observations over two midlatitude sites in opposite hemispheres captures the temperature profiles up to 10 km from the tropopause in all seasons, including the TIL. The implied dynamical warming shows consistency with the Brewer–Dobson circulation. A set of sensitivity tests clarify the relevance of the factors behind the midlatitude tropopause sharpness. Despite the major role played by solar heating due to ozone in upper layers, it is found that ozone accounts for only a small part of the TIL, while its effect is negligible in the strongest part of the TIL. In contrast, the fine-scale distribution of water vapour, in combination with seasonally-varying dynamical warming, accounts for most of the TIL. The definition of tropopause sharpness is thoroughly discussed and then modelled. It is demonstrated that the sharpness of the temperature minimum nearby the tropopause is enhanced by the local rate of decrease of water vapour mixing ratio with increasing height. A wide-ranging definition related to the TIL’s static stability structure allows to show that the tropopause sharpness is also enhanced by dynamical heating. A model calculation of the static stability maximum at the TIL under varied conditions further confirms the main research findings. Overall, the dissertation supports the thesis that both radiation and dynamics are vital to the typical sharpness of the extratropical tropopause.