Water vapour stratification and dynamical warming behind the sharpness of the Earth's midlatitude tropopause

Abstract

Recent studies have highlighted the fact that the extratropical lower stratosphere, up to ∼10 km from the thermal tropopause, is far from being isothermal. The tropopause is sharper than once thought and the vertical temperature profile in the next layers undergoes significant seasonal variations. Here, an analytical model is formulated for studying the seasonal temperature structure of that region of the Earth's atmosphere. A model fitting to over 10 years of stratospheric balloon observations at two midlatitude locations in opposite hemispheres captures the observed temperature profiles, including the tropopause inversion layer. Several sensitivity tests clarify the radiative role of ozone (through solar radiation) and water vapour (through thermal radiation) on the thermal stratification of the midlatitude tropopause region, and further demonstrate the role of heating from atmospheric motions. In spite of the major role played by ozone in upper layers, ozone heating only accounts for a small part (∼1 K) of the temperature maximum of the tropopause inversion layer, and its effect is negligible in the first kilometre right above the tropopause. In contrast, the vertical structure of water vapour between the tropopause and the hygropause combined with dynamical warming is found to explain the sharp change in lapse rate that marks the midlatitude tropopause, and most of its immediate inversion layer. This reveals how both radiation – chiefly thermal absorption and emission by stratified water vapour – and dynamics are behind the mysterious sharpness of the tropopause in midlatitudes.