A drop in upper tropospheric humidity in autumn 2001, as derived from radiosonde measurements at Uccle, Belgium

Simulations of climate models predict a doubling of the amount of upper tropospheric water vapor by the end of this century, caused by the increasing concentrations of greenhouse gases. Observations indicate that the tropopause height has increased by several hundred meters since 1979. In this paper, we verify and link these two results by carrying out a time series analysis on a uniform database of corrected radiosonde vertical profiles gathered at Uccle, Belgium, and covering the 1990–2007 time period. The most remarkable finding of this trend analysis is a significant drop in upper tropospheric humidity (UTH) around autumn 2001, which marks an end to the upper tropospheric moistening of the precedent decade. This UTH drop in autumn 2001 coexists with a sudden lifting and cooling of the tropopause and with a significant stretch‐out of the free troposphere. Therefore, we conclude that these autumn 2001 trends are certainly associated with the dynamical behavior of the troposphere, triggered by the surface warming. Links with the solar variability and the lower stratosphere were investigated but could not be established definitely.