Anomalous behavior of the total hydrogen mixing ratio around the turbopause Original Research Article

In the middle atmosphere the total hydrogen mixing ratio considering all hydrogen-carrying species is about 15 ppmv. It is commonly believed that this is a conservative quantity there and that the mixing ratio monotonically increases above the turbopause due to the action of molecular diffusion on the light hydrogen constituents. On the basis of a global 3D-model of the dynamics and chemistry of the middle atmosphere (0–150 km), we show that this picture is wrong under the condition of a flux through the domain which corresponds to the commonly accepted value of the hydrogen escape flux. The calculations clearly show a distinct minimum around 108 km. The drop of the hydrogen mixing ratio already starts in the upper mesosphere. The cause of this apparently contradictory behavior lies in the combined action of four influences: 1.) There is a flux through the domain. 2.) At the turbopause the eddy diffusion drops off suddenly and the molecular diffusion becomes prevalent above this height. 3.) Due to chemical decomposition the relatively heavy hydrogen constituents H2O and CH4 are converted into the lightest constituents H2 and H just in the mesopause region. 4.) Above the mesopause the temperature increases strongly, influencing the molecular diffusion coefficient. The decrease of the total hydrogen mixing ratio affects different phenomena occurring in the mesopause region, as we will briefly discuss.