Cirrus clouds and ice supersaturation
From many measurements we know that cirrus clouds and the tropopause are closely related: Cirrus clouds occur quite regularly in the tropopause region, leading to widely horizontally extended regions filled with ice crystals; the tops of cirrus cloud layers are often very close to the thermal tropopause. On the other hand, it is not clear how cirrus clouds could modify the structure of the tropopause region or even the distribution of trace gases in this region. In a current study we investigate this question in idealized 2D simulations using the EULAG model together with a state-of-the-art bulk ice microphysics scheme and passive tracers. Here, we concentrate on a specific case, namely patchy cirrus clouds, driven by convective instabilities.
The exchange of air masses across the tropopause plays an important role for the chemical composition of the stratosphere and troposphere. For instance, the injection of stratospheric air into the troposphere can enhance the ozone concentration significantly in the troposphere, even down to the boundary layer. On the other side, the amount of water vapor can be strongly increased when tropospheric air is transported into the stratosphere. Stratosphere-troposphere exchange (STE) occurs in different meteorological environments: e.g., near tropopauselevel jet streams due to turbulent mixing, associated with the diabatic decay of upper-level cutoff lows, or in overshooting deep convective systems. In this study the importance of STE is quantified in the vicinity of (mainly extratropical) cyclones, based upon climatologies of cyclones and STE.