Possible contributions of pileus clouds to TTL dehydration

Measurements from CRYSTAL-FACE show that thin tropopause cirrus (TTC) was frequently present above anvil cirrus. TTC was typically a hundred times more tenuous, about 20 K colder, and had similar horizontal dimensions to the anvil. Photography, as well as analysis of the cloud dynamics, chemistry, and isotopic ratios has led to a hypothesis that the tropopause cirrus formed initially as cap-shaped pileus clouds over deep convective turrets. The pileus spread as stratiform layers over the anvil, where they were shielded from heating from terrestrial infrared radiation. Here we propose possible implications of pileus cloud formation to the process of stratospheric dehydration. In the moist stratified tropopause transition layer (TTL), isentropes are bent upwards ahead of deep convection. A thin pileus cloud may form if isentropic surfaces are raised sufficiently that air is cooled to the point of homogeneous ice nucleation. If the TTL was initially supersaturated with respect to ice (as is often observed), once the convection subsides and the isentropes flatten, a stratiform TTC cloud lingers. As the anvil beneath the TTC thins, the TTC is increasingly exposed to radiative heating from the earth's surface, and it is lofted upwards. If it precipitates it may contribute to the dessication of the lower stratosphere.