An evaluation of the UWNMS treatment of water vapor transport and cirrus formation in the upper troposphere/lower stratosphere

Recent satellite observations suggest frequent and persistent cirrus decks near the tropical tropopause, which are not always related to deep convection (i.e. cirrus anvils). New observations and theories concerning dynamics in the tropical tropopause layer (TTL) indicate that conditions in this layer influence the chemical composition of the tropical lower stratosphere. Also of interest is the observed upward trend in lower stratospheric water vapor. How might cirrus affect conditions within the TTL that relate to the concentration of water vapor in the lower stratosphere? In particular, how may anthropogenic emissions of ice nuclei (IN) affect cirrus properties, and in turn, water vapor transport?

We have used the University of Wisconsin Non-hydrostatic Modeling System (UWNMS) to investigate the relationship between the changing conditions of cirrus formation and water vapor transport. Using a spectrum of idealized IN, we will show that cirrus particles in the presence of IN are reduced in size and are more numerous when compared to cirrus unaffected by IN. We will also show how a shift in the size distribution of cirrus particles weakens the effects of dehydration mechanisms such as sedimentation, allowing for more water vapor to be transported into the lower stratosphere.