The concentration of the water isotope HDO in an air parcel reveals something about that parcel's history. This information is particularly important in the tropical tropopause layer (TTL), where the histories of air parcels -- in particular, how they are dried -- is of great scientific interest. Unfortunately, the concentration of HDO is only a single number, so there are several different histories consistent with that one concentration. To tease out the dominant mechanisms responsible for setting the HDO concentration of air parcels in the TTL, a cloud-resolving simulation is run in a Walker-cell configuration using a new 18-class isotope-enabled (H2O, HDO, and H218O) microphysics package. Enrichment of HDO in the TTL is found to occur through ice injection by convection, while gravity-wave generated cirrus is found to deplete the TTL of HDO.
The microphysical tendency of HDO in the upper troposphere and lower stratosphere (12 km to 20 km) throughout the 8000-km-wide domain. Air is enriched over the "warm pool" by the convective injection of HDO-rich ice (red areas). Air is also depleted throughout the 14-to-17-km range by gravity-wave generated cirrus (blue areas).