A model of HDO in the tropical tropopause layer

[摘要] Any theory of water vapor in the tropical tropopause layer (TTL) must explainboth the abundance and isotopic composition of water there.In previous papers, wepresented a model of the TTL that simulated the abundance of water vapor as well as thedetails of the vertical profile.That model included the effects of"overshooting" convection, which injects dry air directly into the TTL.Here, we present results for themodel after modifying it to include water's stable isotopologue HDO (where Drepresents deuterium, ²H).We find that the model predicts a nearly uniform HDOdepletion throughout the TTL, in agreement with recent measurements.This occursbecause the model dehydrates by dilution, which does not fractionate, instead of bycondensation.Our model shows that this dehydration by dilution is consistent with otherphysical constraints on the system.We also show the key role that lofted ice plays indetermining the abundance of HDO in the TTL.Such lofted ice requires a complementary source of dry air in the TTL; without that, the TTL will rapidly saturateand the lofted ice will not evaporate.

[学科分类] 大气科学