A trajectory model coupled to a simple micro-physical model is used to explore the observed relationship between convection, water vapor and cirrus clouds in the tropical tropopause layer (TTL). Horizontal transport associated with the local Hadley circulation leads to water vapor minima in the winter hemisphere separated from the convective regions and the region of minimum temperatures. These spatial signatures are consistent with observations of water vapor and cirrus in the TTL from the Halogen Occultation Experiment (HALOE). In the simulations, one third of observed ice is formed due to horizontal transport through cold regions. Applied variations in temperature over time scales longer than a few hours, similar to gravity wave induced perturbations, act to lower the simulated water vapor.