What sets the humidity of the tropical stratosphere?

It is not known what processes set the humidity of the air that enters the tropical stratosphere from the tropical tropopause layer (TTL). Candidate processes include convective drying, drying by gravity waves, moistening by convective injection of ice, and drying during slow ascent through the cold-point tropopause. These processes are investigated here using a cloud-resolving model in radiative convective equilibrium with an applied Brewer-Dobson circulation.

The air that enters and exits the bottom and top of the TTL may be described as a mass-source distribution along a specific-humidity axis. For a TTL in a steady state, this distribution must balance the microphysical sources and sinks of air along the humidity axis. By conditioning these microphysical sources and sinks on the presence of convection, we can identify the processes that generate air with stratospheric humidity.