A model for transport across the tropical tropopause

A model of convective and advective transport across the tropical tropopause is described. In this model overshooting convective turrets inject dehydrated tropospheric air into a tropical "tropopause layer'' (TTL) bounded approximately by the 50- and 150-hPa surfaces, a layer similar to the "entrainment zone'' at the top of the planetary boundary layer. The overshooting process occurs only in limited regions. In the TTL, mixtures of overshooting and ambient air undergo buoyancy-driven settling, then slowly loft through the TTL and eventually enter the main stratosphere throughout the Tropics. It is found that for reasonable parameter settings the combined action of convection, isentropic mixing, and advection by the large-scale circulation in the model can produce realistic water vapor and ozone profiles while balancing the energy budget. Some of the observed peculiarities that can be simulated are (i) the widespread absence of vapor saturation at the tropopause despite tropical mean upward motion, (ii) an ozone minimum below the mean tropopause, and (iii) the typical location of stratiform cloud tops below the mean tropopause. In contrast to inferences from typical "cold trap'' models, the relative humidity of air crossing the tropopause is found to be sensitive to ice microphysics.