Simulations of Clouds and Water Vapor in Boreal Summer Tropical Tropopause Layer with a Microphysical Model

The Tropical Tropopause Layer (dubbed the TTL), roughly between 13 and 18km altitude, is one of the coldest parts of the earth's atmosphere. In contrast to the rest of the global tropopause region, radiative heating rates are positive and mean vertical motion is upward. It is thus the pathway for constituents into the stratosphere, and the cold temperatures lead to the well-known very dry stratospheric conditions. This simple picture is made more complicated by the interaction of convective injection, horizontal advection through cold regions (and consequent dehydration), slow ascent, and constraints on the nucleation of ice crystals. All these processes have significant effects on the water vapor distribution. Simulations of the boreal winter TTL have shown that including microphysics and convection in trajectory- based calculations moisten the TTL by about 1 ppmv, improving agreement with observations. Simulated horizontal distributions are also improved. In the boreal summer, the distribution of radiative heating rates strongly affects simulated water vapor and clouds. Generally, simulated water vapor values are wetter than observations, while the model simulates fewer clouds than observed. The implication is that gravity wave amplitudes are too low and that convective injection is inadequate. Results will be presented using: (1) modified convective injection based on analyses of CLOUDSAT and CALIPSO measurements, and (2) modified gravity wave parameterizations.