A model study on the influence of overshooting convection on TTL water vapor

Overshooting deep convection that penetrates into the Tropical Tropopause Layer (TTL) is thought to have an important role in regulating the water vapor content of this region. Yet, the net effect of such convection and the dominant mechanisms remain unclear. This study uses two idealized three-dimensional cloud-resolving model simulations to examine the influence of overshooting convection on water vapor, when it penetrates into two different TTL environments, one supersaturated and the other subsaturated with respect to ice. These simulations show that the overshooting convection plays a direct role in driving the ambient environment towards ice saturation through either net moistening (subsaturated TTL) or net dehydration (supersaturated TTL). Moreover, in these cases the extent of dehydration in supersaturated conditions is greater than the moistening in subsaturated conditions. By using modeled passive tracers and carefully constructing the simulations to have similar storm structures, the relative roles of transport, mixing and ice microphysics are assessed; ultimately, ice sublimation and scavenging processes play the most important role in defining the different TTL relative humidity tendencies. In addition, significant moistening in both cases is modeled well into the subsaturated tropical lower stratosphere (up to 450 K), even though the overshooting turrets only reach approximately 420 K. It is shown that this moistening is the result of jumping cirrus, which is induced by the localized upward transport and mixing of TTL air following the collapse of the overshooting turret.