Assessing the role of overshooting deep convection on water vapour in the TTL

Overshooting deep convection penetrating into the Tropical Tropopause/Transition Layer (TTL, 355-425 K, ~14-18.5 km) is thought to have an important role in regulating the water content of the region. Yet, the relative roles of transport, mixing and microphysics in governing this influence, along with the sensitivity of these processes to the ambient TTL relative humidity remain unclear. In this study, results from two three-dimensional, cloud-resolving Weather Research and Forecasting (WRF) model simulations of overshooting convection are presented. These simulations were performed under idealised conditions and simplified model physics. The aim was to better understand the dominant physical processes governing the role of overshooting deep convection in defining the TTL moisture content. These simulations were initialised with soundings obtained from the Tropical Warm Pool-International Cloud Experiment (TWP-ICE, Darwin, Australia, 2006; May et al., 2008) that were specifically chosen because they reflected subsaturated and supersaturated TTL relative humidity conditions (with respect to ice) below 400 K.

With the aid of passive tracers incorporated into the model as simple proxies for parcel origin, our results suggest that overshooting deep convection plays an important and direct role in driving the ambient TTL environment in which it penetrates towards ice saturation, through either net moistening (subsaturated TTL) or net dehydration (supersaturated TTL). The moisture adjustment effect is most efficiently achieved when the upper-level environment is supersaturated to begin with, indicating the dominant role of ice-phase microphysics (through the vapour-scavenging effect of ice) in comparison to transport and mixing processes. Overshooting convection therefore provides the required catalyst for dehydrating the TTL/lower-stratosphere once it becomes supersaturated by horizontal transport or other processes. Furthermore, significant moistening in both cases was modelled well into the subsaturated tropical lower stratosphere (up to 450 K) at simulation end, even though the turret overshoots had only reached the altitude of \sim420 K. It is shown that this moistening is the result of ‘jumping cirrus’, whose occurrence was induced by the localised upward transport of TTL air near the vicinity of the overshoot following its collapse.