The composition of the air in the Upper Troposphere-Lower Stratosphere (UTLS) region is strongly influenced by the transport of atmospheric constituents across the Tropical Tropopause Layer (TTL). In this work we investigate the dynamical processes which are associated to deep tropical convection and their impact on water transport and mixing across the TTL and into the lower stratosphere.

Strong convective systems, generally known as Hector thunderstorms, develop frequently over the Tiwi Islands (a pair of closely spaced islands off the north coast of Australia) during the pre-monsoon season (Nov-Dec). A demonstrative strong Hector, on the 30th of November 2005, has been selected for this study. The characteristic features of this particular convective system are well documented as part of the ACTIVE and SCOUT-O3 Tropical field campaigns (based in Darwin, Australia), and peak heights up to 19 km (well within the UTLS region) have been recorded.

Mesoscale simulations using the UM (UK Met. Office Unified Model) and WRF model, were performed with domain-nesting down to a horizontal resolution of ~1 km. This fine resolution allows convective-scale processes to be resolved, and convective transport to be calculated explicitly. Results from the two models are compared with standard meteorological observations (e.g. precipitation rate, outgoing long-wave radiation, etc.). Both models are able to represent the onset, development and intensity of the convective system, with the storm being triggered by sea breeze convergence (in line with findings from previous studies).

The vertical distribution of water mixing ratio from model runs (including both gas and condensed phases) are compared to the total water mixing ratios measured by the FISH hygrometer instrument operated on board of the M55-Geophysica research aircraft. The simulated vertical profiles of water agree well with those measured by the FISH hygrometer. A detailed model analysis of the transport and distribution of water from the mid troposphere up to the lower stratosphere is presented at different stages in the development of the Hector convective system. Water is lifted from the troposphere upwards as the convective system develops and is mixed across the TTL. The simulations reveal a net cooling within the UTLS above the convective system. This cooling is due to irreversible mixing, which results from convective entrainment across the TTL.