School of Earth Sciences - Theses
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ItemThe influence and observability of overshooting convection in the tropical tropopause layer( 2012)Intense tropical convection that overshoots deep into the Tropical Tropopause Layer (TTL) is thought to play an important role in influencing the region’s moisture content. However, the net effect of such convection and the details of how it affects TTL humidity are still not well understood. Furthermore, the degree of this influence is dependent on the statistics of overshooting convection, such as its frequency, areal extent and depth, which may differ between different convective regimes. This thesis first examines the key physical processes that govern the net influence of overshooting convection on TTL water vapour when it penetrates into different observed TTL environments, one supersaturated and the other subsaturated (with respect to ice), using idealised three-dimensional cloud-resolving simulations. It then investigates the characteristics of overshooting convection during a convectively active monsoon regime and a ‘break’ period, using ground-based radar data from the Tropical Warm Pool-International Cloud Experiment (TWP-ICE). In addition, observational biases – particularly at TTL levels – are determined, guided by synthetic imagery derived from emulated radar scans of realistic, high-resolution numerical model simulations of the two TWP-ICE regimes. These are then corrected using a simple correction scheme. Overshooting convection is demonstrated to play a direct efficient role in driving the ambient local environment towards ice saturation through either net moistening (subsaturated TTL) or net dehydration (supersaturated TTL). However, the extent of dehydration in supersaturated conditions is greater than the moistening in subsaturated conditions. Ultimately, vapour-scavenging and sublimation processes from convectively-lofted ice play the most dominant role in defining the net TTL relative humidity tendencies, although transport and mixing are implicitly important. Analysis of gridded TWP-ICE radar data reveals that overshooting storms during the two regimes possess slightly different characteristics. ‘Break’ overshooting convection is relatively more intense, has a pronounced diurnal cycle, and reaches above 18 km more often than similar storms during the active monsoon phase. Intense storms during the ‘break’ period also exhibit much less variability in terms of mean overshooting area in the TTL than their active period counterparts. However, the active phase during TWP-ICE produced a larger mean overshooting area with height compared to the ‘break’, although this was significantly influenced by an atypical day of enhanced overshooting activity associated with the passage of a large mesoscale convective system. Due to geometrical constraints and sampling gaps at high altitude, ground-based radar is shown to overestimate certain echo-top heights while consistently underestimating the highest core intensities and the mean overshooting area with height across both TWP-ICE regimes (O(100) km^2). This implies that the radar is underestimating the overshooting flux of water into the TTL during both periods. Yet, corrected profiles of detected mean overshooting area indicate that the average area covered by convective overshoots only differs marginally between regimes. Based on these corrections, ‘break’ overshooting convection is concluded to typically transport slightly more (particulate) water mass into the TTL than archetypal overshooting convection during the active monsoon. This result suggests that each convective regime may exert dissimilar effects on the TTL over the course of one monsoon season.