The role of the sea breeze and synoptic conditions in severe storm formation in Brisbane

Abstract

This thesis assesses the role of the sea breeze and synoptic meteorological conditions in severe storm formation in Brisbane, southeast Queensland. Previous studies suggest that a causal link exists between severe storm initiation and the sea breeze in many regions, including a possible link in southeast Queensland. Numerous thunderstorm climatologies exist that describe Brisbane as a thunderstorm hotspot. The vast majority of these, however, focus on event-based sampling, radar data, reanalysis data or remotely-sensed data. Therefore, a significant knowledge gap exists in radiosonde-based thunderstorm climatologies for Brisbane. This analysis addresses this gap by considering three key radiosonde indicators of atmospheric stability (convective available potential energy, convective inhibition and effective storm relative helicity) for the 16 year period 2000-2015 and comparing these indicators between sea breeze days, non-sea breeze days and non-storm days. The analysis determined that an existing algorithm derived by Soderholm et al. (2015) for identifying the sea breeze in near-coastal inland areas of southeast Queensland was inapplicable to a locality within the ocean-modified coastal environment. Instead, this thesis presents a new algorithm that identifies days in which onshore flow increased whilst also remaining within a particular directional range. This approach has greater applicability to the ocean-modified coastal environment, successfully identifying 60% of a list of manually verified sea breeze days and only 20% of a list of manually verified non-sea breeze days over the 15-year period considered. Analysis of a list of severe storm events from the volumetric radar dataset derived from the dataset of Soderholm et al. (2015) shows that 54% of the severe storm events in this period coincided with either a sea breeze day or a day that exhibited increasing onshore flow. The relationship between storm severity and the rate of increase in onshore flow is not proportional. In general, the sea breeze lowered the threshold value of convective available potential energy required for severe storm initiation and allowed higher convective inhibition values to be overcome. No clear relationship between effective storm relative helicity and the sea breeze is evident. The synoptic-scale circulation conditions over Brisbane for all the storm days over the 16-year period are also analysed. Wind shear, vector wind and geopotential height at various pressure levels are considered, along with surface pressure. This analysis demonstrates that the “conducive storm conditions” reflected in these variables are indeed associated with severe storm formation but are not strictly necessary for all severe storm days, as they are not identified for all storm days. Therefore, instead of creating an idealised set of conditions that describe a severe storm day, these “conducive storm conditions” are ranked according to their relative usefulness in predicting severe storm formation. Show more