Diurnal preconditioning of subtropical coastal convective storm environments

Boundary layer evolution in response to diurnal forcing is manifested at the mesobeta and smaller scales of the atmosphere. Because this variability resides on subsynoptic scales, the potential influence upon convective storm environments is often not captured in coarse observational and modeling datasets, particularly for complex physical settings such as coastal regions. A detailed observational analysis of diurnally forced preconditioning for convective storm environments of South East Queensland, Australia (SEQ), during the Coastal Convective Interactions Experiment (2013-15) is presented. The observations used include surface-based measurements, aerological soundings, and dual-polarization Doppler radar. The sea-breeze circulation was found to be the dominant influence; however, profile modification by the coastward advection of the continental boundary layer was found to be an essential mechanism for favorable preconditioning of deep convection. This includes 1) enhanced moisture in the city of Brisbane, potentiality due to an urban heat island-enhanced land-sea thermal contrast, 2) significant afternoon warming and moistening above the sea breeze resulting from the advection of the inland convective boundary layer coastward under prevailing westerly flow coupled with the sea-breeze return flow, and 3) substantial variations in near-surface moisture likely associated with topography and land use. For the 27 November 2014 Brisbane hailstorm, which caused damages exceeding $1.5 billion Australian dollars (AUD), the three introduced diurnal preconditioning processes are shown to favor a mesoscale convective environment supportive of large hailstone growth. The hybrid high-precipitation supercell storm mode noted for this event and previous similar events in SEQ is hypothesized to be more sensitive to variations in near-surface and boundary layer instability in contrast to contemporary supercell storms.