Lightning in Aotearoa New Zealand : how weather patterns and dynamic forcings influence spatial and temporal patterns of lightning in New Zealand.
Thesis DisciplineEnvironmental Science
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Convective storms can cause significant disruption to human activity, danger to life and damage to property and livelihood. Hazards include heavy rain and associated flash flooding, lightning and associated wild fires, hail, strong wind gusts and tornadoes. While thunderstorms are relatively infrequent in New Zealand compared to other regions around the world, associated hazards like lightning still pose a risk to humans. An understanding of spatial and temporal patterns of severe convective activity is needed to assist in decreasing these risk factors.
Twelve years of lightning data from the ground-based New Zealand Lightning Detection Network were obtained from the New Zealand Meteorological Service. Lightning was used as a proxy for severe convective activity in this research as, unlike automatic weather stations, radar or satellite-based lightning detection data, the dataset encompassed the New Zealand region in its entirety and for a sufficient time period. Lightning spatio-temporal analysis was combined with Kidson synoptic weather types, Southern Annular Mode (SAM) and El Niño Southern Oscillation (ENSO) indices to assess the influence of synoptic and hemispheric scale weather patterns on lightning production. In addition, an individual storm was investigated to gain a better understanding of local influences. The main aims of this research were, firstly, to assess where lightning occurs in New Zealand by virtue of production of a lightning climatology and, secondly, to investigate the hypothesis that convective triggers leading to lightning activity in New Zealand are associated with different synoptic and local forcings depending on time and geographic location.
Results show that lightning was more likely during trough synoptic weather situations, during the negative SAM phase and during La Niña; although the mechanism for lightning differs depending on locality. Troughs were particularly associated with lightning around western parts, where instability was enhanced by orography and lightning occurrence exhibited little seasonal or diurnal patterns. In contrast, lightning in eastern parts, especially in the South Island, exhibited a strong seasonal and diurnal signature. Lightning here occurred predominantly during the afternoon periods in warmer months and was primarily associated with interactions between synoptic-scale post-frontal south-westerly airflows and local winds. Lightning was also most dominant during the afternoon and early evening period during summer months across the central North Island. However, this lightning occurred primarily during blocking synoptic weather situations, where localised areas of convective development were created under high pressure.
This doctoral research investigated the relationship between lightning, as a proxy for severe convective activity, and spatial and temporal atmospheric processes and phenomena. It is the first lightning climatology to provide evidence of local and regional differences in lightning drivers in New Zealand. Furthermore, because lightning can be utilised as a proxy for severe convection, this research also resulted in an improved understanding of when, where and why severe convection occurs in New Zealand. Research outcomes can be applied for risk assessment, to pinpoint the most vulnerable localities / regions for lightning hazards. This can be utilised by groups interested in weather-‐related risk assessment (e.g. local councils) to help mitigate injury, death, damage to property and livelihood. In addition, a detailed knowledge of where and when lightning occurs can strengthen the advancement of nowcasting and forecasting techniques.