Kerryn Hawke

Christchurch city’s primary natural hazard exposure, arguably, is to various kinds of flooding and inundation. Recently the Canterbury Earthquake Sequence (CES) reshaped many aspects of the city’s natural and built environment, with significant changes to the flood-scape. In response to the CES impacts on the city’s waterways and drainage network, the Christchurch City Council (CCC) established the Land Drainage Recovery Programme (LDRP) to consider these impacts on flood risk, and to identify and implement responses to reduce these risks.Cited on a low-lying coastal plain in a seismically active setting, large parts of the city are subject to the effects of, and interactions between, multiple types of natural hazard, with a particular concentration of multi-hazard risk in coastal and riverside areas. This challenging circumstance gives rise to the question: How do we make decisions about flood management in a multi-hazard environment? To address this question, the Council initiated an investigation that aims to develop flood management plans for eastern Christchurch involving a range of sustainable, adaptable and resilient flood management options within a multi -hazard context. The hazards examined in this assessment included fluvial, pluvial and coastal flooding; the likely impacts of climate change on these including the impacts of sea level rise on coastal inundation, erosion and rising groundwater; and the potential cascading effects of future earthquakes (subsidence, liquefaction, lateral spread, ground shaking) and tsunami on flooding risks. The assessment approach employed involves three stages: (i) understanding the multiple hazards and multi-hazard interactions affecting the city’s exposure to flood risks in terms of spatial co-location, temporal coincidence and cascading impacts; (ii) undertaking targeted studies to address key gaps in our understanding of these multiple hazards; and (iii) developing a range of potential management options including engineering, planning and policy responses, within an adaptive planning pathways framework. Stage one has been completed, the stage two studies have largely been completed, and considerable work effort is being undertaken to address the challenges in proceeding to stage three of the study. This presentation overviews the overall approach of the project and presents the findings of the stage one analysis of multi-hazard interactions.

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. Convective storms in New Zealand occur, on average, fifteen to twenty days per year in northern and western parts of the country, while on the east coast of the South Island the average is commonly less than five occurrences per year. While thunderstorms are infrequent in New Zealand when compared to countries such as Australia and the USA, associated hazards like lightning still pose a risk to humans and so a better understanding of spatial and temporal patterns of severe convective activity is valuable to assist in decreasing associated risk factors. A twelve year ground-based lightning dataset was used as a proxy for severe convective activity in this research as, unlike AWS, radar or satellite-based lightning detection data, dataset encompasses the whole New Zealand region The main aims of this research, therefore, were to produce a spatial lightning climatology of New Zealand investigate the hypothesis that convective triggers leading to lightning activity in New Zealand are associated with different synoptic and local situations depending on time and geographic location. The purpose of this poster is to present the results of this high-resolution lightning climatological research in order to aid a better understanding of thunderstorm occurrence across New Zealand. There are different mechanisms for lightning. For example, lightning to the west of the Southern Alps in the South Island can occur under any weather situation at any time of the day or night and in any season of the year. Peak lightning occurrences are over and to the west of the seaward most mountain range, regardless of elevation. Lightning occurrence is highly influenced by topography in many places, most notably over the South Island, where the Southern Alps acts as a barrier. Applications include risk assessment, where research outcomes can be used 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 also strengthen the advancement of nowcasting and forecasting techniques.

Retrospective analysis of severe convective storms and associated lightning is useful for forecasting these extreme events. However, until recently it has been difficult to obtain sufficient datasets for long-term analysis. The use of lightning as a proxy for severe convection has become feasible in the past decade as global and regional lightning detection datasets have become of sufficient longevity for climatological investigation. A twelve-year (2001-2012) climatological analysis of positive and negative cloud-to-ground discharge (+/-CGD) lightning in New Zealand was completed using data from a network of sensors maintained by the New Zealand Meteorological Service. A spatio-temporal lightning analysis in relation to diurnal and seasonal scales, Kidson’s synoptic types and variability introduced by SAM and ENSO was carried out. Results show that -CGD are largely dominant, with clear inter-annual and seasonal variability and topography shaping their spatial variability. Western areas, especially the West Coast of the South Island, experience the highest +/-CGD. They primarily occur under trough situations in response to orographic triggers and can occur at any time of the day or year, although more frequently during spring and autumn months (Sep-Dec, Mar-Jun). Eastern areas are most likely to experience lightning activity during summer (Nov-Feb), have a strong diurnal pattern and are linked to interactions between post-frontal unstable southwesterly flow regimes and smaller scale sea breeze convergence mechanisms. The central North Island also has a strong diurnal pattern, with lightning most likely to occur during the afternoon in the summer months (Dec-Jan). These primarily occur during blocking synoptic conditions, where slack air gradients and strong daytime heating produce small-scale wind interactions, local convection and the production of severe convective storm cells. These results have assisted in the construction of a lightning climatology, while subsequent investigation of the atmospheric processes involved is underway using a meso-scale modeling system (WRF-ARW).