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Extreme weather event real-time attribution machine

As GHGs continue to accumulate in Earth’s atmosphere, the resultant warming of the climate changes the 
nature of extreme weather events (EWEs). Near real-time, scientifically defensible, attribution of such events to human-induced changes in climate not only satisfies the public’s desire to know, but can also highlight the future risks of such events to emergency managers, regional planners, the insurance industry, and policy-makers at all levels of government. Through this project our team is building such a near real-time capability on a foundation of improved understanding of EWE attribution which provides deeper insight into, and confidence in, the many risk calculations that underpin New Zealand’s building codes; land, water, health and flood management; insurance; transport networks, and many additional aspects of daily life. We can be confident that climate warming has increased the moisture-holding capacity of the atmosphere, and evidence for increased severity of extreme rainfall is accumulating. Furthermore, atmospheric circulation changes will affect the nature of EWEs in complex ways. While this creates a greater attribution challenge, for New Zealand in particular, accounting for changes in dynamics is essential to communicating a coherent picture of EWE attribution to stakeholders. 

As the science of ‘event attribution’ develops, interest in generating messages around the contribution of climate change to EWEs in near real-time is growing. Our team will conduct the research necessary to develop an Extreme Weather Event Real-time Attribution Machine (EWERAM) whose outputs will be broadly disseminated to the New Zealand public through standard MetService communication channels. We will analyze the contribution of climate change to both the severity and frequency of target EWEs. There are many methodological choices that need to be evaluated to provide a sound scientific underpinning for EWERAM and these evaluations are forming foci for the proposed research. The general underlying principle for attributing climate change to the severity and/or frequency of an event is to compare two large ensembles of numerical weather prediction (NWP) model simulations under two sets of conditions:
  • the anthropogenic (ANT) ensemble where the context for the simulations mirrors current conditions under which the EWE evolved; often referred to as the ‘factual’ ensemble.
  • the natural (NAT) ensemble where the context for the simulations is sourced from a similar synoptic meteorological state, but where sea surface temperatures, air temperatures, and the moisture holding capacity of the atmosphere have been modified to mimic pre-industrial conditions; often referred to as the ‘counter-factual’ ensemble.
The extent to which the ANT and NAT ensembles of simulations differ provides a quantitative indication of the contribution of climate change to the severity of the event. Methodological choices that need to be made regarding the construction of the ANT and NAT simulations include selection of the initial conditions and selection of the lateral and surface boundary conditions. 

The project is creating a public service where, shortly after the occurrence of an EWE, and in response to media questions about the role of climate change in the event, rather than generic statements, scientifically defensible data will be available to inform quantitative statements about the role of climate change in both the severity and frequency of the event. The project will address the scientific and technical challenges to a level that demonstrates operational functionality for delivering what is required to answer stakeholders’ questions.
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