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Atmospheric rivers' orientation and landfall direction matter for characterising the associated impact on extreme rainfall and water resources
  • Jingxiang Shu,
  • Asaad Yahia Shamseldin,
  • Evan Weller
Jingxiang Shu
University of Auckland

Corresponding Author:[email protected]

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Asaad Yahia Shamseldin
University of Auckland
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Evan Weller
University of Auckland
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Abstract

Atmospheric Rivers (ARs) are filamentary channels of strong poleward water vapour transport in the midlatitudes. Recent studies have found that ARs are northwesterly and northeasterly orientated and can make landfall in all directions over New Zealand. In this study, we further investigate the characteristics, in particular orientation and landfall direction, of detected landfalling ARs based on two atmospheric reanalysis datasets over 35 years. Daily rainfall records from 655 rain gauges between 1979 to 2018 were used to investigate the spatial variability of the AR contribution to annual rainfall and extreme rainfall linked with AR events with different orientations and mean landfall directions. A modified AR impact ranking scale was then evaluated regarding AR-event orientation and mean landfall direction, different “AR impact” sectors, and peak daily AR-event rainfall. We found that landfalling ARs (events) with a northwesterly orientation and northwesterly landfall direction (NW-NW ARs) are the most frequent and relatively stronger, more coherent and concentrated over the country. As a result, NW-NW ARs are major contributors to annual rainfall and extreme rainfall for the country’s West Coast. Generally, the windward side experiences anomalously high rainfall as ARs reach the country from different directions, and the spatial distribution of AR-event heavy rainfall is shown to vary with an AR’s orientation and landfall direction. Moreover, the AR impact ranking scale performs well for NW-NW ARs over the West Coast. However, more factors need to be considered to improve the applicability of the scale on the East Coast.