Atmospheric rivers' orientation and landfall direction matter for
characterising the associated impact on extreme rainfall and water
resources
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.