Atmospheric rivers' direction over land matters for characterising its
impact in New Zealand
Abstract
Atmospheric Rivers (ARs) are filamentary channels of strong poleward
water vapour transport in the midlatitudes. Recent studies have
demonstrated the significant role of ARs in New Zealand’s water
resources and extreme precipitation events. Motivated by a recently
proposed AR-impact ranking scale in the USA to enhance the communication
between scientific communities and water sectors, here the
characteristics of AR events with peak daily rainfall greater than 100
mm over 5 divided sectors are further investigated, and the AR-impact
ranking scale is evaluated for the applicability for such events in New
Zealand. Previous studies have found that the windward side along
coastlines favours locally high rainfall. As such, we show that the
strength and duration of those AR events also vary with event direction,
and NW-AR events are normally stronger and longer than those of other
directions throughout the country. However, over the eastern areas, most
of those events are easterly directed and produce anomalously high
rainfall, despite being ranked as “Weak AR” events based on the
current AR-impact ranking scale. It is found that easterly directed ARs
originating from the west make landfall along the eastern coastline from
the ocean (i.e., from the east) or simply the onshore flow. Therefore,
our results suggest that localised ranking scale or considering more
parameters, such as AR over-land direction, might help improve the
AR-impact ranking scale applicability over eastern regions in New
Zealand.