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.