The Hydrometeorological Impact of Atmospheric Rivers in New Zealand

Abstract

Atmospheric rivers (ARs) are long, narrow corridors of meridional flow that transport 90% of global water vapour from the tropics and higher latitudes in the sky. ARs make landfall in most countries of midlatitude globally, including New Zealand. Their hydrometeorological and associated economic and societal impacts can be beneficial and hazardous. Understanding the hydrometeorological impacts of ARs in the country forms the motivation and directs this research. This thesis comprises studies contributing to four topics: (i) the climatology of ARs in New Zealand, (ii) the beneficial role of ARs – the association between ARs and rainfall accumulations, (iii) the hazardous role of ARs – the association between ARs and extreme rainfall events, and finally (iv) the applicability of AR event classification on heavy rainfall events in the country. The algorithm is presented for detecting ARs in the country. With detected ARs based on multiple atmospheric reanalysis datasets, the climatology and characteristics of landfalling ARs are investigated. Results highlight that the seasonality and spatial distribution of high rainfall accumulations and extreme rainfall events are presumably associated with ARs. The beneficial and hazardous role of ARs in the country is further studied through analysis examining the contribution of ARs to rainfall accumulations and extreme rainfall on different temporal scales based on more than 500 rain gauge sites scattered across the country. The beneficial and hazardous role is demonstrated through statistical comparison tests between rainfall associated with ARs and non-AR events. Finally, the characteristics of AR events leading to heavy rainfall events and the applicability of an AR impact ranking scale are assessed.