Atmospheric rivers (ARs) are an important component of the global water cycle, and AR-related hydrological hazards are across the midlatitudes, including New Zealand. The ability to classify the strength and potential impact of ARs has shown benefits to water resource management in western U.S. regions. In New Zealand, the topography means that the impact of ARs may vary significantly on either side of mountainous areas. In this study, we evaluate whether ARs dominate monthly heavy rainfall events and investigate characteristics of AR events that contribute to heavy rainfall as well as the performance of an AR impact ranking scale concerning orientations for these AR events. We found that different geographical regions favour ARs with different orientations for heavy rainfall generation, and northwesterly ARs are more likely to deliver higher rainfall at certain classification levels. Furthermore, we showed cases where ARs classified at the level could result in different benefits and hazards in different locations. Future research should focus on a more descriptive AR ranking scale regarding hydrometeorological benefits and hazards, considering factors such as the correspondence cyclones and topography.

Laboratory experiments were performed to study the impact of the relative particle protrusion P/D (P is the protrusion height and D is the diameter of the target particle) on the mechanism of entrainment of sediment particles from a spherical rough bed. The target particle to be entrained was instrumented with electronic sensors, which can measure the tri-axis linear acceleration, and consequently the inertial hydrodynamic forces, during the entrainment process. The velocity field was obtained using the two-dimensional Particle Image Velocimetry (PTV) technique and the velocity data were synchronised with the force data relative to the entrainment time. Experimental results show that the magnitudes of inertial drag force and lift force have a decreasing trend as particle protrusion increases. The ratio of inertial lift force to drag force reveal that drag force slightly dominates the entrainment process at P/D > 0.7 while lift force slightly dominates at P/D < 0.62. Also, the inertial drag and lift coefficients were computed by the force data and velocity data. The inertial drag coefficient was found to be independent of P/D when P/D < 0.62 but declined with increasing P/D for P/D > 0.62. Similarly, the variation of inertial lift coefficient with P/D reversed at P/D = 0.7. This variation of force coefficients with P/D is consistent with the independence of inertial forces with respect to P/D when P/D > 0.62-0.7. In summary, the inertial forces demonstrate that the impact of protrusion on the particle entrainment becomes less important when P/D > 0.62-0.7.

This study is motivated by the potential improvement in water supply reliability and better forecasts of extreme rainfall and floods linked to Atmospheric rivers (ARs) in New Zealand. Results indicate that ARs generally dominate monthly rain amounts and wet and dry conditions for the western side of mountainous regions and the north part of the country. Precursor soil moisture conditions and their relationship to runoff are also evaluated. In these regions, ARs are more likely to replenish soil moisture or lead to flash floods in warm-season months but generate severe floods in cool-season months due to low soil moisture deficit. This relationship is further confirmed by the seasonality of the monthly maximum streamflow of the Whanganui River (a major river in the North Island of New Zealand). Additionally, ARs generally lead to high streamflow all year round and are responsible for most top 10% streamflow.

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.

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.