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Tim Kerr, M. S. Srinivasan, and Jeremy Rutherford

Abstract

Stable water isotope concentrations were obtained from samples of stream water at 29 sites in a west–east transect across the Southern Alps of New Zealand, where westerly conditions dominate the precipitation regime. The samples were taken from small catchment streams during a time of extended recession as a means of collecting time and space averages of the source precipitation. The isotopic concentrations from sites at either end of the transect lead to a drying ratio estimate of 22%–34% for this region of the Southern Alps. The isotope concentrations increased from west to east, indicating that precipitation in the lee area originates from higher and/or colder condensation than on the windward side. The transect was divided into three regions according to the deuterium-excess (d excess) results. Increasing d-excess values on the windward side of the mountains were speculated to be a result of some unknown combination of reducing relative evapotranspiration, increasing recycling of water vapor, and increasing nonequilibrating condensation from high-intensity precipitation. Low d-excess values in the region immediately lee of the mountains were consistent with below-cloud evaporation associated with increased hydrometeor drift into the drier lee side. High d excess in the more distant lee side was attributed to a secondary source of moisture (from the east and south). The information gained has supported current concepts of the precipitation processes dominant in the region and has provided additional quantitative measurements with which to validate future precipitation modeling efforts.

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Rasool Porhemmat, Heather Purdie, Peyman Zawar-Reza, Christian Zammit, and Tim Kerr

Abstract

Synoptic-scale moisture transport during large snowfall events in the New Zealand Southern Alps is largely unknown due to a lack of long-term snow observations. In this study, records from three recently developed automatic weather stations (Mahanga, Mueller Hut, and Mt Larkins) near the Main Divide of the Southern Alps were used to identify large snowfall events between 2010 and 2018. The large snowfall events are defined as those events with daily snow depth increase by greater than the 90th percentile at each site. ERA-Interim reanalysis data were used to characterize the hydrometeorological features of the selected events. Our findings show that large snowfall events in the Southern Alps generally coincide with strong fields of integrated vapor transport (IVT) within a northwesterly airflow and concomitant deepening low pressure systems. Considering the frequency of large snowfall events, approximately 61% of such events at Mahanga were associated with narrow corridors of strong water vapor flux, known as atmospheric rivers (ARs). The contributions of ARs to the large snowfall events at Mueller Hut and Mt Larkins were 70% and 71%, respectively. Analysis of the vertical profiles of moisture transport dynamics during the passage of a landfalling AR during 11–12 October 2016 revealed the key characteristics of a snow-generating AR in the Southern Alps. An enhanced presence of low- and midlevel moisture between 700 and 850 hPa and pronounced increases of wind velocities (more than 30 m s−1) with high values of the meridional component between 750 and 850 hPa were identified over the Southern Alps during the event.

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