Abstract
Abstract. A 16-year series of daily snow-covered area (SCA) for 2000–2016 is derived from MODIS imagery to produce a regional-scale snow cover climatology for New Zealand's largest catchment, the Clutha Catchment. Filling a geographic gap in observations of seasonal snow, this record provides a basis for understanding spatio-temporal variability in seasonal snow cover and, combined with climatic data, provides insight into controls on variability. Seasonal snow cover metrics including daily SCA, mean snow cover duration (SCD), annual SCD anomaly and daily snowline elevation (SLE) were derived and assessed for temporal trends. Modes of spatial variability were characterised, whilst also preserving temporal signals by applying raster principal component analysis (rPCA) to maps of annual SCD anomaly. Sensitivity of SCD to temperature and precipitation variability was assessed in a semi-distributed way for mountain ranges across the catchment. The influence of anomalous winter air flow, as characterised by HYSPLIT back-trajectories, on SCD variability was also assessed. On average, SCA peaks in late June, at around 30 % of the catchment area, with 10 % of the catchment area sustaining snow cover for > 120 d yr−1. A persistent mid-winter reduction in SCA, prior to a second peak in August, is attributed to the prevalence of winter blocking highs in the New Zealand region. In contrast to other regions globally, no significant decrease in SCD was observed, but substantial spatial and temporal variability was present. rPCA identified six distinct modes of spatial variability, characterising 77 % of the observed variability in SCD. This analysis of SCD anomalies revealed strong spatio-temporal variability beyond that associated with topographic controls, which can result in snow cover conditions being out of phase across the catchment. Furthermore, it is demonstrated that the sensitivity of SCD to temperature and precipitation variability varies significantly across the catchment. While two large-scale climate modes, the SOI and SAM, fail to explain observed variability, specific spatial modes of SCD are favoured by anomalous airflow from the NE, E and SE. These findings illustrate the complexity of atmospheric controls on SCD within the catchment and support the need to incorporate atmospheric processes that govern variability of the energy balance, as well as the re-distribution of snow by wind in order to improve the modelling of future changes in seasonal snow.
Highlights
Seasonal snow packs represent an important, yet vulnerable water resource (Barnett et al, 2005; Mankin and Diffenbaugh, 2015) and play a major role in earth– atmosphere energy exchange (Estilow et al, 2015)
This paper aims to achieve the following: 1. produce a snow cover climatology for the Clutha Catchment, New Zealand, for the years 2000–2016 – the snow cover climatology is defined here as the characterisation of the expansion and depletion of the seasonal snow cover over the course of an average hydrological year; 2. leverage the snow cover climatology to examine basinwide temporal variability in snow-covered area and snowline elevation and assess whether any negative trend is detectable over the study period; 3. characterise spatial modes of temporal variability in snow cover duration in order to better understand spatiotemporal variability across the catchment; 4. assess the sensitivity of variability of SCD to variability in temperature, precipitation and synoptic-scale climate processes
The mean snowline elevation (SLE) during the winter period was found to be 1263 m a.s.l; this corresponds with an area of 3180 km2 and is substantially higher than the mean catchment elevation of 615 m
Summary
Seasonal snow packs represent an important, yet vulnerable water resource (Barnett et al, 2005; Mankin and Diffenbaugh, 2015) and play a major role in earth– atmosphere energy exchange (Estilow et al, 2015). N. Redpath et al.: Clutha Catchment snow cover variability lishment of the Snow and Ice Network (SIN) of alpine automatic weather stations (Hendrikx and Harper, 2013), longterm records of snow occurrence and persistence are rare in New Zealand (Fitzharris et al, 1999). Redpath et al.: Clutha Catchment snow cover variability lishment of the Snow and Ice Network (SIN) of alpine automatic weather stations (Hendrikx and Harper, 2013), longterm records of snow occurrence and persistence are rare in New Zealand (Fitzharris et al, 1999) This scarcity of data has limited the empirical characterisation of seasonal snow processes. Modelling work undertaken to assess future impacts of climate change on seasonal snow in New Zealand predicts a substantial reduction overall in seasonal snow cover duration (SCD), the snow proportion of precipitation and peak snow accumulation through the 21st Century (Hendrikx et al, 2012; Jobst et al, 2018)
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