Brewster Glacier Research Articles

A new, high‐resolution atmospheric dataset for southern New Zealand, 2005–2020

AbstractThe regional climate of New Zealand's South Island is shaped by the interaction of the Southern Hemisphere westerlies with the complex orography of the Southern Alps. Due to its isolated geographical setting in the south‐west Pacific, the influence of the surrounding oceans on the atmospheric circulation is strong. Therefore, variations in sea surface temperature (SST) impact various spatial and temporal scales and are statistically detectable down to temperature anomalies and glacier mass changes in the high mountains of the Southern Alps. To enable future studies on the processes that govern the link between large‐scale SST and local‐scale high‐mountain climate, we utilized dynamical downscaling with the Weather Research and Forecasting (WRF) model to produce a regional atmospheric modelling dataset for the South Island of New Zealand over a 16‐year period between 2005 and 2020. The 2 km horizontal resolution ensures realistic representation of high‐mountain topography and glaciers, as well as explicit simulation of convection. The dataset is extensively evaluated against observations, including weather station and satellite data, on both regional (in the inner domain) and local (on Brewster Glacier in the Southern Alps) scales. Variability in both atmospheric water content and near‐surface meteorological conditions is well captured, with minor seasonal and spatial biases. The local high‐mountain climate at Brewster Glacier, where land use and topographic model settings have been optimized, yields remarkable accuracy on both monthly and daily time scales. The data provide a valuable resource to researchers from various disciplines studying the local and regional impacts of climate variability on society, economies and ecosystems in New Zealand. The model output from the highest resolution model domain is available for download in daily temporal resolution from a public repository at the German Climate Computation Center (DKRZ) in Hamburg, Germany (Kropač et al., 2023; 16‐year WRF simulation for the Southern Alps of New Zealand, World Data Center for Climate (WDCC) at DKRZ [data set]. https://doi.org/10.26050/WDCC/NZ‐PROXY_16yrWRF).

Applying a distributed mass-balance model to identify uncertainties in glaciological mass balance on Brewster Glacier, New Zealand

AbstractA distributed mass-balance model is used over a 10-year period for the re-analysis of a glaciological mass-balance time series obtained from Brewster Glacier, New Zealand. Mass-balance modelling reveals glaciological mass balance has been overestimated, with an average mass loss of −516 mm w.e. a−1 not captured by observations at the end of the ablation season, which represents 35% of the annual mass balance. While the average length of the accumulation season (199 days) remains longer than the ablation season (166 days), melting is not uncommon in the core part of the accumulation season, with 2–32% of total snowfall being melted. Refreezing of meltwater is also important, with 10% of surface and subsurface melt being refrozen in the present climate. Net radiation, driven primarily by net shortwave radiation, is the main contributor to melt energy, with melt variability mainly influenced by the turbulent heat fluxes, net longwave radiation and the heat flux from precipitation in the ablation season. Snowfalls in summer are an important moderator of melt, highlighting the critical role of the ice-albedo feedback and phase of precipitation on seasonal mass balance. A complete homogenisation of the long-term glaciological mass balance for Brewster Glacier is still required.

A decade of surface meteorology and radiation fluxes at Brewster Glacier in the Southern Alps of New Zealand

AbstractHigh‐altitude observations of mountain meteorology remain extremely rare in the Southern Alps despite their importance for detecting changes in seasonal snow and glacier extent. To address this, we present a unique in situ analysis of surface meteorology, including radiative forcing from clouds, using automatic weather station data obtained near the terminus of Brewster Glacier in the Southern Alps of New Zealand over the period 2010–2020. The average air temperature is 2.1°C at an altitude of 1,650 m above sea level, and seasonal cycles of surface temperature and albedo indicate the automatic weather station (AWS) site is typically snow‐covered between June and November. Snowfall can occur anytime of the year, but rainfall accounts for more than half of the total precipitation (56%). In the absence of strong gradient airflow, a katabatic wind often develops on the glacier, which is strongest at night time and early morning. Partly cloudy conditions are most frequent (43%) at a daily time scale, followed by overcast (34%) and clear‐sky conditions (23%). A distinct diurnal and seasonal cycle of cloud cover is observed, with clouds most frequent in the afternoon during spring. Daily clear‐sky conditions are most common during winter, in particular in June, while the atmospheric transmission of shortwave radiation is lowest in summer. For 4 months of the year (May–August), the increase in incoming longwave radiation by clouds exceeds the decrease in incoming shortwave radiation. Interannual extremes in melt on Brewster Glacier cannot be fully attributed to summer air temperature anomalies and future work should seek to examine the influence of clouds and atmospheric moisture fluxes on seasonal mass balance variations.

A Detailed, Multi‐Scale Assessment of an Atmospheric River Event and Its Impact on Extreme Glacier Melt in the Southern Alps of New Zealand

AbstractNorth‐westerly airflow and associated atmospheric rivers (ARs) have been found to profoundly influence New Zealand’s west coasts, by causing flooding, landslides and extreme ablation and accumulation on glaciers in the Southern Alps. However, the response of local glacier mass balance to synoptic‐scale circulation, including events with ARs, has typically not been investigated by considering mesoscale processes explicitly. In this study, high‐resolution atmospheric simulations from the Weather Research and Forecasting model are used to investigate the mesoscale drivers of an extreme ablation event on Brewster Glacier (Southern Alps), which occurred on February 6, 2011 during the landfall of an AR on the South Island. The following processes were found to be crucial for transferring the high temperature and water vapor contained in the AR into energy available for melt on Brewster Glacier: First, the moist‐neutral character of the air mass enabled the flow to pass over the ridge, leading to the development of orographic clouds and precipitation on the windward side of the orography, and foehn winds on the leeside. These processes fueled melt through longwave radiation and strong turbulent and rain heat fluxes within the high‐condensation environment of the orographic cloud. Second, orographic enhancement occurred due to both cellular convection within the cloud and the combined effect of multiple precipitating systems by the seeder‐feeder‐mechanism. These results indicate the potential importance of AR dynamics for New Zealand’s glaciers. They also illustrate the benefit of mesoscale atmospheric modeling for advancing process understanding of the glacier‐climate relationship in New Zealand.

The Influence of Weather Systems in Controlling Mass Balance in the Southern Alps of New Zealand

AbstractThe recent advance of some fast‐responding glaciers in New Zealand demonstrates that regional variability in atmospheric circulation can counteract the effects of global warming. Until recently, a key challenge in quantifying how weather systems influence glaciers has been the lack of observational data from high‐elevation sites in the Southern Alps. Using high‐quality meteorological and glaciological observations from Brewster Glacier over four summers and two winters, a physically based mass balance model is used to resolve daily ablation and solid precipitation. Composite analysis confirms ablation is largely controlled by changes in meridional airflow, with high ablation associated with north to northwest airflow, while low ablation is associated with south to southwest airflow. The largest snowfall events in the cool season also occur during northwesterly airflow, highlighting the importance of seasonality (i.e., whether an event occurs in the ablation or accumulation period) for mass balance. Application of a synoptic weather typing approach commonly used for the New Zealand region, known broadly as Kidson weather types, revealed that three established regimes (trough, zonal, and blocking) are less effective in resolving variability in ablation and snowfall than a simple procedure that groups weather types using geostrophic flow direction. The frequency of four weather types can explain approximately 40% of the variance in mass balance over a 42‐year period. This highlights that future regional variability or trends in large‐scale circulation and their seasonality, superimposed on background warming, will be important in shaping glacier mass balance in the coming decades.

The Role of Atmospheric Rivers for Extreme Ablation and Snowfall Events in the Southern Alps of New Zealand

AbstractThe role of atmospheric rivers (ARs) for extreme ablation and snowfall is examined at Brewster Glacier in the Southern Alps, the site of the longest glacier mass balance record in New Zealand. By global standards, New Zealand is strongly impacted by ARs. Here it is shown for the first time (in New Zealand) that ARs strongly contribute to extreme snowfall and ablation and thus mass balance overall. Vertically integrated water vapor transport (IVT) exceeds 1,600 (800) kg·m−1·s−1 for the largest ablation (snowfall) events, marking these as very strong ARs. The proximity to the freezing threshold during extreme snowfall events indicates the sensitivity of mass balance to temperature variation. Importantly, similarly high rates of IVT during some extreme ablation and snowfall events also occur outside of conventional AR spatial structures. This finding indicates that AR detection algorithms may substantially underestimate the importance of extreme IVT for New Zealand and elsewhere.

Using structure from motion photogrammetry to measure past glacier changes from historic aerial photographs

ABSTRACTQuantifying historic changes in glacier size and mass balance is important for understanding how the cryosphere responds to climate variability and change. Airborne photogrammetry enables glacier extent and equilibrium line altitudes (ELAs) to be monitored for more glaciers at lower cost than traditional mass-balance programs and other remote-sensing techniques. Since 1977, end-of-summer-snowlines, which are a proxy for annual ELAs, have been recorded for 50 glaciers in the Southern Alps of New Zealand using oblique aerial photographs. In this study, we use structure from motion photogrammetry to estimate the camera parameters, including position, for historic photographs, which we then use to measure glacier change. We apply this method to a small maritime New Zealand glacier (Brewster Glacier, 1670–2400 m a.s.l.) to derive annual ELA and length records between 1981 and 2017, and quantify the uncertainties associated with the method. Our length reconstruction shows largely continuous terminus retreat of 365 ± 12 m for Brewster Glacier since 1981. The ELA record, which compares well with glaciological mass-balance data measured between 2005 and 2015, shows pronounced interannual variability. Mean ELAs range from 1707 ± 6 to 2303 ± 5 m a.s.l., with the highest ELAs occurring in the last decade.

An 11-year record of mass balance of Brewster Glacier, New Zealand, determined using a geostatistical approach

ABSTRACTRecognising the scarcity of glacier mass-balance data in the Southern Hemisphere, a mass-balance measurement programme was started at Brewster Glacier in the Southern Alps of New Zealand in 2004. Evolution of the measurement regime over the 11 years of data recorded means there are differences in the spatial density of data obtained. To ensure the temporal integrity of the dataset a new geostatistical approach is developed to calculate mass balance. Spatial co-variance between elevation and snow depth allows a digital elevation model to be used in a co-kriging approach to develop a snow depth index (SDI). By capturing the observed spatial variability in snow depth, the SDI is a more reliable predictor than elevation and is used to adjust each year of measurements consistently despite variability in sampling spatial density. The SDI also resolves the spatial structure of summer balance better than elevation. Co-kriging is used again to spatially interpolate a derived mean summer balance index using SDI as a co-variate, which yields a spatial predictor for summer balance. The average glacier-wide surface winter, summer and annual balances over the period 2005–15 are 2484, −2586 and −102 mm w.e., respectively, with changes in summer balance explaining most of the variability in annual balance.

Reconstructing the mass balance of Brewster Glacier, New Zealand, using MODIS-derived glacier-wide albedo

Abstract. In New Zealand, direct measurements of mass balance are sparse due to the inaccessibility of glaciers in the Southern Alps and the logistical difficulties associated with maintaining a mass balance record. In order to explore the benefit of remotely sensed imaging to monitor mass balance in the Southern Alps, this research assesses the relationship between measurements of glacier surface albedo derived from Moderate Resolution Imaging Spectroradiometer (MODIS) and mass balance observations using the glaciological method on Brewster Glacier over the 2005–2013 period. We confirm that minimum glacier-wide albedo is a reliable predictor for annual mass balance in this maritime environment (R2 = 0.93). Furthermore, we show that regular monitoring of glacier-wide albedo enables a new metric of winter accumulation to be derived, namely the cumulative winter albedo, which is found to correlate strongly with winter mass balance (R2 = 0.88), thus enabling the reconstruction of separate winter and summer mass balance records. This allows the mass balance record for Brewster Glacier to be extended back to the start of MODIS observations in 2000 and to confirm that the annual balance of Brewster Glacier is largely controlled by summer balance (R2 = 92 %). An application of the extended record is proposed whereby the relationship between mass balance and the photographic record of the end-of-summer snowline altitude is assessed. This allowed the annual balance record of Brewster Glacier to be reconstructed over the period 1977–2013, thus providing the longest record of mass balance for a glacier in New Zealand. Over the 37-year period, our results show that Brewster Glacier gained a significant mass of up to 14.5 ± 2.7 m w.e. by 2007. This gain was offset by a marked shift toward negative balances after 2008, yielding a loss of 5.1 ± 1.2 m w.e., or 35 % of the gain accumulated over the previous 30 years. The good correspondence between mass balance of Brewster Glacier and the phase of the Pacific (Inter-)Decadal Oscillation (PDO/IPO), associated with the fast terminus retreat observed between 1978 and 1998, strongly suggests that the observed mass gain of Brewster Glacier since 1977 is only offsetting a longer sequence of dominantly negative balances.

Cloud effects on surface energy and mass balance in the ablation area of Brewster Glacier, New Zealand

Abstract. The effect of clouds on glacier surface energy balance (SEB) has received increased attention in the last decade, but how clouds interact with other meteorological forcing to influence surface mass balance (SMB) is not as well understood. This paper resolves the SEB and SMB at a site in the ablation zone of Brewster Glacier over a 22-month period, using high-quality radiation data to carefully evaluate SEB terms and define clear-sky and overcast conditions. A fundamental change in glacier SEB in cloudy conditions was driven by increased effective sky emissivity and surface vapour pressure, rather than a minimal change in air temperature and wind speed. During overcast conditions, positive net long-wave radiation and latent heat fluxes allowed melt to be maintained through a much greater length of time compared to clear-sky conditions, and led to similar melt in each sky condition. The sensitivity of SMB to changes in air temperature was greatly enhanced in overcast compared to clear-sky conditions due to more frequent melt and changes in precipitation phase that created a strong albedo feedback. During the spring and autumn seasons, the sensitivity during overcast conditions was strongest. To capture these processes, future attempts to explore glacier–climate interactions should aim to resolve the effects of atmospheric moisture (vapour, cloud, and precipitation) on melt as well as accumulation, through enhanced statistical or physically based methods.

A 22 month record of surface meteorology and energy balance from the ablation zone of Brewster Glacier, New Zealand

AbstractMulti-annual records of glacier surface meteorology and energy balance are necessary to resolve glacier–climate interactions but remain sparse, especially in the Southern Hemisphere. To address this, we present a record from the ablation zone of Brewster Glacier, New Zealand, between October 2010 and September 2012. The mean air temperature was 1.2°C at 1760 m a.s.l., with only a moderate temperature difference between the warmest and coldest months (∼8°C). Long-term annual precipitation was estimated to exceed 6000 mm a−1, with the majority of precipitation falling within a few degrees of the freezing level. The main melt season was between November and March (83% of annual ablation), but melt events occurred during all months. Annually, net radiation was positive (a source of energy) and supplied 64% of the melt energy, driven primarily by net shortwave radiation. Net longwave radiation was often positive during cloudy conditions in summer, demonstrating the radiative importance of clouds during melt. Turbulent sensible and latent heat fluxes were directed towards the surface in the summer months, accounting for just over a third of the energy for melt (34%). The energy gain associated with rainfall was small except during heavy events in summer.

All‐sky radiation over a glacier surface in the Southern Alps of New Zealand: characterizing cloud effects on incoming shortwave, longwave and net radiation

ABSTRACTClouds are important features of many high‐altitude and glaciated areas, yet detecting their presence and specifying their effects on incoming shortwave (SW↓), longwave (LW↓) and net all‐wave radiation (Rnet) remains challenging in these environments. These limitations hamper efforts to understand atmospheric controls on glacier surface mass balance (SMB) in the Southern Alps of New Zealand, as both cloud and airmass forcing accompanies key synoptic controls on SMB. Multi‐year datasets of four‐component broadband radiation from two sites at Brewster Glacier, Southern Alps of New Zealand, are used here to develop cloud metrics to account for the effects of clouds on SW↓, LW↓ and Rnet. On average 23% of top‐of‐atmosphere shortwave radiation (SWTOA) is attenuated by the clear‐sky atmosphere, while clouds attenuate a further 31%, resulting in <50% of SWTOA being received at the surface. The transmission of shortwave radiation by clouds (trc) during overcast conditions is found to vary with season and airmass characteristics. A simple parameterization is developed to account for lower trc observed during periods of higher water vapour pressure. Cloud metrics derived at the site show overcast conditions are frequent (45% of period) and strongly dependent on wind direction, highlighting the dominant role of orography in cloud formation and enhancement in the Southern Alps. The effect of clouds on Rnet exhibits a distinct seasonal variation; during summer when albedo and trc are lower, clouds decrease Rnet by 20–40 W m−2, while during autumn, winter and spring, clouds enhance Rnet by approximately 20 W m−2. Idealized modelling shows that these patterns are strongly dependent on albedo and extend across the elevation range of glaciers in the Southern Alps. Thus, overcast conditions appear to aid the extension of ablation into spring and autumn by increasing the energy available for snow and ice melt.

Constraining turbulent heat flux parameterization over a temperate maritime glacier in New Zealand

AbstractThe turbulent sensible and latent heat fluxes are important components of the surface energy balance over glaciers in the Southern Alps of New Zealand, contributing over half the energy available for ablation during large melt events. To calculate these terms confidently in glacier mass-balance models it is essential to use appropriate parameterizations for surface roughness and atmospheric stability. Eddy covariance measurements at Brewster Glacier were obtained over an ice surface to help facilitate an assessment of the calculation of the turbulent heat fluxes. The roughness length for momentum was found to be 3.6 x 10−3m, while the roughness lengths for temperature and humidity were two orders of magnitude smaller, in agreement with surface renewal theory. A Monte Carlo approach was used to assess the uncertainty in turbulent heat fluxes calculated using the bulk aerodynamic method. It was found that input-data and roughness-length uncertainty could not explain underestimates of observed sensible heat fluxes during periods with low wind speed and large temperature gradients. During these periods a katabatic wind speed maximum alters the formulation of the turbulent exchange coefficient to that typically observed in a neutral atmosphere and this has implications for glacier mass-balance sensitivity.

Atmospheric controls on summer ablation over Brewster Glacier, New Zealand

AbstractMicrometeorological data collected from an automatic weather station over a total of 85 days during the summer of 2007/2008 in the ablation zone (1770 m a.s.l.) of the Brewster Glacier, New Zealand, were used to determine the main atmospheric processes controlling the magnitude and variability of daily ablation. During the field season, ablation was measured and modelled using an energy balance model (EBM) and a degree‐day model (DDM). Calculation of the energy balance over the glacier using the EBM revealed that net radiation provided the largest source of energy for ablation (52%). The turbulent sensible heat flux was the next largest energy source (25%), followed by the turbulent latent heat flux (20%) and the rain heat flux (3%). While daily measured and modelled (EBM) ablation were both equal to approximately 40 mm w.e. d−1, the EBM was used to demonstrate that rates were highly variable, ranging from less than 10 mm w.e. d−1 to 101 mm w.e. d−1, controlled primarily by changes in synoptic conditions. Days with ablation above 60 mm w.e. d−1 accounted for almost 25% of total ablation, despite such events occurring on only 11 of the 85 days monitored. The contribution of the individual terms of the energy balance to observed air temperature is complex on Brewster Glacier, leading to a larger uncertainty in DDM ablation estimates (under prediction of total ablation by 11%). Advection of warm and moisture‐laden air over Brewster Glacier, which is not uncommon given its close proximity to the ocean, is not well accounted for by the DDM. This is because the turbulent latent heat flux and the rain heat flux are poorly correlated to air temperature but are, nonetheless, important sources of energy during large ablation events. Copyright © 2010 Royal Meteorological Society

Climate sensitivity of a high-precipitation glacier in New Zealand

AbstractThe sensitivity of glaciers to climatic change is key information in assessing the response and sea-level implications of projected future warming. New Zealand glaciers are important globally as an example of how maritime glaciers will contribute to sea-level rise. A spatially distributed energy-balance model is applied to Brewster Glacier, New Zealand, in order to calculate glacier mass balance, run-off and sensitivity to climate change. The model successfully simulates four annual mass-balance cycles. Close to half (52%) of the energy available for melt on the glacier is supplied by turbulent heat fluxes, with radiation less important, except during the winter. Model sensitivity to temperature change is one of the largest reported on Earth, at −2.0 m w.e. a−1 °C−1. In contrast, a 50% change in precipitation is required to offset the mass-balance change resulting from a 1 °C temperature change. Meltwater runoff sensitivity is also very high, increasing 60% with a 1°C warming. The extreme sensitivity of mass balance to temperature change suggests that significant ice loss will occur with even moderate climate warming.

Subglacial drainage system structure and morphology of Brewster Glacier, New Zealand

AbstractA global positioning system and ground penetrating radar surveys is used to produce digital elevation models of the surface and bed of Brewster Glacier. These are used to derive maps of subglacial hydraulic potential and drainage system structure using three different assumptions about the subglacial water pressure (Pw): (i) Pw = ice overburden; (ii) Pw = half ice overburden; (iii) Pw = atmospheric. Additionally, 16 dye‐tracing experiments at 12 locations were performed through a summer melt season. Dye return curve shape, together with calculations of transit velocity, dispersivity and storage, are used to infer the likely morphology of the subglacial drainage system. Taken together, the data indicate that the glacier is underlain by a channelised but hydraulically inefficient drainage system in the early summer in which water pressures are close to ice overburden. By mid‐summer, water pressures are closer to half‐ice overburden and the channelised drainage system is more hydraulically efficient. Surface streams that enter the glacier close to the location of major subglacial drainage pathways are routed quickly to the channels and then to the glacier snout. Streams that enter the glacier further away from the drainage pathways are routed slowly to the channels and then to the snout because they first flow through a distributed drainage system. Copyright © 2008 John Wiley & Sons, Ltd.

Exceptional ablation arising from rainfall- induced slushflows: Brewster Glacier, New Zealand

An exceptional ablation event resulted in the destruction of a network of glacier survey stakes on the Brewster Glacier, New Zealand. Up to 5 m of surface lowering occurred over 26 days, releasing at least 1·6×106 m3 of water. The episode is attributed to saturation of the snowpack by rainfall and consequent slushflows. Slushflows permit surface lowering at rates greater than expected from the surface energy balance on snow. Resulting slushflow events may significantly influence glacier mass balance, and may constitute a hazard. Little is known of their physical characteristics, but some descriptive background is provided. Copyright © 2000 John Wiley & Sons, Ltd.

A Preliminary Assessment of Glacial Ice Profiling Using Vlf Surface-Impedance Measurements

AbstractVLF surface-impedance measurements have been used in the past for sub-surface mapping. The application of this technique to glacial ice probing is discussed theoretically and results of measurements on Brewster Glacier, New Zealand, are presented. Results were fitted to a three-layer model and a section profile is given. Dramatic changes in the phase of the surface impedance were observed in the vicinity of crevasses. Results indicate that the technique has potential as a tool for quick, reliable, and non-invasive ice-thickness measurements.

A Preliminary Assessment of Glacial Ice Profiling Using Vlf Surface-Impedance Measurements

AbstractVLF surface-impedance measurements have been used in the past for sub-surface mapping. The application of this technique to glacial ice probing is discussed theoretically and results of measurements on Brewster Glacier, New Zealand, are presented. Results were fitted to a three-layer model and a section profile is given. Dramatic changes in the phase of the surface impedance were observed in the vicinity of crevasses. Results indicate that the technique has potential as a tool for quick, reliable, and non-invasive ice-thickness measurements.