Temperature-index Melt Research Articles

Estimating glacier snow accumulation from backward calculation of melt and snowline tracking

AbstractEstimating precipitation to determine accumulation is challenging. We present a method that combines melt modelling and snowline tracking to determine winter glacier snow accumulation along snowlines. The method assumes that the net accumulation is zero on the transient snowlines and the maximum winter accumulation at the snowline can be calculated backwards with a temperature-index melt model. To verify the method, the accumulation model is applied for the year 2004 on Storglaciären, Sweden, for which extensive meteorological and mass-balance data are available. The measured mean snowline accumulation is 0.94 ± 0.10mw.e. for 2004. Modelled accumulation, using backward melt modelling, at the same snowlines is 0.82 ± 0.25 m w.e. The accumulation model is also compared with an often used linear regression accumulation model which yields a mean snowline accumulation of 1.02 ± 0.38 m w.e. The reduction in standard error from 0.38 m w.e. to 0.25 m w.e. shows that the backward melt modelling applied at snowlines can provide a better spatial representation of the accumulation pattern than the regression model. Importantly, the applied method requires no field measurements of accumulation during the winter and snowlines can be readily traced in remotely sensed images.

Relative contribution of solar radiation and temperature in enhanced temperature-index melt models from a case study at Glacier de Saint-Sorlin, France

AbstractA large set of ice ablation data from a glacier in the French Alps is used to investigate the sensitivity of ice melting to solar radiation and temperature. The data come from 7 years of observations on a small network of 16 stakes set up on Glacier de Saint-Sorlin. The high spatial variability of ice ablation is shown to be strongly dependent on potential solar radiation. On the other hand, temporal variations are highly correlated with air temperatures measured both at a nearby and at a remote meteorological station, but poorly correlated with incoming shortwave radiation measured at an automatic weather station located close to the glacier terminus. Spatial variations of ice ablation therefore appear to be mainly driven by potential solar radiation, while temporal variations are driven by temperature. This result suggests that minimizing the influence of temperature in an enhanced temperature-index melt model may compromise the model’s ability to simulate interannual changes in melt. These new results may help to improve the calibration, and thus the performance, of these empirical melt models used for long-term simulations of glacier mass balances.

Distributed temperature-index snowmelt modelling for forested catchments

► We adapt extended temperature-index snowmelt algorithms for forested catchments. ► A simple model for canopy interception reasonably predicts snow accumulation. ► Extended temperature-index algorithms can be as accurate as an energy balance model. ► Accounting for cold-content was required to realize the increased performance. ► One algorithm is also capable of simulating the diurnal cycle of meltwater outflow. This work was motivated by the need to understand the hydrologic consequences of rapid and widespread forest disturbance to assist forest management and operational forecasting. The objective was to compare three extended temperature-index-based melt models to a basic temperature-index model, as well as to the physically based snow model in the Distributed Hydrology-Soil-Vegetation Model (DHSVM). One model uses ad hoc parameterizations to represent the variation of the melt factor with slope-aspect, forest cover and time of year. The other two incorporate either potential direct or measured solar radiation; they were originally developed for open sites and were adapted to account for the effects of forest cover. The models were calibrated and tested using lysimeter data and spatially distributed snow surveys conducted at the Cotton Creek Experimental Watershed located in southeast British Columbia, Canada. A model for canopy interception that included a maximum canopy storage and a fixed sublimation rate provided reasonable predictions of peak snow accumulation. Accounting for cold content in the upper layer of a two-layer snowpack model was required to realize the increased predictive performance of the extended temperature-index melt models, compared to the most basic model. The model that incorporated measured solar radiation provided the best performance, and was almost as accurate as the DHSVM snow model. The model that uses potential direct radiation also performed well in spatial testing and, in addition, closely matched day-time highs and night-time lows of the measured lysimeter outflow. The model with ad hoc representations did not perform as well as the other extended temperature-index models outside the calibration period. Spatially distributed snowpack measurements assisted in constraining the parameters in the extended temperature-index models. Further research should test the extended temperature-index melt models at the catchment scale, particularly in catchments where there have been rapid and well documented changes in forest cover.

Spatial distribution of surface ablation in the terminus of Rhonegletscher, Switzerland

AbstarctThe spatial pattern of glacier surface melt was measured with a resolution of 20–100m within a region extending 1 km up-glacier from the terminus of Rhonegletscher, Switzerland. The melt rate was monitored from 6 July to 6 September 2009 using 44 ablation stakes. We also measured the surface albedo near the stakes to investigate the importance of this parameter for the melt-rate distribution. The melt rate varied from 32.8 to 71.9 mm w.e. d–1 in the study area. Our measurements suggest that the spatial variation of the melt rate can be explained by (1) shading of the ice surface by neighbouring mountains, (2) surface albedo and (3) effects of microclimate (e.g. radiation from sidewalls) on the surface energy balance. The observed melt-rate distribution was compared to the results of a temperature-index melt model, which takes into account shading of direct solar illumination but not the other two effects. The model reproduces some important features of the field data, but its spatial variations are generally less than the measured values. Our study shows the importance of albedo and other local conditions in the accurate estimation of the small-scale melt-rate distribution.

Snow accumulation distribution inferred from time‐lapse photography and simple modelling

AbstractThe spatial and temporal distribution of snow accumulation is complex and significantly influences the hydrological characteristics of mountain catchments. Many snow redistribution processes, such as avalanching, slushflow or wind drift, are controlled by topography, but their modelling remains challenging. In situ measurements of snow accumulation are laborious and generally have a coarse spatial or temporal resolution. In this respect, time‐lapse photography shows itself as a powerful tool for collecting information at relatively low cost and without the need for direct field access. In this paper, the snow accumulation distribution of an Alpine catchment is inferred by adjusting a simple snow accumulation model combined with a temperature index melt model to match the modelled melt‐out pattern evolution to the pattern monitored during an ablation season through terrestrial oblique photography. The comparison of the resulting end‐of‐winter snow water equivalent distribution with direct measurements shows that the achieved accuracy is comparable with that obtained with an inverse distance interpolation of the point measurements. On average over the ablation season, the observed melt‐out pattern can be reproduced correctly in 93% of the area visible from the fixed camera. The relations between inferred snow accumulation distribution and topographic variables indicate large scatter. However, a significant correlation with local slope is found and terrain curvature is detected as a factor limiting the maximal snow accumulation. Copyright © 2010 John Wiley & Sons, Ltd.

Assessing the transferability and robustness of an enhanced temperature-index glacier-melt model

AbstractWe investigate the transferability of an enhanced temperature-index melt model that was developed and tested on Haut Glacier d’Arolla, Switzerland, in the 2001 season. The model’s empirical parameters (temperature factor, TF, and shortwave radiation factor, SRF) are recalibrated for: (1) other locations on Haut Glacier d’Arolla; (2) subperiods of distinct meteorological conditions; (3) different years on Haut Glacier d’Arolla; and (4) other glaciers in different years. The model parameters are optimized against simulations of an energy-balance model validated against ablation observations. Results are compared with those obtained with the original parameters. The model works very well when applied to other sites, seasons and glaciers, with the exception of overcast conditions. Differences are due to underestimation of high melt rates. The parameter values are associated with the prevailing energy-balance conditions, showing that high SRF are obtained on clear-sky days, whereas higher TF are typical of locations where glacier winds prevail and turbulent fluxes are high. We also provide a range of parameters clearly associated with the site’s location and its meteorological characteristics that could help to assign parameter values to sites where few data are available.

The influence of resolution and topographic uncertainty on melt modelling using hypsometric sub‐grid parameterization

AbstractModelling of physical processes such as ablation or runoff at continental or global scales provides a key challenge: a high degree of abstraction is required in order to minimize computational demands, while spatial and temporal variability of key processes, often at the sub‐scale level, need to be adequately captured and reproduced within a lower resolution model. For some approaches, such as temperature index models, downscaling to lower resolutions is straightforward. However a key issue when using these downscaled models is to assess the impact of scaling on model behaviour and results, including the associated uncertainties. We assess the impact of scaling on both a simple and an enhanced temperature index melt model from 100 m to 1, 5 and 10 km resolutions. Different sub‐grid parameterization approaches are applied to both models across all resolutions and tested for their suitability against high‐resolution reference data, with the aim of developing a robust, scalable and computationally undemanding parameterization. Results show patterns of over‐ and underestimation of potential melt rates for both models, with clear dependencies on scale, terrain roughness and variations of temperature thresholds, among other quantities. The sub‐grid parameterizations tested in this article are found to effectively compensate these effects at little additional computational cost. Copyright © 2008 John Wiley & Sons, Ltd.

Glacier-dammed lake outburst events of Gornersee, Switzerland

AbstractGornersee, Switzerland, is an ice-marginal lake, which drains almost every year, subglacially, within a few days. We present an analysis of the lake outburst events between 1950 and 2005, as well as results of detailed field investigations related to the lake drainage in 2004 and 2005. The latter include measurements of lake geometry, water pressure in nearby boreholes and glacier surface motion. A distributed temperature-index melt model coupled to a linear-reservoir runoff model is used to calculate hourly discharge from the catchment of Gornergletscher in order to distinguish between the melt/precipitation component and the outburst component of the discharge hydrograph. In this way, drainage volume and timing are determined. From 1950 there is a clear trend for the outburst flood to occur earlier in the melt season, but there is no trend in lake discharge volumes. Peak discharges from the lake lie significantly below the values obtained using the empirical relation proposed by Clague and Mathews (1973). The shapes of the 2004 and 2005 lake outflow hydrographs differ substantially, suggesting different drainage mechanisms. From water balance considerations we infer a leakage of the glacier-dammed lake in 2005, starting 1 week prior to the lake outburst. During the drainage events, up to half of the lake water is temporarily stored in the glacial system, causing substantial uplift of the glacier surface.

Temperature index melt modelling in mountain areas

Temperature index or degree-day models rest upon a claimed relationship between snow or ice melt and air temperature usually expressed in the form of positive temperatures. Since air temperature generally is the most readily available data, such models have been the most widely used method of ice and snow melt computations for many purposes, such as hydrological modelling, ice dynamic modelling or climate sensitivity studies. Despite their simplicity, temperature-index models have proven to be powerful tools for melt modelling, often on a catchment scale outperforming energy balance models. However, two shortcomings are evident: (1) although working well over long time periods their accuracy decreases with increasing temporal resolution; (2) spatial variability cannot be modelled accurately as melt rates may vary substantially due to topographic effects such as shading, slope and aspect angles. These effects are particularly crucial in mountain areas. This paper provides an overview of temperature-index methods, including glacier environments, and discusses recent advances on distributed approaches attempting to account for topographic effects in complex terrain, while retaining scarcity of data input. In the light of an increasing demand for melt estimates with high spatial and temporal resolution, such approaches need further refinement and development.

Modelling changes in the mass balance of glaciers of the northern hemisphere for a transient 2×CO 2 scenario

A climate forecast provided by a General Circulation Model (GCM), a glacier mass balance model and a glacier flow model is applied to a sample of 11 small glaciers. Another sample of six glaciers and six large, heavily glacierized areas in the arctic were modelled using only the climate forecast and the glacier mass balance model. The climate forecast of two different GCM's with identical experimental setup takes into account a gradual increase in atmospheric content of CO 2 and other greenhouse gases to a doubled CO 2 equivalent in 2050 corresponding to the IPCC Scenario IS92a. The differences between the two GCM's are significant for the future development of the glacierization of arctic areas. The glacier mass balance model consists of a temperature index melt model, which uses potential direct clear sky radiation to obtain a better spatial and temporal resolution, combined with imposed snow precipitation. Static mass balance sensitivities span −0.2 to −1.5 mwe a −1 °C, the higher and lower sensitivities apply for glaciers in more maritime and continental climates, respectively. All of the modelled glaciers and glacierized regions show a strong decrease in the net mass balance. Temperature changes dominate the effect of snow precipitation on the net mass balance. A temperature increase results in substantial increase in melt especially through the extension of the melt season in spring and fall. The mass balance projections for the sample of 11 glaciers are then used in a glacier flow model to simulate the dynamic reaction of the glacier to the changing climatic conditions. An iteration procedure accounts for possible feedback of changes in the glacier area and the ice surface elevation to the projected mass balance. For the modelled glaciers, an average volume loss of 60% until 2050 is predicted. Without the above iteration and assuming a constant glacier area, the volume change is overestimated by about 20%. A comparison with the climate predictions of the two different GCMs shows only a difference of about 10% in the mass loss obtained by flow modelling of four glaciers.

Modeling Climate Conditions Required for Glacier Formation in Cirques of the Rassepautasjtjåkka Massif, Northern Sweden

Timing of cirque formation and the climate necessary to initiate glaciation are fundamental to the understanding of the landscape of the northern Scandinavian mountains. Empty cirques in the Rassepautasjtjåkka massif are located near a glaciated area and thus appear near the glaciation limit. In order to investigate the climate conditions necessary for glacier formation in the cirques, we applied a spatially distributed temperature index melt model. After calibration under present climate conditions, the model was run with different combinations of increased initial winter snow cover and lowered summer air temperatures to assess the climate conditions needed for snow to survive summer and hence form a base for glaciation. Results indicate that a significant increase in precipitation or decrease in summer air temperature or a combination of both is necessary to initiate glaciation. Thus current climate conditions are far from favorable for glaciation. If summer temperature is decreased by 4°C or winter snow cover is more than doubled, only 10% of cirque areas remain snow covered, which is considered as a minimum condition for glacier formation. According to climate reconstructions such conditions have not occurred during the Holocene suggesting that the cirques have not been glaciated during this period. Consequently glaciation of the cirques must have occurred during other parts of the glacial cycles.

Comparison of Modeled Water Input and Measured Discharge Prior to a Release Event: Unteraargletscher, Bernese Alps, Switzerland

A distributed temperature index melt model including potential clear-sky solar radiation was applied to Unteraargletscher, Bernese Alps, Switzerland, to quantify water input to the glacial hydrological system during the ablation season 1999. Model parameters were determined by calibrating calculated melt with ablation measurements. Discharge was measured in the proglacial stream for 18 days until the station was destroyed by an outburst flood. Comparison of modeled melt and measured discharge reveals that routing processes of water through the glacier vary with time as the water is transferred through a dynamic drainage system. Furthermore, an imbalance of water input and output suggests that water was stored in or beneath the glacier during this period. The culminating outburst flood presumably released this en– or subglacially stored water and may be related to a change in the configuration of the glacial drainage system as inferred from measurements of subglacial water pressure.

Snowmelt runoff models

Models that predict meltwater runoff at a daily timescale are important in water resource management, flood hazard assessment and climate-change impact studies. This article identifies four basic components of such models: meteorological extrapolation, snowmelt estimation at a point, snow-cover depletion and runoff routing. Alternative ways of handling these are discussed, with emphasis on the contrasting treatments in two widely used models: HBV and SRM. Many of the issues in meltwater modelling reflect wider debates in hydrological and environmental modelling, including problems of complexity vs. simplicity, the appropriate level of spatial disaggregation, parameter identification and calibration, and internal validation. In reviewing current trends emphasis is placed on the potential and limitations of fully distributed models, problems in using energy-balance rather than temperature-index melt models at basin scale, ways to deal with spatial variability in snow cover, and the value and limitations of earth observation data.