Light-absorbing impurities in snow and ice control glacier melt as shortwave radiation represents the main component of the surface energy balance. Here, we investigate the long-term effect of snow impurities, i.e., mineral dust and black carbon (BC), on albedo and glacier mass balance. The analysis was performed over the period 1914–2014 for two sites on Claridenfirn, Swiss Alps, where an outstanding 100-year record of seasonal mass balance measurements is available. Information on atmospheric deposition of mineral dust and BC over the last century was retrieved from two firn/ice cores of high-alpine sites. A combined mass balance and snow/firn layer model was employed to assess the effects of melt and accumulation processes on the impurity concentration at the surface and thus on albedo and glacier mass balance. Compared to pure snow conditions, the presence of Saharan dust and BC lowered the mean annual albedo by 0.04–0.06 depending on the location on the glacier. Consequently, annual melt was increased by 15–19 %, and the mean annual mass balance was reduced by about 280–490 mm w.e. BC clearly dominated absorption which is about 3 times higher than that of mineral dust. The upper site has experienced mainly positive mass balances and impurity layers were continuously buried whereas at the lower site, surface albedo was more strongly influenced by re-exposure of dust and BC-enriched layers due to frequent years with negative mass balances.
Deposition of mineral dust and BC have a fundamental impact on the energy balance of glaciers and snow-covered areas by increasing the absorption of solar radiation. Along with the enhanced melting due to the darkening of the snow surface, the growth of snow grains is accelerated, which further reinforces snow melt rates (Painter et al., 2007). While light-absorbing impurities control the snow albedo mainly in the visible wavelengths, the snow grain size affects the albedo in the near-infrared. Shortwave radiation is the dominant energy source for the melting of snow, firn and ice, and consequently the surface albedo has an important influence on the mass budget of glaciers (Oerlemans et al., 2009).
Repeated years with negative glacier mass balances lead to a shift in the equilibrium line to higher elevations and to the re-exposure of dust and BC-enriched firn layers at the surface. Snow impurities are mainly retained at the surface during conditions of melt and surface concentrations might be enhanced by up to 1 order of magnitude resulting in a pronounced melt amplification (Sterle et al., 2013).
Absorptive impurities consist of mineral dust, carbonaceous particles and colored organic matter (Warren, 1984). Advection of dust-loaded air masses from the Saharan desert leads to episodic deposition of large amounts of mineral dust in the Alps. Analyses of firn cores from high-alpine sites, resolving the signal of the continental background aerosols, indicated that long-range transported crustal impurities account for about two-thirds and local impurities for about one-third of the total mineral dust deposited (Wagenbach and Geis, 1989). One single Saharan dust deposition event may even supply 30 % of the total annual dust budget at high-altitude mountain glaciers (Schwikowski et al., 1995). Most prominent Saharan dust episodes in the Alpine region occurred in the years 1936–1937, 1977, 1990 and 2000 leaving marked dust horizons in firn/ice cores (Schwikowski et al., 1995; Jenk et al., 2009; Sigl, 2009).
BC refers to the strongly light-absorbing component of soot and is emitted naturally and anthropogenically by incomplete combustion of fossil fuels and by biomass burning. BC has become a focus of interest as it has been identified recently as one of the major contributors to global climate change (Ramanathan and Carmichael, 2008; Bond et al., 2013). BC contributes to global warming by absorption of sunlight, firstly when it is suspended in the atmosphere, and secondarily when it is deposited on snow and ice by reducing the albedo and hence accelerating melting. Along with the beginning of the era of industrialization, global BC emissions sharply increased and continued to rise into the 21th century. In the European region, BC concentrations started to decrease in the mid-20th century and have stabilized over the last few decades (Bond et al., 2007).
Recently different studies investigated the impact of light-absorbing impurities on snow albedo and the melting of snow. Based on a 10-year record of mineral dust and BC concentrations, retrieved from an ice core at Mera Peak, Nepalese Himalaya, Ginot et al. (2014) found that light-absorbing particulates cause up to 26 % of the total annual surface melting. Another study performed at Mera Peak shows that mineral dust dominates absorption and may reduce the albedo of snow by up to 40 % (Kaspari et al., 2014). Investigations for the Colorado River basin, western US, show that the radiative forcing of mineral dust deposition may shorten the duration of snow cover by several weeks (Skiles et al., 2012) and also affects the timing and magnitude of runoff (Painter et al., 2010). It was suggested that increasing anthropogenic emissions of black carbon during the Industrial Revolution have forced the end of the Little Ice Age in the Alps (Painter et al., 2013).
In this study we assess the significance of natural mineral dust and anthropogenic BC particles in snow and firn on the mass balance of a high-mountain glacier over a centennial period (1914–2014). Using a unique 100-year record of seasonal glacier mass balances, ice core records of past atmospheric deposition of Saharan dust/BC and a sophisticated modeling approach, we examined the contribution of light-absorbing impurities to glacier melt for (1) a site with accumulation conditions over the entire period, where dust is predominately buried by winter snow, and (2) a site at the glacier's equilibrium line involving a re-exposure of buried dust and BC layers at the surface in years with negative mass balance. We have chosen Claridenfirn (Swiss Alps) for which the worldwide longest data series of seasonal glacier mass balance exists. This comprehensive data set enables an accurate and field data-based simulation of ablation and accumulation processes. In order to simulate the feedback between melt, accumulation and snow impurities, a mass balance model was coupled with a snow density model, which tracks the position and the thickness of deposited snow layers and impurities. The mass balance model incorporates an enhanced temperature-index melt model including the shortwave radiation balance and a parameterization for albedo, which is based on the specific surface area of snow and the impurity concentration in the surface snow.
Claridenfirn is a mountain glacier with an area of approximately
5
First mass balance measurements date back to 1914 and provide a unique data
set covering a period of 100 years. At two different sites, at
a
Study site overview. The red dots on Claridenfirn indicate the lower and the upper stake. The inset shows the location of the study site within Switzerland (red point), of the ice core sites Colle Gnifetti and Fiescherhorn (black dot). The aerosol measurement station Jungfraujoch and the weather stations used to derive meteorological time series (black triangles) are indicated.
The forcing of the mass balance model requires daily air temperature and
precipitation data for 1914–2014. We used air temperature recorded by the
MeteoSwiss weather station Säntis (2490
Furthermore, our model requires daily time series of incoming shortwave radiation that were derived from a simple parameterization based on daily temperature ranges (see Sect. 3). Daily maximum and minimum temperature were provided from the MeteoSwiss weather station at Davos over the period 1914–2014 (Fig. 1). For calibrating the parameterization, daily values of incoming solar radiation of 1981–2014 recorded at the same station were used. In addition, a unique data set of monthly means of global solar radiation for Davos, covering the period 1936–2014, is provided by the Global Energy Balance Archive (GEBA; Ohmura et al., 1989), and was employed to improve the performance of the cloud factor parameterization.
A firn/ice core from the cold glacier saddle of Colle Gnifetti
(4455
In order to determine the impact of Saharan dust events on surface albedo and glacier melt, a mass balance model including a parameterization for snow, firn and ice albedo was coupled with a snow/firn density model to track the position and thickness of the snow layers and dust. The physical albedo parameterization is based on the evolution of the specific surface area of snow grains and includes the option to simulate the effect of snow impurities on pure snow albedo. Atmospheric input of mineral dust and BC was derived based on the ice/firn core data. The mass balance model was forced by daily time series of air temperature, precipitation and incoming shortwave radiation and was run over a 100-year period (1 October 1914 to 30 September 2014). Hereafter, the data series of mineral dust and BC and the individual components of the employed mass balance and snow density model are described in detail.
The absorption of mineral dust in the visible spectrum is highly sensitive to the content of iron oxides. Kaspari et al. (2014) determined light absorption of mineral dust in snow and ice of a Himalayan glacier based on gravimetrically determined Fe concentrations. Accordingly, we used records of iron (Fe), provided by the ice core, to infer mineral dust concentrations. Iron oxides mainly consist of the minerals goethite and haematite (Sokolik and Toon, 1999; Lafon et al., 2006). Since they have different light absorption spectra (e.g., Lafon et al., 2006) their relative proportion has to be known for calculating the radiative properties of dust. According to Shi et al. (2011) the mass ratio of the mineral haematite to the minerals haematite plus goethite for Saharan dust is 0.42 on average. Based on the assumption that about 45–64 % of the total Fe is encompassed in light-absorbing oxides (Lafon et al., 2004), the mass of goethite and haematite is calculated following Kaspari et al. (2014) and used as proxy for the absorption of mineral dust.
Most of the dust peaks can be related to long-range transported crustal impurities, which account for about 70 % of the total deposited mineral dust (Wagenbach et al., 1996). This is a conservative assumption since local dust contains a lower portion of Fe-oxides. We therefore assume that all mineral dust is made up by Saharan dust.
Errors in the annual layer counting of the ice cores might involve
uncertainties of
The annual amount of mineral dust (i.e., Fe-oxide) was distributed over the
year according to the Saharan dust climatology reported by Collaud Coen
et al. (2004). They analyzed the number and duration of Saharan dust events
per month based on measurements of the aerosol scattering coefficient
performed at Jungfraujoch in the years 2001–2002. Higher probability of
occurrence was observed in the March–June and the October–November period.
Extended time series of the years 2001–2012 confirm this distribution
(MeteoSwiss, 2014a). Three different classes of Saharan dust events were
defined: Saharan dust events lasting between 4 and 10
While Saharan dust transport has an episodic character, deposition of BC is controlled by seasonal variations in atmospheric stability, which is higher in winter than in summer. In order to mimic the yearly cycle of BC input, daily ambient BC measurements at Jungfraujoch, performed in the frame of the GAW monitoring programme, were used. Based on these measurements, daily anomalies averaged over the period 2002–2013 were derived and applied to the annual BC concentrations provided by the firn/ice core in order to infer daily atmospheric deposition rates of BC (Fig. 2b).
Several studies performed detailed investigations of the regional and altitudinal distribution of major ions in the high Alpine region (e.g., Nickus et al., 1997; Rogora et al., 2006). They found a marked regional variability but no clear trends, neither in distance nor in altitude. Due to a lack of clear indication, we assumed that Fe/BC concentrations at Claridenfirn are in a similar range as the concentrations observed on Colle Gnifetti and Fiescherhorn, respectively, and employed measured Fe/BC concentrations directly without a transfer function. In order to estimate the influence of potential differences in their input concentration, we performed a sensitivity analysis (see Sect. 5).
For simulating snow and ice melt, the enhanced temperature-index (ETI) model
(Pellicciotti et al., 2005) was employed. This model computes melt as
a function of air temperature and shortwave radiation and accounts for the
effects of albedo and cloudiness on melting:
Snow accumulation was computed by the station precipitation and a correction factor,
Comparison of mean annual global radiation measured at Davos (bold grey, 1936–2014) and global radiation modeled by the cloud factor parameterization (blue). The dashed black line shows annual averages of modeled daily radiation adjusted by the measurements to fit measured monthly means which are used to force the mass balance model.
The cloud transmissivity factor, cf, accounts for the attenuation of solar radiation by clouds and
is derived as a function of daily temperature ranges (
Snow albedo was derived according to the physical snow albedo
parameterization by Gardner and Sharp (2010) as the sum of pure snow albedo
and its change due to impurities. The albedo of ice was kept constant at 0.2
(Pellicciotti et al., 2005). Pure snow albedo,
The specific surface area of the snow grains was calculated relying on the
approach by Roy et al. (2013) that considers both dry and wet snow
metamorphism. In the case of dry snow conditions, the evolution of SSA is
computed according to Taillandier et al. (2007), as a logarithmic function of
snow age and snow temperature,
A snow densification model is required to determine the position and the thickness of each snow layer. The simple point model by De Michele et al. (2013) for bulk snow density and snow depth was employed and applied to each snow layer. The two-constituent model solves mass balance equations for the dry and liquid mass of the snow pack, as well as momentum balance and rheological equations for the dry part. It results in a system of three differential equations for depth and density of the dry part of the snowpack, and the depth of liquid water. Sublimation and evaporation are not considered. The main characteristics of the model are shortly described in the following. For more detailed information see De Michele et al. (2013).
A simplified energetic description of the snowpack assuming thermal
equilibrium between constituents is used. The temperature profile,
The change in snow depth,
The height of liquid water,
The momentum balance equation,
Each precipitation event was considered as a single snow layer which is stacked atop of the snow pack. Snow layers with
a thickness of less than 1
Mineral dust and BC entered the system by liquid or solid precipitation, as wet deposition is expected to be the
predominant mechanism (Raes et al., 2000; Koch, 2001). Particulate impurities were supposed to be evenly distributed
in precipitation and consequently also in the snow layers. Particulates remained in the corresponding snow layer as
long as there was no melt. When melt occurred, impurities of the melted snow were accumulated in the top 2
Melt water percolation may lead to vertical redistribution of snow impurities. Different studies have investigated the
removal of particulate impurities by melt water (e.g., Conway et al., 1996; Flanner et al., 2007). They found that larger
particles (
The melt parameters, TF and SRF, and the accumulation parameter,
Average Fe-oxide and BC concentrations in the surface snow at the upper and lower measurement site on Claridenfirn for the period 1914–2014. The crosses mark years with exceptionally high Saharan dust activity. Note that the scales for upper and lower stake are different.
Total Fe-oxide amount of the surface snow layer of each year and the amount of Fe-oxides of previous years
emerging at the surface through melt-out over the period 1914–2014 for
At the upper measurement site, located in the accumulation area of Claridenfirn, the mean annual Fe-oxide concentration
in the surface layer was
At the lower measurement site, located near the glacier's equilibrium line altitude (ELA), the mean Fe-oxide
concentration was more than twice as high as at the upper stake and was
In the accumulation area (upper stake) most of the mineral dust exposed at the surface originated from deposition occurring during the same year. Only in the few years with negative mass balances mineral dust of previous years reappeared and reinforced the darkening of the glacier surface. In specific years (e.g., 1947 and 1991), mineral dust of previous years accounted for 45–65 % of the total mineral dust at the surface (Fig. 5a). On average, however, the fraction of mineral dust of preceding years becoming albedo relevant was small and made up only 8 % of the total surface dust budget. At the ELA (lower stake) mineral dust of previous years more effectively influenced surface dust concentrations and accounted for about 30 % of the total exposed mineral dust. Particularly in the 1940s and 2000s, but also in the early 1960s and the 1990s, large quantities of previously buried dust were re-exposed at the surface. Up to 97 % of the total surface dust in 2006 and 2007 originated from deposition in preceding years (Fig. 5b). Accordingly, mineral dust of much older layers was re-exposed at the surface of the lower measurement site in comparison to the upper stake. While at the stake in the accumulation area, surface dust had a maximum age of 3 years, mineral dust at the lower stake was found to have an age of up to 21 years (Fig. 5c, d).
Mean surface concentrations of BC showed a distinctly different pattern than mineral dust
concentrations. BC concentrations at the glacier surface were mainly controlled by the melt regime
and were less influenced by episodic deposition compared to Saharan dust (Fig. 4). An exception was
the year 1982, when exceptionally high deposition of BC were recorded (see Fig. 2a). Mean
concentrations of BC over the entire period were 0.26
In contrast to BC, mineral dust concentrations at the surface were up to 5 times larger. However, as BC is much more absorptive than mineral dust (mass absorption coefficient about 10 times higher), the overall absorption by BC and dust are in a similar range. In individual years with extraordinarily high Saharan dust input, such as in 1936, 1977, 1990 and 2000, mineral dust dominated the absorption of solar radiation (Fig. 6). In all other years, the absorption of BC outweighed the absorption of mineral dust and over the entire period BC was clearly the dominant absorber. While at the upper stake, the absorption due to BC was 3.3 times higher on average compared to mineral dust, at the lower stake BC resulted in a 2.2 times higher absorption. These statements are based on the assumption that BC is more efficiently removed by melt water than mineral dust and therefore depend on the chosen removal efficiency. If removal rates of BC and mineral dust would be in a similar range, the influence of BC on the absorption would be even larger.
Mean annual absorption (optical depth) of mineral dust and BC over the period 1914–2014 for
On average the reduction of mean annual surface albedo due to Saharan dust
was less than 0.01 compared to snow with BC only. At the upper measurement
site the mean annual albedo was reduced by
The overall impact of BC on the surface albedo was substantially higher than
that of Saharan dust. Our results suggest that BC reduced the albedo over
1914–2014 by
Cumulative mass balance over the period 1914–2014 at
Effect of Saharan dust on annual mass balance for the period 1914–2014 for
The impact of Saharan dust on the total mass change over the 100-year period
was in the order of a few meters and was less pronounced in the accumulation
area than at the ELA (Fig. 7). At the upper measurement site, the difference
in total cumulative mass balance due to Saharan dust was 2.8
Maximum deviations in annual mass balance due to Saharan dust were up to
The BC-induced albedo changes led to an average reduction in annual mass
balance of 183
The combined effect of Saharan dust and BC reduced the mean annual mass
balance by 282
Converting changes in annual mass balance caused by absorption of dust/BC
into the energy consumed for melt allowed calculating the radiative forcing
of snow impurities. The radiative forcing (RF, W
For the measurement site in the accumulation area we found a mean radiative
forcing over the 100-year period of
At a global scale, the mean radiative forcing from BC in snow is reported to
be in the range of 0.02–0.08
Painter et al. (2013) suggested that the rapid retreat of Alpine glaciers at
the end of the Little Ice Age was forced by increasing BC concentration due
to industrialization. They found BC-induced mass balance anomalies in the
order of
Parameters of the impurity, the SSA and the snow density model and the corresponding parameter ranges (
In order to assess the sensitivity of the model results to the chosen input
parameters, we performed a sensitivity analysis. Four parameters of the snow
impurity model were examined: (1) removal rates of BC by melt water, (2)
fraction of Fe which is presented as Fe-oxides, (3) the proportion of
haematite and goethite in the Fe-oxides, and (4) the ratio of the MAC of BC
vs. MAC of Fe-oxides. In addition, another four parameters of the SSA model
(
Sensitivity of annual surface mass balance (i.e., the percentage change in the parameter value vs. the percentage change in annual mass balance) to the different parameters of the impurity, the specific surface area and the snow density model, as well as the sensitivity to the input of mineral dust and BC.
Results of the sensitivity analysis are shown in Fig. 9. The mass balance was
most sensitive to the amount of snow impurities and the parameters of the
snow density model, while the parameters of the SSA model were clearly less
relevant. In contrast to the input quantity of BC, mineral dust had a less
pronounced impact on modeled mass balance. A change of 10 % in the BC
concentration in precipitation led to a 5.8 % change in mass balance,
whereas the same change in the mineral dust concentration in precipitation
only resulted in a 1.6 % change in mass balance. The reason for this
difference in sensitivity is the stronger absorption of solar radiation by BC
compared to mineral dust. An even higher sensitivity could be assigned to the
removal efficiency of BC with melt water. A 10 % change in the BC removal
rate leads to a 1.5 times larger change in the mass balance. This is
particularly important since the removal rates are subject to considerable
uncertainty (see Sect. 5.5). Hence, the removal of BC by melt water seems to
be the most critical point of the simulation and strongly controls the impact
of BC on the long-term glacier mass balance. Besides the impurity model, also
the performance of the density model affected the simulations. In particular,
parameter
The extent of the impact of Saharan dust and BC on the glacier mass balance is spatially variable and strongly depends on prevailing conditions. According to our results, the effect of light-absorbing impurities increase from the accumulation area towards the equilibrium line as higher melt rates lead to a re-exposure of old firn layers bearing light-absorbing impurities. In the ablation area where most glacier mass loss occurs, however, the processes are different. Winter accumulation is not preserved over multiple years and thus light-absorbing snow impurities affect only the albedo of the winter snow cover until it has been melted away. During the summer season when bare ice is exposed at the surface, snow impurities are removed by melt water which might limit the impact of impurities on glacier melt, although a darkening on gently sloping glacier tongues has also been observed (Oerlemans et al., 2009). Hence, we suppose that the effect of Saharan dust and BC in the ablation area is lower compared to areas near the equilibrium line.
Our analysis is based on the general assumption that concentrations of Saharan dust and BC in precipitation at Colle Gnifetti and Fiescherhorn (Fig. 1) are comparable to those at Claridenfirn. In order to receive undisturbed records of past aerosol concentrations, only few sites in the Alpine region are suitable. Prerequisites are high elevation to exclude chemical disturbance by melt water percolation, sufficient ice thickness to ensure long enough records and flat terrain to limit the effect of ice flow (Wagenbach and Geis, 1989). For this reason, we relied on time series at locations other than Claridenfirn and had to transpose the measurements to the study site for which long-term mass balance measurements were available.
In the 1990s a large-scale study about the chemical composition of high-alpine winter snow packs was carried out in the Alps with the aim of detecting the regional and altitudinal distribution of major ions (SNOSP; Nickus et al., 1997). It was found that the concentration of most ionic species in winter snow increases by about one-third from west to east and that ionic loads show no regional preference due to opposite gradients in the prevailing precipitation patterns. The same also applies to variations with altitude: at higher elevation, ion concentrations are lower compared to valleys, but the general increase in precipitation with elevation compensates for this effect, so that ionic loads are expected to be in the same order of magnitude independent of absolute elevation. A more detailed investigation of selected sampling locations in the vicinity of Colle Gnifetti/Fiescherhorn (Breithorn, Gorner-/Theodulgletscher, Colle Vincent, Jungfraujoch) revealed that there is no distinct altitudinal trend in ionic loads (Nickus et al., 1997). A recent study about atmospheric deposition in alpine and subalpine areas confirms these results and concludes that there are no clear regional gradients, but a significant spatial variability of atmospheric ion deposition over the Alps (Rogora et al., 2006). Another study concluded that sites with large quantities of precipitation exhibit highest ion concentrations because they receive generally the first, more contaminated fraction of a precipitation event (Nickus et al., 1998).
The above mentioned studies are mainly focused on anthropogenic impurities of winter snow packs and can not be directly transferred to the situation at Claridenfirn. We suppose that concentrations at Claridenfirn might be higher than at Colle Gnifetti/Fiescherhorn due to (1) its eastern location, (2) the lower elevation and thus proximity to the polluted mixing layer, and (3) the higher precipitation rates (MeteoSwiss, 2014b). However, we are unable to conclude with certainty whether and to what extent ion concentrations differ between the ice cores and our study site. For these reasons we adopted the impurity concentrations measured at Colle Gnifetti/Fiescherhorn directly to the study site without a transfer function. Our assumption is supported by a supplementary analysis carried out on Claridenfirn. Kappenberger and Steingruber (2014) collected and analyzed winter snow samples for major ions between 1995 and 2013. Comparison of bulk winter snow concentrations with those at Colle Gnifetti revealed that concentrations at both locations are in the same order of magnitude. In contrast to Saharan dust, which is transported by large-scale upper air flows, BC concentrations are more influenced by the regional environment. Therefore, the assumption of using concentrations from a remote location might be less valid for BC than for Saharan dust. Actual BC input concentrations are thus subject to a higher uncertainty.
When a snowpack begins to melt, the insoluble snow impurities are partly retained and concentration of impurities in the surface snow increases as snow melt proceeds, thus reducing snow albedo. Consequently, melt is amplified and therefore provides a positive feedback on radiative forcing by light-absorbing impurities. To what extent snow impurities are removed by melt water percolation has not been fully clarified and only a small number of studies has addressed this issue so far. Important contribution is made by Conway et al. (1996), who found that particles of volcanic ash remained at or near the surface throughout the melting process, while a large part of soot particles was flushed through the snow with the melt water. They suppose that the difference in the particle's diameters is responsible for the different behavior of ash and soot during the melting process. Doherty et al. (2013) concluded that removal rates due to melt water percolation of BC are in the order of 10–30 % which is in agreement with the results by Conway et al. (1996). Based on the limited information available, a removal efficiency of 20 % for BC seems to be a reasonable assumption. The sensitivity study indicated that the results are more sensitive on the removal rate than the amount of mineral dust and BC input. Hence, this issue needs further investigation. However, ignoring flushing-out of BC with melt would lead to an overestimation of surface concentrations and thus to an excessive melt amplification through BC (Doherty et al., 2013).
The performance of the albedo parameterization mainly depends on an accurate
modeling of the specific surface area of snow grains and the fraction of
snow impurities in the surface snow. Roy et al. (2013) demonstrate that the
simulated snow grain sizes are in good agreement with measurements and that
despite the simplicity of the SSA model results are comparable to
well-established snow models (i.e., Crocus, Brun et al., 1989, 1992). The
root-mean-square error in the overall SSA is 8.0
In this study we analyzed the impact of Saharan dust and black carbon on the mass balance of an Alpine glacier over a centennial period (1914–2014) covered by exceptional observational data sets. A mass balance model including a parameterization for albedo was combined with a snow density model in order to track snow layers and impurities over time. The combined model was forced with temperature and precipitation time series in daily resolution, the latter being assigned by mineral dust and BC concentrations retrieved from ice/firn cores.
On average the presence of Saharan dust at the glacier surface reduced mean
annual albedo by less than 0.01. The associated decrease in the mean annual
mass balance was 28–58
Our study demonstrates that the influence of snow impurities on glacier melting should be taken into consideration, when modeling the mass balance of alpine glaciers over long-term periods in order to increase the reliability of the simulations. Particularly in years with large deposition of Saharan dust or BC and during periods with negative glacier mass balance, re-exposure of old firn layers can importantly impact on the rate of snow and ice ablation and thus enhance the albedo feedback. Furthermore, the study emphasizes the crucial role of BC in melt processes taking place on Alpine glaciers.
This study was supported by the National Research Programme NRP61. Many thanks go to all those who performed mass balance measurements at Claridenfirn with great effort and care. We particularly thank the two key players, H. Müller and G. Kappenberger, for collecting and homogenizing the mass balance measurements. T. M. Jenk and M. Sigl are gratefully acknowledged for providing ice core data of Colle Gnifetti and Fiescherhorn. The analysis of EC in the upper part of the Fiescherhorn ice core (1940–2002) was funded by the EU FP7 project PEGASOS. We thank M. Funk for his helpful comments on an earlier version of the manuscript. Comments by two anonymous reviewers were helpful to finalize the manuscript. Furthermore, we thank MeteoSwiss for providing meteorological time series and the Global Energy Balance Archive (GEBA), ETH Zurich, for the radiation data of Davos. Aerosol measurements at the high-alpine research station Jungfraujoch were conducted by Global Atmosphere Watch (GAW). Edited by: V. Radic