Earth Syst. Sci. Data, 10, 53–60, 2018
https://doi.org/10.5194/essd-10-53-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Modulation of glacier ablation by tephra coverage from
Eyjafjallajökull and Grímsvötn volcanoes, Iceland: an
automated field experiment
Rebecca Möller
1,2
, Marco Möller
3,4,1
, Peter A. Kukla
2
, and Christoph Schneider
4
1
Department of Geography, RWTH Aachen University, Aachen, Germany
2
Geological Institute, Energy and Minerals Resources Group, RWTH Aachen University, Aachen, Germany
3
Institute of Geography, University of Bremen, Bremen, Germany
4
Geography Department, Humboldt-Universität zu Berlin, Berlin, Germany
Correspondence:
Rebecca Möller (rebecca.moeller@geo.rwth-aachen.de)
Received: 21 June 2017 – Discussion started: 6 July 2017
Revised: 22 November 2017 – Accepted: 23 November 2017 – Published: 10 January 2018
Abstract.
We report results from a field experiment investigating the influence of volcanic tephra coverage
on glacier ablation. These influences are known to be significantly different from those of moraine debris on
glaciers due to the contrasting grain size distribution and thermal conductivity. Thus far, the influences of tephra
deposits on glacier ablation have rarely been studied. For the experiment, artificial plots of two different tephra
types from Eyjafjallajökull and Grímsvötn volcanoes were installed on a snow-covered glacier surface of Vat-
najökull ice cap, Iceland. Snow-surface lowering and atmospheric conditions were monitored in summer 2015
and compared to a tephra-free reference site. For each of the two volcanic tephra types, three plots of variable
thickness ( ∼ 1.5, ∼ 8.5 and ∼ 80 mm) were monitored. After limiting the records to a period of reliable mea-
surements, a 50-day data set of hourly records was obtained, which can be downloaded from the Pangaea data
repository (https://www.pangaea.de; doi:10.1594/PANGAEA.876656). The experiment shows a substantial in-
crease in snow-surface lowering rates under the ∼ 1.5 and ∼ 8.5 mm tephra plots when compared to uncovered
conditions. Under the thick tephra cover some insulating effects could be observed. These results are in contrast
to other studies which depicted insulating effects for much thinner tephra coverage on bare-ice glacier surfaces.
Differences between the influences of the two different petrological types of tephra exist but are negligible com-
pared to the effect of tephra coverage overall.
1
Introduction
Deposits of sedimentary materials on the surface of glaciers
are known to have significant influence on glacier melt as
they alter the energy exchange processes at the surface (e.g.,
Nicholson and Benn, 2013; Mattson et al., 1993; Østrem,
1959). The thickness of the layer controls whether the domi-
nant factor at the glacier surface is the decrease in albedo or
the increase in thermal resistance (Möller et al., 2016). The
former implies an increase in the energy gain to the glacier
from solar radiation while the latter implies a decrease be-
cause of reduced heat conduction to the glacier surface. As
a result, thin layers of supraglacial deposits lead to increased
glacier melt, while thick layers imply decreased glacier melt
or even insulation. With increasing layer thickness glacier
melt peaks at the so-called effective thickness. With further
increasing layer thickness, glacier melt decreases again and
returns to the level of uncovered conditions at the so-called
critical thickness. Beyond this thickness, glacier melt de-
creases further towards the limit of complete insulation (Ad-
hikary et al., 1997).
The influence of tephra on glacier melt is usually
parametrized using in situ data for calibration. However,
most of the formulations developed thus far are designed to
capture the effects of moraine debris deposits which are usu-
ally formed by layers with thicknesses on the order of meters
Published by Copernicus Publications.
54
R. Möller et al.: Modulation of glacier ablation by tephra coverage
or at least decimeters or centimeters. In recent years there
have been numerous studies dealing with the relationship be-
tween debris thickness and resulting modification of ablation
(e.g., Collier et al., 2015; Juen et al., 2014; Pratap et al., 2015;
Rounce et al., 2015).
Volcanically active regions of the world in sub-polar and
polar environments episodically experience the deposition of
tephra on glacier surfaces after explosive volcanic eruptions.
Volcanic tephra deposits show a wider range of depositional
thicknesses than moraine debris, i.e., from sub-millimeter to
meter scale. They also feature distinctly different thermal
properties (Brock et al., 2007). The model formulation of
Evatt et al. (2015) is valid for all thicknesses from dust to
meter scale. However, dedicated studies dealing with the re-
lationship between tephra thickness and the intensity of in-
duced ablation change are remarkably less numerous than
those dealing with moraine debris, even if supraglacial tephra
deposits are known to significantly influence glacier surface
processes and mass balance (e.g., Kirkbride and Dugmore,
2003; Möller et al., 2014; Nield et al., 2013). So far, only
three recent studies have carried out a systematic, quantita-
tive investigation of the influence of tephra deposits of vary-
ing thickness on glacier ablation (Dragosics et al., 2016; Juen
et al., 2013; Möller et al., 2016). However, these studies were
carried out on bare-ice surfaces and only rely on results ob-
tained over short periods. The experiments covered periods
of only 17 (Dragosics et al., 2016) or 13 days (Möller et al.,
2016) of regular daily measurements. Moreover, the experi-
ment of Dragosics et al. (2016) was carried out in an ex situ,
non-local environment under controlled, partly laboratory-
like conditions. The experiment of Juen et al. (2013) lasted
for about 1 month, but ablation measurements were mostly
carried out at an irregular frequency.
Here, we present data from automated, continuous mea-
surements of meteorological conditions and snow-surface
lowering under artificially installed plots of volcanic tephra
of different type and thickness. The measurements were ob-
tained from a field experiment which was carried out on
Vatnajökull ice cap, Iceland, over the 2015 summer season.
Snow-surface lowering rates under different thicknesses of
tephra during days with and without precipitation are com-
pared to illustrate the variability of snow-surface lowering
with tephra thickness and the influence of different meteoro-
logical conditions. It has to be noted that our measurements
are only a proxy for snow ablation, as snow density changes
beneath the tephra plots (which also impact snow-surface
lowering) were not quantified due to logistical limitations.
2
Field experiment
2.1
Study site
The field experiment was carried out at an elevation of
∼
970 m a.s.l. on Tungnaárjökull (64.3253
◦
N, 18.0476
◦
W),
a glacier which is part of the western Vatnajökull ice cap,
Iceland (Fig. 1a). The site was situated on a slightly inclined
surface, facing approximately west-southwest. It was char-
acterized by wind-compacted snow coverage with a homoge-
neous depth of ∼ 2.7 ± 0.2 m throughout the site according to
snow-depth probing. Layering of the snowpack was not well
pronounced and snow density showed little variability over
the vertical profile with an integrated mean of ∼ 410 kg m
−
3
,
which was obtained by stepwise measurements along a ver-
tical profile.
2.2
Design and setup
The field experiment was designed to quantify the influ-
ence of volcanic tephra (with variable type and thickness) on
snow-surface lowering and to relate the measured lowering
to meteorological conditions. A set of six artificial plots of
tephra coverage with a diameter of 0.7 m were installed at the
study site. Three of these plots were made from tephra of Ey-
jafjallajökull volcano (EYV) and the other three from tephra
of Grímsvötn volcano (GRV; Fig. 1a). Both types of tephra
were spread out at thicknesses of ∼ 1.5, ∼ 8.5 and ∼ 80 mm.
This was done by weighing out tephra material according to
its bulk density (1276 kg m
−
3
for EYV and 791 kg m
−
3
for
GRV) as dispersal by thickness was not feasible at the mil-
limeter scale. The three thicknesses approximately match the
effective thickness (1.5 mm), the critical thickness (8.5 mm)
and a thickness under which the dominance of insulation can
be considered. These values were chosen according to results
of a short, 13-day field experiment by Möller et al. (2016)
carried out on bare glacier ice using tephra of GRV.
Contiguous to the tephra plots where snow-surface lower-
ing was recorded, standard meteorological parameters were
measured and recorded by an automatic weather station
(AWS). The parameters include air temperature and relative
humidity at two levels (initially 0.3 and 1.1 m above snow,
but increasing according to snow-surface lowering), wind
speed and direction (initially 2.1 m above snow), liquid pre-
cipitation and incoming and reflected shortwave radiation.
For measuring snow-surface lowering at the tephra plots,
an aluminum structure for sensor installation was mounted
(Fig. 1b). Over each of the six plots ultrasonic height gauges
measured snow-surface lowering at hourly intervals. In ad-
dition, sensors for surface temperature measurements were
installed over the two ∼ 80 mm plots. Table 1 gives an
overview of all sensor and measurement specifications for
both the AWS and the tephra plots. The snow-surface lower-
ing measurement at the AWS provides a reference represent-
ing non-tephra covered conditions.
A camera system, taking photographs hourly, was setup
to monitor and document the conditions of tephra plots and
AWS. Unfortunately, it stopped working after a few days and
we do not use these data here.
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R. Möller et al.: Modulation of glacier ablation by tephra coverage
55
Figure 1.
Overview of the field experiment. The locations of tephra sampling at the calderas of Eyjafjallajökull volcano (EYV) and
Grímsvötn volcano (GRV) and the location of the field experiment are shown in (a). The installation of the field experiment is shown
in (b). The three plots in the foreground are covered by EYV tephra and the three plots in the back by GRV tephra.
Table 1.
Measured quantities at the field experiment installation and at the automatic weather station. For each variable the type of the sensor
is given along with its uncertainty and the type of data aggregation over each 1 h record interval.
Variable
Sensor
Uncertainty
Aggregation
Air temperature
Vaisala HMP35C
±
0.4 K
Average
Relative humidity
Vaisala HMP35C
±
3 %
Average
Incoming SW radiation
Campbell Scientific CS300
±
5 %
Average
Reflected SW radiation
Campbell Scientific SP1110
±
5 %
Average
Rainfall
RM Young 52203
±
2 %
Total
Wind speed
RM Young 05103
±
0.3 m s
−
1
Average
Wind direction
RM Young 05103
n.a.
Sample
Snow-surface lowering (reference)
Campbell Scientific SR50
±
1 cm
Sample
Surface temperature
Campbell Scientific IRTS-P
±
0.3 K
Average
Snow-surface lowering (tephra plots)
Campbell Scientific SR50A
±
1 cm
Sample
2.3
Tephra sampling
The tephra material was directly sampled at the calderas
of EYV and GRV (Fig. 1a) in order to obtain pristine ma-
terial. At EYV the tephra was acquired from inside the
caldera (63.6314
◦
N, 19.6373
◦
W). This sampling was car-
ried out on 7 May 2015. At GRV the tephra was collected
at rocky outcrops near the southern caldera rim (64.4061
◦
N,
17.2741
◦
W). Here, sampling was done on 8 May 2015. At
both locations, the tephra was taken from active geothermal
areas.
2.4
Measurements and data preparation
The experiment started on 10 May 2015 and recorded hourly
means and samples from the sensors described in Table 1 un-
til 8 September 2015. Measurements stopped on 9 Septem-
ber 2015, when ablation was so advanced that the aluminum
structure collapsed. During the collapse, the lowermost parts
of the structure were still anchored inside the ice, but the
center of mass of the overlying installation was probably
too high above ground. The timing of the collapse was eas-
ily identifiable from abnormal radiation and distance mea-
surements. For studying the influences of tephra coverage on
snow-surface lowering, the records had to be narrowed down
to a period without snow cover on top of the tephra. The se-
lection of the suitable period is based on measured surface
temperatures on the tephra packs of the two ∼ 80 mm plots
(Fig. 2).
Surface temperature is generally closely related to the
intra-day cycles of air temperature and shortwave radiation.
However, snow or ice surfaces cannot exceed 0
◦
C. This im-
plies that surface temperatures which follow a regular above-
zero intra-day cycle indicate a completely snow- or ice-free
surface. In our field experiment, this is the case for the period
after 15 June 2015 (Fig. 2). Up until this date, sub-zero sur-
face temperatures prevail despite the presence of intra-day air
temperature cycles which regularly exceed 0
◦
C. This indi-
cates at least partly snow covered conditions on the surfaces
of the tephra plots.
From 4 August 2015 onwards, the intra-day cycles of sur-
face temperature start to become irregular. In addition, the
periodic, substantially positive offsets of surface tempera-
ture over air temperature, which occurred consistently over
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Earth Syst. Sci. Data, 10, 53–60, 2018
56
R. Möller et al.: Modulation of glacier ablation by tephra coverage
June
July
August
September
−8.0
0.0
+8.0
+16.0
Δ
T (K)
−16.0
−8.0
0.0
8.0
16.0
ST (°C)
EYV
GRV
EYV
GRV
(a)
(b)
Figure 2.
Records of measured hourly surface temperatures at the two ∼ 80 mm tephra plots (a) and calculated differences between these
surface temperatures and air temperatures measured at the automatic weather station (b) over 11 May to 8 September. The air temperatures
are calculated as the mean of upper and lower air temperature sensor at the AWS. The types of tephra (EYV for Eyjafjallajökull volcano and
GRV for Grímsvötn volcano) on which the surface temperatures were measured are indicated by color code. The grey shading in the center
of the time series indicates the period considered in the final data set, i.e., 15 June to 3 August.
15 June to 3 August, were replaced by rather irregular, pre-
dominantly negative offsets (Fig. 2). This combination of ob-
servations suggests that the tephra packs started to disinte-
grate, providing space for snow or bare-ice outcrops which
destroyed the homogeneous surfaces of the tephra plots. Over
homogeneous, low-albedo tephra coverage, shortwave radia-
tion adds considerably to the energy gain at the surface and
thus drives surface temperatures far above the air temperature
level. Over rather patchy tephra coverage with high-albedo
bare-ice outcrops, the integrated energy gain due to absorbed
shortwave radiation is much lower. In addition, the surface
temperature of the outcrops is capped at 0
◦
C. The integrated
surface temperature of the tephra plots might thus lie well
below the air temperature level.
Based on these considerations, we limit the observations
to the 50-day period covering 15 June to 3 August (Fig. 2).
The final data set contains hourly averaged data for all mete-
orological parameters measured at the AWS (Fig. 3a). More-
over, it contains hourly data from all seven ultrasonic height
gauges, i.e., from snow-surface lowering measurements at
the six tephra plots and at the reference site at the AWS
(Fig. 3b).
We compared the snow-surface lowering rates at the dif-
ferent plots. To facilitate this analysis, running 24 h differ-
ences, i.e., running daily snow-surface lowering rates, were
calculated for the data of each of the seven sensors when-
ever valid measurements existed at all six tephra plots and
at the reference site. This was undertaken in order to assure
full comparability of the 24 h snow-surface lowering values.
These running 24 h differences are also part of the published
data set.
3
Results
Snow-surface lowering measurements over the chosen time
period (15 June to 3 August 2015) reveal a loss of 2.25 m
of snow cover at the reference site and between 2.21 and
2.97 m at the tephra plots (Fig. 3b). During almost the en-
tire period the study site showed snow coverage. Only for
the plots with ∼ 1.5 mm tephra coverage it cannot be ruled
out that the snowpack beneath the plots disappeared just be-
fore the end of the study period. For the reference site, the
snowpack completely disappeared during the second week
of August according to the measured albedo values. The pro-
gressive snow-surface lowering led to an increasing measure-
ment uncertainty towards the end of the study period because
the sensors’ footprints might have extended beyond the bor-
ders of the tephra plots and erosion of the tephra material
might have destroyed the previously homogeneous disper-
sal across the plots. Nevertheless, the running daily snow-
surface lowering rates, i.e., the slopes of the snow-surface
lowering curves (Fig. 3b), show small variability with time,
even if ephemeral increases sporadically occur at the end of
June and during mid-July. The lowering rates at the six dif-
ferent plots become more similar over the second half of July,
suggesting an incipient disintegration of the different tephra
packs presumably due to erosion by meltwater.
Major disturbances occur in the snow-surface lowering
curves of two of the GRV tephra plots (∼ 8.5 and ∼ 80 mm)
in mid-July (Fig. 3b). On 14 July the measured distance at the
∼
8.5 mm GRV tephra plot increased by ∼ 0.20 m, followed
by an increase of ≥ 0.15 m at the ∼ 80 mm GRV tephra plot
on 16 July. These disturbances coincide with a major rain
event (Fig. 3a). It can thus not be ruled out that partial de-
structions of the tephra plots and of the upper layers of the
snowpack occurred at this date subsequently distorting the
distance measurements at the six tephra plots.
The relationships between tephra thickness and running
daily snow-surface lowering rates (Fig. 4) resemble the find-
ings of previous studies dealing with bare-ice ablation (Kirk-
bride and Dugmore, 2003; Mattson et al., 1993; Möller
et al., 2016). At the thin (∼ 1.5 mm) tephra plots, snow-
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R. Möller et al.: Modulation of glacier ablation by tephra coverage
57
−8.0
0.0
8.0
16.0
ST (°C)
0.0
1.0
2.0
3.0
SSL
EYV
(m)
15
June
1
July
15
July
1
August
0.0
1.0
2.0
3.0
SSL
GRV
(m)
b)
(
0.0 mm (ref.)
1.5 mm
8.5 mm
80.0 mm
0.0 mm (ref.)
1.5 mm
8.5 mm
80.0 mm
−3.0
0.0
3.0
6.0
T (°C)
40
60
80
100
RH (%)
0
10
20
W
S
(m
s
–1
)
270
0
90
180
270
WD
(°)
0
300
600
900
SW
R
(W
m
-
)
2
15
June
1
July
15
July
1
August
0.0
1.0
2.0
3.0
P (mm)
(a)
Figure 3.
Hourly records of the measurements of all sensors installed at the automatic weather station are shown in (a), and measurements
of all sensors mounted at the field experiment installation are shown in (b). Records are shown for 15 June to 3 August. For air temperature
(T ) and relative humidity (RH) the records of the upper sensor (blue line) are shown together with those of the lower sensor (red line).
Wind speed (blue line) is shown together with wind direction (red line); note the different y axes here. Incoming shortwave radiation (SWR,
blue line) is shown together with reflected shortwave radiation (red line). For precipitation (P ), only the liquid fraction has been measured.
Surface temperatures (ST) are shown for the ∼ 80 mm plots of tephra from Eyjafjallajökull volcano (EYV, blue line) and from Grímsvötn
volcano (GRV, red line). Cumulative snow-surface lowering (SSL) is shown over the different plots (indicated by color codes) of EYV tephra
and GRV tephra.
surface lowering was substantially increased by a factor of
1.49 ± 0.88 (mean ± 1σ over time) under EYV tephra and by
a factor of 1.51 ± 0.71 under GRV tephra. At the tephra plots
geared to the critical thickness of the tephra (∼ 8.5 mm),
snow-surface lowering was equal to uncovered conditions
under EYV tephra (1.00 ± 0.61) and slightly increased un-
der GRV tephra (1.17 ± 0.57). However, at the thick tephra
plots (∼ 80 mm) the observed snow-surface lowering did not
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Earth Syst. Sci. Data, 10, 53–60, 2018
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R. Möller et al.: Modulation of glacier ablation by tephra coverage
Mean (all days)
Mean (dry days)
Mean (wet days)
Mean (all days)
Mean (dry days)
Mean (wet days)
0.0
1.5
8.5
80.0
Tephra thickness (mm)
0
1
2
3
4
5
6
SSL relative to 0.0 mm tephra
GRV
0.0
1.5
8.5
80.0
Tephra thickness (mm)
0
1
2
3
4
5
6
SSL relative to 0.0 mm tephra
EYV
Figure 4.
Running 24 h snow-surface lowering (SSL) rates at the different plots of tephra from Eyjafjallajökull volcano (EYV) and from
Grímsvötn volcano (GRV) relative to the reduction rates measured at the non-tephra covered reference site. The box plots give an overview
of the data spread across all running 24 h values recorded during the field experiment period (15 June to 3 August). Outliers are indicated
as open circle symbols. Mean values over the entire field experiment period are indicated by yellow triangles, and the mean values over wet
(precipitation > 0.1 mm) and dry (precipitation ≤ 0.1 mm) days are shown as color-coded line graphs.
match expectations drawn from previous bare-ice knowl-
edge. Under EYV tephra, snow-surface lowering was close
to uncovered conditions (0.98 ± 0.73) and under GRV tephra
only a slight insulation effect was present (0.85 ± 0.59). The
rather high standard deviations, however, suggest a consid-
erable, misleading influence of sporadic, anomalously high
and potentially erroneous values. Our assumption, which is
supported by the distinctly more moderate medians of 0.93
(EYV) and 0.76 (GRV; Fig. 4), is of insulating conditions
under both ∼ 80 mm tephra covers. Nevertheless, the high
snow-surface lowering rates at the two sites with ∼ 80 mm
tephra cover suggest substantially different snowpack behav-
ior than bare glacier ice behavior under tephra coverage.
This unexpected and thus important finding cannot be ex-
plained in full detail here because of limitations in the exper-
imental setup. One obvious explanation is the fact that pure
snow ablation is masked by additional processes in the mea-
surements conducted. Snow-surface lowering resulting from
settling and compaction of the snowpack as well as from
metamorphism on the snow-crystal level also definitely im-
pact the measurements. Moreover, the rather small horizontal
extent of the tephra plots probably permits lateral influences
of weather conditions on the snowpack beneath the plots. Ex-
planations beyond these influences cannot be given, because
the pure, energy-balance-controlled ablation signal cannot be
isolated from measured snow-surface lowering. It is thus rec-
ommended that future experiment setups at least account for
snow density variations.
Distinct differences were observed between snow-surface
lowering rates during periods with and without precipitation
(Fig. 4). On wet days the increase in snow-surface lower-
ing rates under the thin tephra covers compared to uncov-
ered conditions is even more pronounced than it is on dry
days. This finding is in clear contrast to short-term measure-
ments by Möller et al. (2016) on bare glacier ice. Their study
shows that on wet days sub-tephra ice ablation rates are even
decreased when compared to uncovered conditions. The in-
crease in snow-surface lowering under the ∼ 8.5 mm tephra
covers compared to uncovered conditions is also higher on
wet days than on dry days. This implies that the critical
thickness of wet tephra is generally higher than that of dry
tephra. The strength of the small insulation effect at the thick
∼
80 mm tephra plots is, however, independent of the alloca-
tion to dry or to wet days.
There were average summer meteorological conditions
during the field experiment period (15 June to 3 Au-
gust 2015; Fig. 3a). Air temperature mostly fluctuated be-
tween 0 and +4
◦
C (with few outliers) and showed a mean of
+
2.1 ± 1.4
◦
C (mean ± 1σ ). Thereby, mean (± 1σ ) air tem-
perature gradients between lower and upper sensors amount
to +0.20 ± 0.15 K m
−
1
. Daily albedo means decreased from
∼
0.71 during the first week of the field experiment period
to ∼ 0.58 during its last week. The associated daily mean of
net shortwave radiation fluxes was 86.0 ± 22.4 W m
−
2
. The
mostly undisturbed daily cycles of incoming shortwave radi-
ation suggest little cloud coverage. Accordingly, total rain-
fall over the period sums up to only 40.2 mm. However, high
wind speeds of 5.65 ± 3.34 m s
−
1
(mean ± 1σ ) with peak
wind periods reaching 12–19 m s
−
1
might have led to con-
siderable undercatch of precipitation by the tipping-bucket
rain gauge (Sugiura et al., 2006). The by far most frequently
occurring wind directions (ENE to ESE) resemble the kata-
batic flow direction down the western slope of Vatnajökull.
4
Data availability
The final data set is organized in one single csv file
which is available for download from the Pangaea Earth
Earth Syst. Sci. Data, 10, 53–60, 2018
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R. Möller et al.: Modulation of glacier ablation by tephra coverage
59
and environmental sciences data repository (Möller et al.,
2017; https://doi.org/10.1594/PANGAEA.876656). It con-
tains 2904 hourly samples (11 May to 8 September) of 18
variables. Among these, the six variables related to snow-
surface lowering measurements at the artificial tephra plots
are limited to 1200 hourly samples (15 June to 3 August)
only (see Sect. 2.4).
5
Summary and outlook
A field experiment, studying the influences of different types
of volcanic tephra on snow-surface lowering, was conducted
on Vatnajökull ice cap, Iceland, in summer 2015. Two types
of Icelandic tephra were compared, one from Eyjafjallajökull
volcano and one from Grímsvötn volcano. Both tephras
were sampled right before the start of the experiment at the
calderas of the respective volcanoes. For the experiment,
three different artificial plots of different thickness (∼ 1.5,
∼
8.5 and ∼ 80 mm) were installed from both tephras. Snow-
surface lowering at all six tephra plots and at a tephra-free
reference site was monitored automatically over the summer
season jointly with surface temperature on the two ∼ 80 mm
tephra plots and concurrent atmospheric variables.
The experiment ran from mid-May to mid-September.
Snow-surface lowering could be determined for 50 days
(15 June to 3 August) at hourly resolution. The data set com-
prises records of air temperature and relative humidity at two
levels, wind speed and direction, rainfall, incoming and re-
flected shortwave radiation and snow-surface lowering (in
terms of distance from sensor to surface) over a non-tephra
covered reference site and over the six tephra plots. Surface
temperature was additionally measured at the two ∼ 80 mm
tephra plots. We presented a comparison of snow-surface
lowering rates under the different tephra plots.
Snow-surface lowering showed substantial median in-
creases at the two ∼ 1.5 mm tephra plots (∼ 17 % under Ey-
jafjallajökull tephra and ∼ 40 % under Grímsvötn tephra).
However, snow-surface lowering was also considerably in-
creased at the ∼ 8.5 mm Grímsvötn tephra plot (median of
∼
11 %), which contrasts with results of previous studies
on bare-ice glacier surfaces. Insulation was small even un-
der the thick ∼ 80 mm plots (median reductions of ∼ 7 %
under Eyjafjallajökull tephra and ∼ 24 % under Grímsvötn
tephra). This also stands in contrast to earlier bare-ice re-
sults, where almost full insulation was found under compa-
rably thick tephra covers. The increase in snow-surface low-
ering on days with rainfall under thinner tephra covers com-
pared to uncovered conditions is markedly higher than on
days without rainfall. This is in contrast to bare-ice condi-
tions, where no ablation increase is present on rainfall days
at all. This finding leaves room for further investigation. In-
fluence of tephra type is small compared to the other factors.
For potential future experiments, the results and our expe-
rience in the field suggest that frequent snow profile analyses
or at least snow density measurements over the experiment
period are required to interpret the snow-surface lowering
measurements obtained with regards to snow ablation. How-
ever, this is logistically challenging, as would be the sug-
gested use of larger tephra plot diameters, which would bet-
ter prevent snow-surface lowering measurements from being
influenced by lateral energy fluxes from the surface to the
sub-tephra snowpack. Installing the six tephra plots with a
diameter of 2.0 m instead of 0.7 m would have required the
transport of over 320 kg of tephra (instead of ∼ 115 kg) from
the two sampling sites to the field experiment site.
In conclusion, the experiment delivers a data set which
clearly illustrates that the influences of supraglacial tephra
cover on glacier ablation are considerably different, depend-
ing on the surface of the glacier, i.e., snow or bare ice. To
our knowledge, this data set is the first to continuously mea-
sure snow-surface lowering under different types and thick-
nesses of volcanic tephra. Together with the simultaneously
acquired meteorological conditions, this data set allows for
further in-depth study of the influence of weather conditions
on sub-tephra snowmelt. Moreover, it can readily be included
as a calibration or validation data set in broader studies on the
influences of supraglacial particle cover on ablation.
Competing interests.
The authors declare that they have no con-
flict of interest.
Acknowledgements.
The field experiment was funded by grant
no. SCHN680/6-1 and no. KU1476/5-1 of the German Research
Foundation (DFG). We thank the Vatnajökull National Park
administration for granting permission to carry out the experiment
and the associated tephra sampling at Grímsvötn caldera. Helpful
comments on the manuscript by Jan Lenaerts and Christoph Mayer
are gratefully acknowledged.
Edited by: Reinhard Drews
Reviewed by: Jan Lenaerts and Christoph Mayer
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