Permafrost, Phillips, Springman & Arenson (eds)
© 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7
Monitoring of ground-ice formation in a block slope at
Mt. Nishi-Nupukaushinupuri, Hokkaido, Japan
Y. Sawada
Graduate School of Environmental Earth Science, Hokkaido University, Japan
ABSTRACT: The depth of the ground-ice surface was monitored for one year in the lower part of block slope,
where permafrost had been detected although the annual mean air temperature was positive. In winter, ground
temperature dropped far below 0°C due to the air circulation between the block slope and atmosphere, while the
ice surface level was unchanged. The ground-ice surface began to aggrade upwards just after the snow-melt
occurred in spring, and ice growth continued until middle April, when ground temperatures approached 0°C.
From mid summer to early fall, the ice surface melted and deepened, and ground temperatures showed synchronous increases with heavy rainfall events. These results suggest that subsurface water plays an important role in
growth and melting of ground-ice. Thermal processes of seasonal ice growth and melting are important in the
maintenance of extra-zonal permafrost in a block slope at low latitudes/altitudes.
upward and vents from top, drawing cold air into the
lower slope where snow cover is thin or absent due to
the lacking of forest vegetation and prevailing wind
(Sawada et al., in press). In spring, the ground-ice
grows by refreezing of snowmelt water (Sawada et al.,
in press). This paper reports on the results of a whole
year monitoring of the ground-ice surface, carried out
between November 2000 and November 2001, and
discusses the preservation mechanisms of ground ice
in the block slope in warmer climates.
1 INTRODUCTION
Generally, areas where the permafrost is in equilibrium with climate are characterized by a mean annual
air temperature (MAAT) below 0°C. However, in
middle latitudes, “extra-zonal permafrost” may exist
sporadically in block slope (e.g. Kneisel et al., 2000 in
Switzerland), and in ice caves (e.g. Ohata et al., 1994
in Japan), despite MAAT above 0°C. Ohata et al.
(1994) investigated the formation and preservation
of perennial ice in a lava tube cave (1120 m a.s.l.)
on Mt. Fuji. They found that ice was formed mainly
in spring, when melt-water and rainwater percolated
into the subfreezing cave, and concluded that the
perennial ice is maintained primarily by the balance
between strong cooling by inflow of cold-dry air in
the winter and weak warming by inflow of warm air
in the summer.
The mechanism of ice preservation in block slopes
is still unclear, since previous studies ware based
mainly on indirect phenomena. In block slopes, the
ground ice is formed in inaccessibly narrow spaces,
making direct observation much more difficult than in
tube-like caves. Tanaka et al. (2000) reported that
clear ice masses and icicles fill interstices of block
slopes in northern Japan (Nakayama wind holes in
Fukushima prefecture). They investigated temperatures of air that flew out from “wind holes”, and concluded that the selective air circulation system is
necessary to preserve ground ice in summer. Sawada
et al. (in press) measured both air circulation and
ground-ice formation in a block slope in Hokkaido
Island, northern Japan. This revealed an air ventilation
system in winter, such as the one reported by Tanaka
et al. (2000); warm air in the block slope moves
2 DESCRIPTIONS OF STUDY AREA
The study area is located on the summit slope of
Mt. Nishi-Nupukaushinupuri (1254 m a.s.l.) in the central part of Hokkaido Island, Japan (Fig.1). This mountain is one of the lava domes of the Shikaribetsu volcano
group erupted during the last glacial period. The
mountain slope is widely covered with coarse blocks,
0.3–3 m in diameter. Mean annual air temperature
(MAAT) in the study area is positive (1.7°C in 1999,
1.3°C in 2000) and much higher than MAAT at the
lower limit of discontinuous alpine permafrost in the
Daisetsu Mountains (2°C: Sone, 1992; Ishikawa and
Hirakawa, 2000). Annual precipitation is 1175.8 mm,
which is an average for the 1979–1990 period at the
nearest meteorological station (Nukabira, 10 km NW
from study area).
The study area is divided into a block slope where
coarse blocks cover most of the ground surface, and
a non-block slope covered by forest (Fig. 1). The
northeast-facing non-block slope is dominated by
mixed forest with a ground cover of bamboo grass
bush (Sasa senanensis). The southwest-facing block
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3 METHODS
slope reaches down the valley bottom. The middle part
of the block slope is covered by spruce (Picea glehnii).
The lower and the upper parts of the block slope, however, lack forest (shaded area in Fig. 1), and are covered by lichen, dwarf pine (Pinus pumila), and alpine
shrubs. The valley bottom is dominated by spruce and
dwarf pine, while the forest floor is covered mostly by
Sphagnum sp. and alpine shrubs. During the summer
and autumn, cold air blows from the hollows among
the moss-covered blocks in the valley bottom. In the
valley bottom (valley bottom site in Fig. 1), perennial
ground ice was found at 1.5 m in a pit in October 1999
and October 2000.
In winter, the local topography and prevailing
wind creates uneven snow cover. The regional maximum average of snow depth is about 1m. However, the
westerly prevailing wind removes snow from the rough
block slope, creating a thin and uneven snow cover,
particularly on the middle part of the slope.
This thin snow cover barely covers the coarse blocky
surface, allowing cold air penetration into the voids
of block slope (Sawada et al., in press). At the
lower end of block slope, the maximum snow cover
is 1–3 m thick.
Figure 1. Study area and monitoring sites.
3.1 Monitoring of the depth of ground-ice surface
In the valley bottom (Fig.1), the depth of the groundice surface was monitored from November 2000 to
November 2001. In October 2000, a perforated plastic
pipe was buried down to the depth of ground-ice
surface (157 cm, as shown in Fig. 2). The perforations allowed ground water penetration into the pipe,
enabling measurement of the depth of the ground-ice
surface. A still rod was used to detect the ice surface
in the pipe. Monitoring started on the 4th of November
2000, and the most intensive investigations were
carried out between 7 and 14 April 2001, during the
snow-melt period.
3.2
Air and ground temperature measurements
Ground temperatures were measured at two sites in
the block slope. One of the sites is in the lower end
of slope, where the perennial ice was found. The other
is at the top of the slope, where the warm funnels
are formed in winter (Fig. 1). In the valley bottom,
thermistor sensors were installed between boulders
beside the ice-monitoring pipe at depths of 0, 20,
50, 100 and 157 cm (Fig. 2). All sensors were
calibrated at 0°C by the apparent ‘zero curtain’
that occurs in spring. At the top of the slope, where
funnels emitting warm air have been observed in the
winter, sensors were buried at 50 and 100 cm
depth between boulders. The air temperature was
measured at 1.5 m above the block field (Fig. 1).
Small data loggers (TR-52, T&D corp.) were used to
monitor air and ground temperature data at one-hour
intervals.
Figure 2. Instruments for ground-ice monitoring.
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4 DATA ANALYSIS
Temperature(ºC)
30
Figure 3 shows vertical changes in the ground-ice
surface over the one year monitoring period. Changes
in depth of the ground-ice surface can be divided into
four stages: stable (winter), rapid growth (spring
melt), slow growth (spring and early summer), and
ablation (late summer and early fall) period (Fig. 3).
Air temperature
a
0
-30
D
N
J
F
M
A
M
J
J
A
20
b
Temperature(ºC)
Stable period
The stable period corresponds to winter, when the air
temperature was below 0°C (except early November
and mid-March). No ice accumulates in the pipe during this period (Fig. 3). Ground temperatures in the
valley bottom dropped well below 0°C and showed
significant short-term fluctuations (Fig. 4c, d and e),
although the snow depth was considerably thick
(174 cm measured in 4/9 2001). Therefore, the shortterm temperature fluctuations imply a circulation of
air from the atmosphere through block slope.
Ground temperatures at the top of the slope
remained positive throughout the winter (Fig. 4b), and
the snow funnels emitting warmer air were found at
the top of block slope (Fig. 5). Hoar-frost crystals
grew around the funnel edge, indicating the sublimation of vapor of warmer air. Similar funnels have been
reported at Murtel rock glacier in the Engadin,
Switzerland (Keller and Gubler, 1993; Bernhard et al.,
1998). This warm funnel is thought to be an indicator
of a “chimney effect” in the block slope (Sawada
et al., in press). Interstitial warmer air moves upwards
through the block slope due to buoyancy effects
(interstitial air in the block slope is warmer than the
air at the top of the slope). This air movement draws
cold air into the lower part of block slope where the
snow cover is thin or absent. This cold air is warmed
by heat conducted from the blocks.
4.2
O
-100cm
Ground temperatures
at the top of slope
10
-50cm
0
-10
D
Temperature(ºC)
5
Temperature(ºC)
N
5
Temperature(ºC)
4.1
S
5
J
F
M
A
M
J
J
A
S
O
Ground temperatures at the valley bottom
c
0
-50cm
-10
d
0
-100cm
-10
e
0
No data
-157cm
-10
N D
2000
J F
2001
M
A
M
J
J
A
S
O
Figure 4. Yearly results of air and ground temperatures
monitoring.
Rapid growing period
Figure 6 illustrates the air and ground temperature
changes, and the depth of the ground-ice surface in
3
2
-130
4
1
depth(cm)
Measurements
-140
-150
-160
N D
2000
Stages
J F
2001
M
1 Stable
2 Rapidgrowth
A
M
J
J
A
S
O
N
3 Slowgrowth
4 Ablation
Figure 3. Changes on the depth of ground-ice surface
from November 2000 to November 2001.
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Figure 5. Warm funnel at the top of block slope. Photograph taken on 14 January 2002.
5.0
+20
-20
1April
4.0
11
21
temperature(ºC)
0ºC
A
1May
0ºC
B 0cm
A
-50 cm
-100 cm
3.0
2.0
1.0
0.0
-157 cm
-1.0
May
-10
June
July
Aug
Sep
Oct
0ºC
150
C -20cm
precipitation(mm)
-10
0ºC
D -50cm
-10
0ºC
E -100cm
50
0
May
-10
0ºC
22, 23August
B
100
June
July
Aug
Sep
Oct
June
July
Aug
Sep
Oct
-130
F -157cm
1April
11
-130
-140
G
-150
-160
cm
1April
21
depth(cm)
-10
1May
C
-150
May
11
21
1May
Figure 6. Air temperature (A), ground temperatures at the
valley bottom (B–F), and depth of ground-ice surface at
the valley bottom (G) in April 2001.
the valley bottom through April 2001. A sudden
temperature rise (experienced in all depths) occurred
on April 9, and the amount of the temperature rise
increases with depth into the block slope. Synchronously, the ground-ice surface began to rise rapidly.
The synchronous changes between ground temperatures and ice surface depth indicate heat input to the
ground from snow-melt water and latent heat emission by the refreezing of the melt water. Since the
ground warmed just after the air temperature rose
above 0°C and snow-melting began, the availability of
snow-melt water controls ground-ice accretion. Abrupt
ground-ice growth continued until the middle of April,
when ground temperatures approached 0°C.
4.3
-140
Figure 7. Ground temperature at the valley bottom (A),
Precipitation at nearest meteorological station (B), and
changes on depth of the ground-ice (C) from 1 May 2001
to 16 Oct 2001. Star symbol indicates an apparent synchronous change. Temperature data at the 157 cm position
was not recorded between early June until mid-August due
to the cable having been cut by wild animals.
water due to conductive cooling on contact with the
ground-ice. In late August, following heavy rainfalls
on the 22nd and 23rd, the ground-ice surface began to
deepen; i.e. the ice was melting. These results indicate
that the increase in summer air temperatures raised the
temperature of the ground-ice to the zero curtain, but
it was the input of heat from the rainfall on the 22nd
and 23rd that triggered ground-ice melting (Fig. 7).
5 DISCUSSION
In winter, ground-ice did not accumulate in the monitoring pipe, indicating that water availability is the
limiting factor for ice growth. Ground temperatures
at all depths are negative through mid-November
to early April (Fig. 4c, d and e). The ground-ice forms
mainly from the refreezing of snow-melt water in
spring. The synchronized accretion of ground-ice with
the increase in ground temperature (Fig. 6) indicates
warming by latent heat released during melt-water
refreezing. This idea is supported by the greater
Slow growing and ablation periods
The ground-ice surface continued to grow slowly until
early August (Fig. 7), when air temperature peaked
(Fig. 3). Ground temperatures slowly increased, punctuated by short positive fluctuations, followed by a
return to 0°C. The abrupt fluctuations in ground
temperature are caused by the percolation of rainfall
(Fig. 7). However, ground-ice continued to accrete
during this period, indicating some freezing of ground
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increase in ground temperature with depth. Following
the rapid accretion of ground-ice, there is a slow
growth period, which continues until air temperatures
reach the annual maximum (Fig. 7). At this time the
ground-ice temperature has risen to 0°C and percolating rainfall now promotes rapid melting of the groundice. However, during the one year monitoring, melting
did not appear to go deeper than the newly precipitated
ice formed during that spring and summer, thus maintaining a positive net balance of ground-ice.
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6 CONCLUSION
Three major thermal processes control the preservation of the ground-ice in the studied block slope. The
first is air circulation from atmosphere into the block
slope conducted by density difference (Sawada et al.,
in press), cooling the block slope through winter. The
second is the latent heat exchange between refreezing
melt water and the ground occurring during the spring
thaw when snow melting occurred. The third is conductive and latent heat transfer onto the ice by the
rainfall. The latter two processes require water flow
into the valley bottom, suggesting the drainage system
plays an important role in preserving the perennial ice
in the block slope. These thermal processes probably
occur not only in extra-zonal permafrost in low altitude, but also in discontinuous permafrost zone in
high mountain slopes, since blocky landform is widespread in steep mountainous region (c.f. Rock glacier). Additional field experiments are planned to
further investigate the permafrost preservation in the
block slopes.
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