3 Results
3.1 Air temperature and Ground
surface temperature
(Ts)
Results presented in this paper are from 1 September 2016 to 31 August
2019 (Figure 6a). The annual mean air temperature and derived values are
presented in Table 1. There were no significant differences in air
temperature (Ta ) between the sunny and shady
slope. The annual mean air temperatures
(\(\overset{\overline{}}{T}\)a) for 2016-17 and 2017-18
were -2.90 and -2.56 °C at the sunny slope and -2.63 and -2.36 °C at the
shady slope. The differences between the sites in the two years were
0.27 and 0.20 °C. Although the annual mean air temperature in 2018-19
was more than 1 °C lower than the previous two years (-4.02 °C at the
sunny slope and -3.82 °C at the shady slope), the air temperature
difference between both sites was still only 0.2 °C, near the sensor
accuracy.
There was little difference in annual freezing degree days (FDD) and
thawing degree days (TDD) for the three years at both sites,
particularly in the thawing season where the difference was
~20 degree days (Table 1). The difference in calculated
air frost number (F) was only 0.01 (0.62 and 0.63), indicating the air
temperature conditions at the two sites are very similar.
Daily variations in Ts are presented in Figure
6b. Compared to air temperature, Ts values were
significantly different between the sites during the monitoring period.
The mean annual Ts for the three years were
0.28 ±0.40 °C at the sunny slope and -1.02 ±0.40 °C at the shady slope,
a difference of ~1.3 °C.
3.2 Ground temperatures
(Tns, Tps,
Tg)
Daily variations in ground temperature at 5 cm (Tns),
2.5 m (Tps), and 5 m (Tg) depth are
shown in Figure 7. Variations in Tns over the three
years were similar to Ts because the thermistors were
only 3-5 cm apart. The daily mean Tns for the three
years at the sunny slope was always higher than at the shady slope, but
the difference in Tns was not as great as for
Ts. The average daily temperature gradient between
Ts and Tns was lower at the shady slope
(0.02 ±0.06 °C) than that at the sunny slope (0.10 ±0.04 °C). The
difference in daily mean Tns (Tns at the
sunny slope minus Tns at the shady slope) was 0.1-3.6
°C, with a mean value of 1.43 °C. And the annual mean temperature
difference ranged 1.3 to 1.6 °C.
The difference in Tps between the sites was significant
up to 2.5 m depth (Figure 7b). The daily mean Tpsfluctuated about 0 °C at the sunny slope, but was stable below 0 °C at
the shady slope. The daily mean Tps for the three years
at the sunny slope was also always higher than at the shady slope, with
differences between 0.4-3.4 °C, and a mean value of 1.44 °C. The annual
mean Tps values at the sunny slope in 2016-2019 were
0.24, 0.30, and -0.12 °C, and -1.21, -1.15, and -1.53 °C at the shady
slope, respectively. The annual temperature difference was
~1.4 °C.
At 5.0 m depth (Figure 7c), the daily mean Tg was
<0 °C at both sites all year. Daily mean Tg at
the sunny slope was always ~-0.1°C over the three years.
Because the precision of the sensor is ±0.21 °C, we don’t think this
value is very accurate. However, it seems that local environmental
factors have little effect on Tg, and conclude that
there is a strong temperature control effect at this depth. The daily
mean Tg at the shady slope fluctuated between -1 to -2
°C over the study period, with a mean annual value of -1.4 ±0.02 °C. The
1.4 °C difference in Tg between the sunny and shady
slopes indicate that the thermal regime is strongly affected by slope
aspect.
3.3 Depth of seasonal
thawing
The ground surface begins to thaw when Ts rises above 0
°C with the increase of Ta, and the thaw depth usually
reaches its annual maximum at the end of August on QTP (Luo et al.,
2019). The maximum seasonal thawing depth at both sites in 2016 was
approximated during the drilling campaign in July-August, and the
observed results were 2.70±0.1 m among nine boreholes at the sunny slope
and 1.74±0.1 m at the shady slope. A nearly 1.0 m difference of the
maximum depth of seasonal thawing between sites is likely to result from
the large difference in Ts between sites. In 2016-19,
the mean annual Tps measured at 2.5 m depth was above 0
°C (~0.14 °C) at the sunny slope, indicating that the
maximum depth of seasonal thawing was >2.5 m. At the shady
slope, the site mean Tps values and accompanying maximum
and minimum values remained below 0 °C (~-1.30 °C),
indicating that the maximum depth of seasonal thawing was <2.5
m. The difference in depth of seasonal thawing between the sites is
related to the difference in thawing period duration. The thawing period
was about 30-40 days shorter at the shady slope than at the sunny slope
in 2016-19, resulting in a shallower depth of seasonal thawing at the
site.
3.4 Soil moisture content
Daily variations in soil moisture content at four different depths
within the depth of seasonal thawing (0.25, 0.5, 1.0, and 1.5m) are
presented in Figure 8. The difference in moisture content between sites
was significant, with the ground at the shady slope always wetter than
at the sunny slope. The difference was maintained despite frequent
summer precipitation events affecting both sites. The near-surface
ground at the shady slope remained relatively moist during the thawing
periods. The mean soil moisture content at 0.25 m depth at the shady
slope was 0.374±0.003 m3/m3 during
the three thawing periods, and was 0.250±0.002
m3/m3 at the sunny slope (Figure
8a). The difference in moisture content between sites occurred up to 1.5
m depth (Figure 8d). The soil moisture content decreased dramatically
after ground freezing commencing at the end of October. The residual
moisture content may reflect unfrozen water content, but as the sensors
are not calibrated for measurement below 0 °C, unfrozen water is not
discussed further.
3.5Radiation
The four components of the radiation budget all showed variation due to
seasonal changes in solar altitude (Figure 9). The seasonal variation in
shortwave downward radiation (DR) is evident at both sites (Figure 9a).
The maximum value of daily mean DR reached ~400
W·m-2 in May-July, then gradually decreased to the
minimum value of ~100 W·m-2 in
mid-December. In contrast with DR, although shortwave upward radiation
(UR) exhibited relatively little seasonal variation, there were many
peaks caused by snowfall increasing the surface albedo, especially in
March-May (the gray area in Figure 9b). The daily mean UR at the sunny
slope was much higher than at the shady slope, and during the cold
period, the monthly mean UR was 40% higher (~12.0
W·m-2) (Table 2).
The daily mean downward long-wave radiation (DLR) showed a similar
seasonal pattern as DR, but variations in DLR were smaller and less
scattered over the year. The DLR lagged behind DR by a few weeks (Figure
9c) due to the thermal inertia of the Earth system. Seasonal fluctuation
in upward long-wave radiation (ULR) was well correlated with DLR. The
daily mean ULR was higher at the sunny slope than that at the shady
slope in most months, except in March-May of each year (Table 2). The
resulting net long-wave radiation (Rl) was always
~7 W·m-2 lower at the sunny slope than
at the shady slope during the warm season and ~5.5
W/m2 lower during the cold season.
The magnitudes of variation in net radiation (Rn) for
both sites were different in each month. However, the daily mean
Rn in most months was lower at the sunny slope than that
at the shady slope (Figure 10 and Table 2). The mean daily
Rn for the whole study period (n=969 days) was 12.5±0.65
W·m-2 lower at
the sunny slope than at the shady slope. This may be related to
differences in surface albedo and surface moisture conditions at the
sites. In the warm season, the maximum daily Rn was >850
W·m-2 (Figure 10h), and <350
W·m-2 in the cold season (Figure 10k).
3.6 Surface
albedo
Variations in daily mean surface albedo were consistent with variations
in UR (Figures 9b, 11), and similar at both sites. However, the daily
mean surface albedo was slightly higher at the sunny slope (n=1095,
0.182±0.003) than at the shady slope (n=969, 0.176±0.003). As a result,
most of the monthly mean albedo values, and the annual mean albedo was
higher are the sunny slope (Table 2). The surface albedo at the two
sites fluctuated greatly over the year, with snowfall in winter causing
a sharp increase up to a maximum of 0.99. The surface albedo was not
significantly higher over the whole cold season (0.175±0.005 and
0.165±0.005) than the warm season (0.178±0.002 and 0.180±0.003)
indicating that the snowfall did not persist on the ground for long at
the sites. This was confirmed by field observations in winter (Jan.
10-15, 2017, Dec. 9-22, 2018, and Dec. 16-25, 2019).