Jökull - 01.01.2019, Side 116

Remote sensing of snow patches on Tröllaskagi Peninsula, N-Iceland
very high (0.5 m) compared to the Landsat images
(30 m). The snow patches around and on the Kerling
rock glacier are well identified, while smaller snow
patches, visible in the orthophoto, are not classified
on the satellite image. The snow patch alongside the
rock glacier is only 6% smaller in the orthophoto than
in the satellite image. The cross-shaped snow patch is
26% smaller in the satellite image, probably because
the thin cloud cover hampers the identification of parts
of the snow patches.
The orthophoto from 2000 only partly covers Búr-
fellsdalur but the complete glacier and its forefield are
visible. Unfortunately, thin clouds are dulling the im-
age therefore some snow patches in the satellite image
are not identified. However, for Búrfellsdalur most
of the snow patches are successfully identified around
the glacier in the satellite image, except the smallest
ones (< 30 x 30 m). Two snow patches are selected
on both images: in the orthophoto the snow patch on
the west side of Búrfellsjökull is 10% smaller and the
snow patch in front of the glacier is 31% smaller com-
pared to the snow patches in the satellite image.
Several field photos are available for intercompar-
ison with the PSP classification based on the satellite
images. These photos are used for comparing and vi-
sual validation of the PSP identification. In 2016 and
2017 intensive field work was conducted in Trölla-
skagi peninsula to study the snow patches in the se-
lected study areas in more detail, such as snow stratig-
raphy and occurrence of ice layers. The layers of
ice were between 10 up to 25 cm thick and on the
bottom of the PSP and found in snow patches in all
study areas. The sizes of selected snow patches were
measured and several snow-pits were dug and anal-
ysed. In 2016 the landscape in available field photos
from Brimnesdalur is partly covered with fog but the
debris-covered glacier and large snow patches were
still identifiable. The area of the snow patches was
found to have decreased by 13% between 9th Septem-
ber 2016 and 14th of October 2016 and 34% between
9th September 2016 and 30th August 2017. However,
the pattern of the snow patch distribution is similar in
2016 and 2017.
In Kerling late summer 2015 and 2016 snow
from previous winters encircled the front of the rock
glacier, but this had melted by the end of summer
2017 (Figure A3), when no snow is identified in front
of the rock glacier. In 2016, the time interval between
the field work photo and the satellite images is al-
most two and a half months and the snow in the lower
part of the photo almost completely melted during this
time. However, if a satellite image closer to the date of
field work is selected (e.g. 08.08.2016) then the snow
patch in front of the glacier is still visible and the pat-
tern in the region is very similar. Between 08.08.2016
and 14.10.2016 about 50% of the snow patches melted
away.
Photographs of the Úlfsdalir valley, taken from the
same location on 31st July 2016, 31st July 2017, and
10th August 2018 (Figure 7), show a substantial de-
crease in snow patches from 2016 until 2018. The
orange circle marks the location of a snow patch, ob-
served in all satellite images from 1986 until 2017.
In 2017 it disappeared but reappeared in 2018 at the
same location.
The comparison of the PSPs classification based
on the satellite images and the field photos is only sat-
isfying if the acquisition date is the same. In the field
the snow patches can be analyzed (e.g. by creating a
snow profile) and used to check if the satellite classi-
fication also works for PSPs situated in the shadow or
covered with a fine layer of dirt.
DISCUSSION
We compared the snow pattern with available temper-
ature, precipitation and wind data from the weather
stations at Öxnadalsheiði and in Ólafsfjörður (Table
A2 – appendix). The warmest year at Öxnadalsheiði
(installed in 2005) was 2016, with a mean annual tem-
perature of 1.8◦C, followed by 2014 and 2017 with
annual mean temperatures of 1.5◦C and 1.4◦C respec-
tively. In Ólafsfjörður a similar pattern was observed:
2014 (4.8◦C), 2016 (4.2◦C), and 2017, (4.3◦C). The
coldest years in Ólafsfjörður were 1997, 1998 and
1999. In Brimnesdalur and Kerling a large amount of
snow patches were identified from 1999 until 2002. In
Grímsey the warmest years, (0.5–1.0◦C), were 2014,
2016 and 2017.
Precipitation in Ólafsfjörður has been automati-
cally measured at hourly intervals since 1997 (Ta-
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