Jökull - 01.07.2003, Page 42

Achim A. Baylich
Figure 5. Mean number of freeze-thaw days per month and their minimum temperatures, in Latnjajaure. –
Meðalfjöldi frost-þíðu daga og lágmarkshitastig þeirra, í hverjum mánuði, í Latnjajaure.
ness of the silt-rich regoliths daily freeze-thaw events
cause creep movements with single stone movements
on vegetation-free talus cones. In June and Septem-
ber minimum air temperatures will usually not be be-
low -5ÆC, with the daily freeze-thaw cycles and creep
processes being limited to near-surface substrates. In
May daily freeze-thaw events with higher frost in-
tensities also occur. Field observations showed that
daily freeze-thaw cyles with minimum air tempera-
tures around -10ÆC can trigger rockfalls from snow
free but wet rockwalls and rock ledges. Rockfall ac-
tivity is caused by both the annual freeze-thaw cycle
and daily cycles and reaches its highest intensity in
May and June. Avalanches occurring mainly in May
and June on the east-facing slope are very important
for triggering rockfalls and also boulder falls at this
slope. On slopes without avalanche activity boulder
falls were only observed in July and August and were
mainly caused by melting of segregation ice devel-
oped deeper within rockwalls and rock ledges. These
boulder falls are mainly caused by the annual freeze-
thaw cycle.
Information on frequencies and intensities of frost
events to be expected at Latnjajaure in different
months is presented by magnitude-frequency analy-
ses (Ahnert 1986; 1987; 1998) of daily air temper-
ature minimum below -15ÆC, as shown in Figure 6.
Most severe frost intensities occur during the winter
frost phase between December and March, and par-
ticularly in February. The long-lasting winter frost
phase with its high frost intensities and the consid-
erable aeolian redistribution of snow are the reasons
that, in some areas, frost penetrates deeply into the
regolith and rocks, and that permafrost exists at least
sporadically in the area, despite relatively high winter
precipitation (Kling 1996; 2003; Beylich et al. 2003).
Larger areas, especially on the gentle, W-facing slope
of the valley, are characterized by solifluction lobes
and sheets.
Due to the characteristics of the temperature
40 JÖKULL No. 52, 2003