Apparent Short-Term Glacier Velocity Variations

Abstract

In connection with a glacier-hydrological project at a sub-polar glacier in West Greenland, short-term glacier velocity variations were measured. Both the horizontal and the vertical velocity components showed distinct diurnal variations. Close examination indicates that these variations are caused by the change in atmospheric refraction during the day, with the vertical component as the most important.

Seasonal Surface-Velocity Variations on a Sub-Polar Glacier in West Greenland

“…It is known that atmospheric refraction varies systematically throughout the day, and Andreasen (1985) found that apparent regular vertical displacements through the day or for a few days were the result of variations in the refraction. In a similar way, the apparent discrepancy between the “uplift” at stake 950 determined from A s and B s , respectively, and the apparent uplift of the fixed point F is explained as a result of changes in atmospheric refraction throughout the period.…”

“…When readings to fixed points were taken during the summer, at the same time as readings to stakes, the accuracy of vertical angles was determined as ±0.0008 g ( n = 33) and ±0.0015 g ( n = 26) for the fixed points on the same side (B s ) and on the opposite side (F) of the glacier, respectively. The discrepancy between the accuracy at a given time and the accuracy during the summer is interpreted as a result of changes in refraction during the longer period, and the greater standard deviation for the measurements to F as a result of greater refraction over snow and ice surfaces (Andreasen, 1985). The standard deviation of horizontal angle measurements was determined using the fixed angle between the two points B s and F. Readings were taken during the summer, at the same time as readings to stakes.…”

“…The discovery of vertical velocity variations has led to the development of a new theory of basal sliding (Iken, 1981) and to calculations of glacier-bed geometry (Hooke and others, 1983; Iken and others, 1983). However, some apparent short-term velocity variations may be due to the effect of variations in atmospheric refraction (Andreasen, 1985).…”

“…As part of a glacier-hydrological investigation at a sub-polar glacier (“Kitdlerssuaq glacier”) at Tasersiaq in West Greenland, measurements of glacier-velocity variations were carried out. An investigation of short-term (12–24 h) glacier-velocity variations on “Kitdlerssuaq glacier” in 1982 showed that both horizontal and vertical variations were probably the result of variations in atmospheric refraction (Andreasen, 1985); consequently, only weekly variations were determined in 1983.…”