TEMPORAL VARIABILITY OF ICE FLOW ON HOFSJÖKULL, ICELAND, OBSERVED BY ERS SAR INTERFEROMETRY

Florian Müller(1), Helmut Rott(2)

(1) ENVEO IT, Environmental Earth Observation GmbH, Technikerstrasse 21a, A-6020 Innsbruck, AUSTRIA (2) Institute of Meteorology and Geophysics, University of Innsbruck, Innrain 52, A-6020 Innsbruck, AUSTRIA

ABSTRACT Hofsjökull is the second largest ice cap in Iceland with five known surge type outlet glaciers. Ice motion studies were carried out on the outlet glaciers Þjorsarjökull and Múlajökull using InSAR techniques. Because of the rapidly changing meteorological conditions and fast ice flow, only short repeat intervals, as provided by ERS 1/2 tandem SAR images, provide coherence suitable for InSAR analysis. In winter 1995/96 Þjorsarjökull was in the final stage of a surge that had started in 1994. The surface motion of the lower part of the terminus decreased from 0.45 m/day in October 1995 to 0.05 m/day in January 1997. On the other hand, three ice flows, descending from the accumulation to the ablation area, accelerated by more than 50 % from October 1995 to March 1996, and slowed down later on. Furthermore significant motion increase of Múlajökull was observed during winter 1995/96. In an area at about 1000 m a.s.l., just below the equilibrium line, an increase of surface velocity of about 80 % was observed between October 1995 and January 1996.

1 INTRODUCTION Hofsjökull is located in Central Iceland at the active volcanic zone of the North Atlantic ridge. The ice cap covers an area of 923 km² and has a volume of 208 km³. The highest part forms an approximately circular plateau at about 1800 m a.s.l., above a large central volcano with a 250 km² base [1]. For Hofsjökull five surge type outlets are known [2]. Fig. 1 gives a topographic overview of Hofsjökull and its surrounding area.

Figure 1. Sketch map of Hofsjökull with investigated surge type outlet glaciers Múlajökull (M) and Þjorsarjökull (P). ERS-SAR acquisition geometries for ascending/descending orbits are indicated by arrows. Glacier boundary is represented by a red line. Elevation contour line interval is 100 m.

Þjorsarjökull is located on the eastern side of Hofsjökull ice cap, north of Múlajökull. The ice flows towards south-east, and the lower part is characterized by a relatively flat ice surface with an average slope of about 4 degrees. The last known surges occurred in 1991 in the southern section, and 1994 in the central section of the outlet. During the surge in 1994, an estimated area of 35 to 100 km² was affected and an advance of the terminus of 200 to 300 m was observed [2]. In Section 3.1 the final stage of this surge is discussed by means of InSAR ice motion analysis. Múlajökull, a southern outlet of Hofsjökull, is a surge-type glacier which surges frequently with a period of about 6 to 12 years. It covers an area of about 94 km², the elevation ranges from 610 to 1800 m a.s.l. The last observed surge occurred in 1992 [2] and is not covered by the ERS tandem data set, but surface motion variations could be observed during winter 1995/96 and are presented in Section 3.2.

2 METHOD For ice motion analysis of Hofsjökull by means of InSAR 42 ERS-1/2 tandem scenes were used. Significant deviations from the steady state ice flow assumption required an external digital elevation model (DEM) for estimation of the interferometric motion phase. Long time intervals between ascending and descending tandem pairs prohibited the combination of interferograms from different look directions. Therefore the surface velocity was estimated from single interferograms with additional information on flow direction. The surface parallel ice flow assumption, which is often applied to estimate the ice flow vector, yields in some areas poor results, for example on the relatively flat ablation area of Þjorsarjökull. Therefore a method to derive the ice flow directions from manually analyzed flow lines in Landsat ETM+ images was applied. This was only possible in ablation areas. In accumulation areas, where no distinct flow lines are visible, the surface parallel flow assumption was applied to estimate surface motion. For this purpose we estimated the flow direction from the maximum slope angle of the low-pass filtered DEM. On non-stationary glaciers this assumption may introduce errors [3]. However, also in these cases the analysis of InSAR derived velocities at different dates along flow lines and transverse profiles enables good characterization of the temporal behaviour of ice flow.

Figure 2. Motion only interferogram of Hofsjökull, 28/29 November 1995 (frame 2295, track 324, descending). One fringe represents 2.8 cm displacement in line of sight direction (LOS). P – Þjorsarjökull (magenta), M – Múlajökull (green). Elevation contour line interval is 25 m. Image is in slant range geometry.

LOOK

FLIGHT

3 ANALYSIS OF ICE MOTION Several maps of ice motion were generated between October 1995 and February 1999 [4]. An example of a wrapped motion phase image covering nearly the entire Hofsjökull is given in Fig. 2. This descending interferogram (frame 2295, track 324), acquired on 28/29 November 1995, has a perpendicular baseline of 37 m and comparatively high coherence, good conditions for ice motion analysis. An elevation error in the DEM of about 270 m would yield only one additional fringe in the motion interferogram. Open fringes on the glacier front of Þjorsarjökull indicate an advance or uplift of the ice body. A maximum of about 4 fringes is observed on the central part of Þjorsarjökull terminus. The catchments of Þjorsarjökull and Múlajökull are well defined, due to the alignment of ice flow approximately in ground range of the SAR.

3.1 Þjorsarjökull The analyzed scenes cover the time between 7/8 October 1995, one year after surge onset, and 27/28 February 1999. The focus of the analysis is on ascending frame 1305/ track 044 and descending frame 2295/track 281. A time series of surface motion maps is shown in Fig. 3. Three main ice flows, labeled by IF1, IF2, and IF3, descending from the accumulation to the ablation area of Þjorsarjökull can be identified. In October 1995 (Fig. 3(a)) a high velocity zone is apparent between the glacier front and an elevation of 800 m a.s.l. in the central part of Þjorsarjökull. An area of about 20 km² is affected. The maximum surface velocity exceeds slightly 0.5 m/day. The fastest ice flow is IF2 with a maximum of about 0.6 m/day, which is not exactly retrievable because the ice flow direction deviates significantly from SAR ground range. Two months later (Fig. 3(b)) the high motion area at low elevations slowed down to velocities below 0.2 m/day, only in a few spots faster motions of up to 0.3 m/day are apparent. The ice flows IF1 to IF3 show a velocity increase of about 0.05 to 0.1 m/day over large areas compared to October 1995. In contrast, the area near the terminus slows down further, to velocities below 0.15 m/day in March 1996 (Fig. 3(c)). This results in a deceleration of 0.45 m/day at the maximum compared to October 1995. In the surface velocity map of March 1996 (track 044, frame 1305) the uncertainty of the motion estimate is higher, because the SAR scenes have a different acquisition geometry with less favorable angles compared to motion maps in October and December 1995 (track 281, frame 2295). Additional motion maps (not shown here), generated from different frames/tracks, confirm this phenomenon of accelerating inflows during winter 1995/96 [4]. During the next three years the ice velocity slowed down in the ablation area of Þjorsarjökull. The extent of stagnant areas increased at elevations below 1000 m a.s.l. to a maximum of about 15 to 20 km². After 1996 the velocities of the ice flows IF1 to IF3 slowed down to values comparable to those of October 1995, and the ice flows covered smaller areas. The ice motion pattern seems to be stable between 1997 and 1999, no significant changes can be detected. The motion behaviour of the northern section of Þjorsarjökull seems to have little connection with the changing motion pattern on the central section. A bedrock ridge between these parts may explain this effect.

Figure 3. Maps of magnitude of surface motion at Þjorsarjökull, geocoded in UTM projection. Black areas are masked out due to estimated velocity errors greater than +/- 20 %. The broken line in Fig. 3(b) shows the position of transverse profile T1.

(b) 30/31 December 1995 (c) 27/28 March 1996 (a) 21/22 October 1995. IF1, IF2 and IF3 are the main ice flows to the ablation zone of Þjorsarjökull.

For better interpretation of the velocity values and their temporal variability, velocity data along a transverse profile are plotted in Fig. 4. Transverse profile 1 (T1) has on overall length of 17.8 km and crosses all main ice flows into the ablation area of Þjorsarjökull (Fig. 3(b)). All three ice flows exhibit similar temporal behaviour. In October 1995 the velocity of ice flow IF1, located between 1 and 5 km of T1, has an average surface velocity value of 0.13 m/day. Towards northeast, on ice flow IF2, velocities between 5 and 9 km range from 0.15 to 0.3 m/day, on ice flow IF3, between 10 and 13 km, they range from 0.15 to 0.25 m/day. All ice flows accelerate until 30/31 December 1995 and 27/28 March 1996. For example, the maximum velocity on ice flow IF1 increased from 0.15 m/day in October 1995 to 0.28 m/day in March 1996. In December 1995 a slight acceleration compared to October is apparent as well, but with the mean velocity being higher only by about 0.03 m/day. Later on, an interferogram is available from January 1999, showing the surface motion along transverse profile T1 reduced by about 50 % compared to March 1966. On IF2 the maximum surface motion increases from 0.3 m/day in October 1995 to nearly 0.4 m/day in March 1996. Moreover, at 8.5 km in T1 an additional peak with a velocity about two times higher than on other dates is apparent in March 1996. The northeastern section of Þjorsarjökull, located between 15 and 17 km in Fig. 4, is relatively unaffected by the rapidly changing motion pattern in the central and southern part of the outlet glacier, but nevertheless shows an overall velocity variation of about 30 % in the analyzed data set.

Figure 4. Plot of magnitude of surface velocity along transverse profile 1 (see Fig.2). IF1, IF2, and IF3 are the main ice flows from the accumulation to the ablation area of Þjorsarjökull.

3.2 Múlajökull Múlajökull shows sufficient coherence for detailed InSAR analysis in one ascending and five descending inter-ferograms acquired between October 1995 and January 1996. Parts of the available time series of surface motion maps are shown in Fig. 5. Area A1 (Fig. 5(b)), located near the equilibrium line at about 1000 m a.s.l. where the ice flow converges to a channel of about 2.5 km width, exhibits the most significant velocity increase during winter 1995/96. In October 1995 (Fig. 5(a)) the average surface motion at A1 is below 0.2 m/day, increasing to 0.35 m/day one month later (Fig. 5(b)). During the following month the ice flow accelerated further, showing a velocity maximum of about 0.35 m/day in January 1996 (Fig. 5(c)). Since October 1995 the overall velocity increase amounted to about 0.15 m/day. This is a maximum surface motion increase of about 80 %. The most significant acceleration occurred between 21/22 October 1995 and 28/29 November 1995. On the terminus of Múlajökull below area A1 the observed acceleration is weaker than in A1. Maxi-mum velocities of 0.3 m/day are observed there in January 1996, increasing from about 0.13 m/day since October 1995.

Figure 5. Maps of magnitude of surface motion of Múlajökull, geocoded in UTM projection. Black areas are masked out due to estimated velocity errors greater than +/- 20 %.

For better intercomparison of different dates, surface velocity data are plotted along a flow line (Fig. 6). Flow line F1 is about 8 km long and located between the center of Múlajökull and the eastern glacier boundary (F1, Fig. 5(a)). In the upper part, 7 to 8 km above the front, some gaps are apparent, which result from the defined velocity error bounds. The acceleration between October 1995 and January 1996 shows up as persistent velocity increase along the entire flow line F1. The motion offset from one interferogram to the next reaches up to 5 cm/day. In October 1995 the maximum at 5 km is about 0.18 m/day, in January 1996 the motion reaches 0.28 m/day. From 21/22 October 1995 to 02/03 January 1996 an average surface motion increase by 72% along flow line F1 is observed.

Figure 6. Profile of magnitude of surface velocity along flow line F1. Gaps – no data.

4 CONCLUSIONS InSAR analysis reveals significant spatial and temporal variability of ice motion on Hofsjökull. The results should be of great interest for assessing and modelling the ice dynamic properties of Þjorsarjökull and Múlajökull. In addition, the observations point out that single term measurements of ice motion are not able to properly describe the dynamic behaviour of these glaciers.

(b) 28/29 November 1995 (c) 02/03 January 1996 (a) 21/22 October 1995

Of particular interest is the acceleration of ice flow in upper areas of Þjorsarjökull, one year after surge onset, that coincides with post-surge slowing down on the terminus. This effect differs from typical surge behaviour, as, for example, observed by InSAR on Sylgjujökull and Dyngjujökull of Vatnajökull [5]. The behaviour of Múlajökull, with acceleration of ice flow during winter, is opposed to the expectation of faster ice flow during summer due to presence of more water in the glacier. The observed temporal velocity variations are a clear indication that the glaciers were not in steady state during the observation period. This points out that deviations from the surface parallel flow may be significant [3]. For this reason the magnitudes of the retrieved surface velocity may locally reveal some errors. However, the velocity variations are of such magnitude and the patterns of variations show up also clearly in the wrapped motion phase images, that possible errors due to non-surface parallel motion components would not change the major findings.

ACKNOWLEDGEMENTS The satellite images were made available by ESA through the projects AO3.108 (VECTRA) and AO3.239. The digital elevation model of Hofsjökull was kindly provided by the Icelandic Science Institute (H. Björnsson, F. Palssón).

REFERENCES 1. Björnsson H., Surface and bedrock topography of ice caps in Iceland, mapped by radio echo sounding, Ann. Glaciol., Vol. 8, 11-18, 1986. 2. Björnsson H., et al., Surges of glaciers in Iceland, Ann. Glaciol., Vol. 36, 82-90, 2003. 3. Mohr J., et al., Accuracy of three-dimensional glacier surface velocities derived from radar interferometry and ice-sounding radar measurements., J. Glaciol., 49(165), 210-222, 2003. 4. Müller F., Ice motion analysis of Hofsjökull, Iceland, by interferometric SAR, Diploma Thesis, Institute of Meteorology and Geophysics, University of Innsbruck, Austria, 2005. 5. Fischer A., Rott H., Björnsson H., Observation of recent surges of Vatnajökull, Iceland, by means of ERS SAR interferometry, Ann. Glaciol., Vol. 37, 69-76, 2003.