Yushin Ahn

Seasonal variability in the dynamics of marine-terminating outlet glaciers in Greenland

Recent studies indicate that the dynamics of fast-flowing, marine-terminating outlet glaciers of the Greenland ice sheet may be sensitive to climate and ocean forcing on sub-annual timescales. Observations of seasonal behavior of these glaciers at such high temporal resolution, however, are currently few. Here we present observations of front position, flow speed, near-surface air temperature and ocean conditions for six large marine-terminating glaciers in the Uummannaq region of West Greenland, to investigate controls on short-term glacier dynamics. As proposed by other studies, we find that seasonal front advance and retreat correlates with the formation and disappearance of an ice melange. Our data suggest that high sea-surface temperature, anomalously low sea-ice concentration and reduced melange formation in early 2003 have triggered multi-year retreat of several glaciers in the study area, which is consistent with other regions in Greenland. Of the stable glaciers, only Rink Isbrae exhibits a seasonal speed variation that correlates with variations in front position, with the others undergoing mid-summer deceleration that indicates the effects of subglacial meltwater discharge and drainage system evolution. Drainage of supraglacial lakes and water-filled crevasses results in substantial decreases in speed (40-60%) on fast-flowing glaciers. Our results demonstrate that attempts to model ice-sheet evolution must take into account short-timescale flow dynamics resulting from drainage events and oceanographic conditions.

Efficient Automated Glacier Surface Velocity Measurement From Repeat Images Using Multi-Image/Multichip and Null Exclusion Feature Tracking

Observations of ice motion are critical for constraining ice sheet mass balance and contribution to sea level rise, as well as predicting future changes. A wealth of imagery now exists for measuring ice motion from space, but existing repeat-image feature-tracking (RIFT) algorithms require the selection of several location- and data-specific parameters and manual data editing and are therefore not efficient for processing large numbers of image pairs for differing regions. Here, we present the multiple-image/multiple-chip RIFT algorithm which does not involve any user-defined local/empirical parameters and has a higher matching success rate than conventional single-image single-chip correlation matching. We also present an efficient method for applying RIFT to null-value striped data, such as the Landsat-7 Enhanced Thematic Mapper Plus. This method offers the potential for fully automated processing of large data sets.

Oceanic mechanical forcing of a marine-terminating Greenland glacier

Abstract Dynamics of marine-terminating major outlet glaciers are of high interest because of their potential for drawing down large areas of the Greenland ice sheet. We quantify short-term changes in ice flow speed and calving at a major West Greenland glacier and examine their relationship to the presence of the sea-ice melange and tidal stage. A field campaign at the terminus of Store Gletscher (70.40˚N, 50.55˚W) spanning the spring and summer of 2008 included four broadband seismometers, three time-lapse cameras, a tide gauge, an automatic weather station and an on-ice continuous GPS station. Sub-daily fluctuations in speed coincide with two modes of oceanic forcing: (1) the removal of the ice melange from the terminus front and (2) tidal fluctuations contributing to speed increases following ice melange removal. Tidal fluctuations in ice flow speed were observed 16km from the terminus and possibly extend further. Seismic records suggest that periods of intensive calving activity coincide with ice-flow acceleration following breakup of the melange in spring. A synchronous increase in speed at the front and clearing of the melange suggests that the melange directly resists ice flow. We estimate a buttressing stress (~30–60 kPa) due to the presence of the ice melange that is greater than expected from the range of observed tides, though an order of magnitude less than the driving stress.

Glacier velocities from time-lapse photos: technique development and first results from the Extreme Ice Survey (EIS) in Greenland

Automated digital cameras were installed in May-June 2007 beside major West Greenland marine-terminating glaciers as part of the Extreme Ice Survey (EIS). EIS cameras began imaging the lowest 4 km 2 of the glacier at hourly intervals throughout sunlit periods of the year. This study presents the development of techniques for quantifying glacier velocity from a single camera perspective. A Multi-Image/Multi-Chip matching procedure yields higher matching skill than conventional matching, and facilitates false-match rejection via a clustering scheme. The matching of motionless on-land features facilitates compensating camera motion. Ray projection to a known terrain elevation allows the assigning of scale to convert pixel displacements to velocity units. With the 10.2-megapixel camera system, velocities on relatively fast glaciers can be resolved at distances up to -4 km. At a distance of 2 km, a demonstrated precision of ∼0.5 pixels yields a ∼0.5 m footprint size. Daily velocities indicate progressive multi-day velocity accelerations associated with calving. Deceleration trends are associated with the regrowth of resistive stress after major calving. The higher observation frequency available to terrestrial photogrammetry indicates higher observed intra-seasonal velocity range than observable by the at-best weekly satellite snapshots.

Improved Multiple Matching Method for Observing Glacier Motion With Repeat Image Feature Tracking

Repeat image feature tracking is commonly used to measure glacier surface motion from pairs of images, most often utilizing normalized cross correlation (NCC). The multiple-image multiple-chip (MIMC) algorithm successfully employed redundant matching (i.e., repeating the matching process over each area using varying combinations of settings) to increase the matching success rate. Due to the large number of repeat calculations, however, the original MIMC algorithm was slow and still prone to failure in areas of high shearing flow. Here, we present several major updates to the MIMC algorithm that increase both the speed and the matching success rate. First, we include additional redundant measurements by swapping the image order and matching direction; a process we term quadramatching. Second, we utilize a priori ice velocity information to confine the NCC search space through a system we term dynamic linear constraint, which substantially reduces the computation time and increases the rate of successful matches. Additionally, we develop a novel postprocessing algorithm, pseudosmoothing, to determine the most probable displacement. Our tests reveal the complementary and multiplicative nature of these upgrades in their improvement in the overall MIMC performance.