Seasonal Variations in Surface Velocity, Storglaciären, Sweden

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Abstract
In order to study short-term variations in velocity and strain rate in the upper part of the ablation area of Storglaciären, twenty stakes, set in a 375 × 500 m rectangular grid, were surveyed at approximately 10-day intervals during the summers of 1981 and 1982 and at approximately 45 day intervals during the 1981–82 winter. The mean horizontal velocity over the net, U, was about 30 mm·d-1, during the winter. U increased during periods of warm weather in the spring and summer, reaching a maximum value of 43 mm·d-1 during the summer of 1981. The mean vertical velocity, W, was 2 mm·d-1 during the winter, and increased to a maximum of 11 mm·d-1 during the summer of 1981. The vertical velocity at the bed. Wc, was determined by subtracting the velocity due to vertical strain and that due to flowup an adverse bed slope from the measured vertical velocity at the surface. The size of cavities at the bed was then determined by integrating Wc over time. The cavity height, averaged over the area of the bed beneath the net, reached maxima of 270 mm in August 1981 and 180 mm in August 1982. Increases in W and Wc lagged those in U by about 2 weeks, and U began to decrease when W and Wc began to increase. Thus the maximum cavity size occurred in late August or early September, when U had decreased nearly to its winter value. These observations suggest that changes in U are largely due to changes in longitudinal strain rate resulting from changes in velocity of the lower part of the glacier. Cavities closed at linear rate over the winter, resulting in a slight decrease in U.