Analysis of time series of glacier speed: Columbia Glacier, Alaska
During the summers of 1984 and 1985, laser measurements were made of the distance from a reference location to markers on the surface of the lower reach of Columbia Glacier, Alaska. The data have numerous gaps, mostly because of inclement weather. The laser measurements were corrected for variations in atmospheric temperature and pressure and for the trajectory of the marker. The marker speed was calculated by using a combination of cubic splines and digital filters to arrive at a data set with 1-hour time intervals. The speed varies from 7 to 15 m/d and has three noteworthy components: (1) a low-frequency perturbation in speed with a time scale of days related to increased precipitation, (2) semidiurnal and diurnal variations related to sea tides, and (3) diurnal variations related to glacier surface melt. The low frequency and tidal period variations are separated by low-pass filtering and subtracting the filtered data from the original data. The variations from melt runoff and tides have the same frequency range and are examined using harmonic analysis and multiple regression with the various forcing functions. The high-frequency portion of the ice speed signal is dominated by tidal influences and to a lesser extent by meltwater influences. The low-frequency portion of the signal is dominated by the effects of precipitation.