Observations of seasonal and diurnal glacier velocities at Mount Rainier, Washington, using terrestrial radar interferometry

We present surface velocity maps derived from repeat terrestrial radar interferometry (TRI) measurements and use these time series to examine seasonal and diurnal dynamics of alpine glaciers at Mount Rainier, Washington. We show that the Nisqually and Emmons glaciers have small slope-parallel velocities near the summit (< 0.2 m day(-1)), high velocities over their upper and central regions (1.0-1.5 m day(-1) 1), and stagnant debris-covered regions near the terminus (< 0.05 m day(-1)). Velocity uncertainties are as low as +/- 0 :02-0.08 m day(-1). We document a large seasonal velocity decrease of 0.2-0.7m day(-1) (25 to 50 %) from July to November for most of the Nisqually Glacier, excluding the icefall, suggesting significant seasonal subglacial water storage under most of the glacier. We did not detect diurnal variability above the noise level. Simple 2-D ice flow modeling using TRI velocities suggests that sliding accounts for 91 and 99% of the July velocity field for the Emmons and Nisqually glaciers with possible ranges of 60-97 and 9399.5 %, respectively, when considering model uncertainty. We validate our observations against recent in situ velocity measurements and examine the long-term evolution of Nisqually Glacier dynamics through comparisons with historical velocity data. This study shows that repeat TRI measurements with > 10 km range can be used to investigate spatial and temporal variability of alpine glacier dynamics over large areas, including hazardous and inaccessible areas.