Ice Accumulation and the Apparent Seasonal Variation of GPS Stations in Alaska

Many GPS stations in Alaska have apparent seasonal variations with amplitudes between 5 and 10mm. This motion is usually in phase with regional snowfall and has been attributed to hydrological loading (Fu et al. 2012). We studied the phase of vertical seasonal motion for fifty stations in the PBO network across Alaska and Washington State and found six stations which move two to four months out of phase with snowfall with amplitudes greater than 4mm. The mean date at which stations' seasonal movement reached peak height was October 21 with a standard deviation of 49.7 days. 59% of this variation is created by the six stations with phases furthest from the mean. These stations are also distinguished by discontinuous winter movements, including jumps of more than 10mm/day, and they have the six most asymmetric time-series in the study. Three of these stations, AB11, AB12, and AB14, are local high points on Alaska's west coast. These locations have high wind speeds and humidity and we expect that in freezing conditions they accumulate thick frost and rime. This hypothesis is supported by multipath values at the sites, which show increased signal scattering during the winter. We modelled signal delays for partially ice-covered GPS stations, and predicted that asymmetric horizontal ice growth will cause apparent vertical motion of GPS stations with a magnitude determined by ice thickness and orientation. Rime grows horizontally into the wind, so we estimated rime directions using wind records from nearby airports. We compared these results to our simulation, and predicted upwards apparent motion for the stations that was consistent with the stations' observed winter movement. The apparent vertical seasonal motion of these stations is not caused by loads but is an artefact of signal delay from ice accumulation.