|Title||Snow accumulation variability on a West Antarctic ice stream observed with GPS reflectometry, 2007-2017|
|Publication Type||Journal Article|
|Year of Publication||2017|
|Authors||Siegfried M.R, Medley B., Larson K.M, Fricker H.A, Tulaczyk S.|
|Journal||Geophysical Research Letters|
Land ice loss from Antarctica is a significant and accelerating contribution to global sea level rise; however, Antarctic mass balance estimates are complicated by insufficient knowledge of surface mass balance processes such as snow accumulation. Snow accumulation is challenging to observe on a continental scale and in situ data are sparse, so we largely rely on estimates from atmospheric models. Here we employ a novel technique, GPS interferometric reflectometry (GPS-IR), to measure upper (<2 m) firn column thickness changes across a 23-station GPS array in West Antarctica. We compare the results with antenna heights measured in situ to establish the method's daily uncertainty (0.06 m) and with output from two atmospheric reanalysis products to categorize spatial and temporal variability of net snow accumulation. GPS-IR is an effective technique for monitoring surface mass balance processes that can be applied to both historic GPS data sets and future experiments to provide critical in situ observations of processes driving surface height evolution.
We applied GPS-IR methods at 42 stations across the Whillans and Mercer ice streams, West Antarctica, to retrieve estimates of net snow accumulation, and validated the retrievals with in situ field measurements. We demonstrated that GPS-IR surface height estimates in the region are accurate to 0.02 m and precise to 0.06 m. We then used our GPS-IR observations to investigate spatial and temporal variability in snow accumulation at our study site and compared our results to past results and two widely used atmospheric reanalysis products. Our GPS-IR observations indicated that both reanalysis products effectively capture subannual structure of accumulation, but both have persistent absolute biases and are likely underestimating interannual variability.
The success of the GPS-IR technique and the relative ease of GPS deployments (in terms of both cost and logistics) imply that future experiments can be readily developed and executed to isolate individual processes that have been difficult or prohibitively expensive to previously assess with field measurements, such as wind scour and deposition, firn compaction, and time variability of volume scattering of satellite radar altimeters. While our study site consists of a simple geometry for the GPS-IR method (i.e., flat), it is representative of a large portion of Antarctica, especially the East Antarctic plateau, where cm-level precision of surface height is required for robust ice sheet mass balance estimates.
This study is the first attempt at a detailed in situ validation of GPS-IR surface height change measurements on an ice sheet; only one previous qualitative comparison of this type was performed with an ultrasonic snow logger at a single location in Greenland, but no quantitative results were given [Larson et al., 2015]. Our results demonstrate that GPS-IR is an effective method for determining surface height change over the Antarctic ice sheet at daily resolution with cm-level precision that can yield unique and significant insights into ice sheet surface processes. This implies that historic GPS records acquired over the past two decades from Greenland and Antarctica could be revisited, and future experiments should be designed with this application in mind.