Multisensor validation of tidewater glacier flow fields derived from synthetic aperture radar (SAR) intensity tracking

Abstract

Abstract. Following the general warming trend in Greenland, an increase in calving rates, retreat and ice flow has been observed at ocean-terminating outlet glaciers. These changes contribute substantially to the current mass loss of the Greenland Ice Sheet. In order to constrain models of ice dynamics as well as estimates of mass change, detailed knowledge of geometry and ice flow is needed, in particular on the rapidly changing tongues of ocean-terminating outlet glaciers. In this study, we validate velocity estimates and spatial patterns close to the calving terminus of such an outlet derived from an iterative offset-tracking method based on synthetic aperture radar (SAR) intensity data with a collection of three independent reference measurements of glacier flow. These reference datasets are comprised of measurements from differential GPS, a terrestrial radar interferometer (TRI) and repeated unmanned aerial vehicle (UAV) surveys. Our approach to SAR velocity processing aims at achieving a relatively fine grid spacing and a high temporal resolution in order to best resolve the steep velocity gradients in the terminus area and aims to exploit the 12 d repeat interval of the single-satellite Sentinel-1A sensor. Results from images of the medium-sized ocean-terminating outlet glacier Eqip Sermia acquired by Sentinel-1A and RADARSAT-2 exhibit a mean difference of 11.5 % when compared to the corresponding GPS measurements. An areal comparison of our SAR velocity fields with independently generated velocity maps from TRI and UAV surveys showed good agreement in magnitude and spatial patterns, with mean differences smaller than 0.7 m d−1. In comparison with existing operational velocity products, our SAR-derived velocities show an improved spatial velocity pattern near the margins and calving front. There 8 % to 30 % higher surface ice velocities are produced, which has implications on ice fluxes and on mass budget estimates of similarly sized outlet glaciers. Further, we show that offset tracking from SAR intensity data at relatively low spatio-temporal sampling intervals is a valid method to derive glacier flow fields for fast-flowing glacier termini of outlet glaciers and, given the repeat period of 12 d of the Sentinel-1A sensor (6 d with Sentinel-1B), has the potential to be applied operationally in a quasi-continuous mode.

Calving event size measurements and statistics of Eqip Sermia, Greenland, from terrestrial radar interferometry

“…5a. Figure 5b shows the complete velocity field including the areas of radar line-of-sight shadow, which has been derived from repeated unmanned aerial vehicle (UAV) surveys from August 2016 (Rohner et al, 2019). Speeds increase towards the calving front, with the highest values reaching 16 m d −1 .…”

“…Terrestrial laser scanning allows the measurement of the volume of individual calving events (Pętlicki and Kinnard, 2016), but it requires suitable meteorological conditions and lacks the temporal resolution to detect individual calving events. Terrestrial radar interferometers can overcome most of the mentioned limitations and have been used to study the effects of tidal forcing on the front of an outlet glacier (Voytenko et al, 2015); to investigate calving rate and velocity (Rolstad and Norland, 2009); to determine calving event frequency (Chapuis et al, 2010), velocity variations and grounding line motion (Xie et al, 2018), pro-glacial mélange thickness (Xie et al, 2019), and glacier response to calving (Cassotto et al, 2018); or to estimate the volume of a single large calving event .…”

Applications of Unmanned Aerial Vehicles in Cryosphere: Latest Advances and Prospects

“…For multitemporal studies, illumination differences between data collection times will, understandably, effect optical band reflectance values. To overcome this issue, Rohner et al [104] used DSMs to derive shaded reliefs rather than reflectance for inputs to Pix4D's image matching algorithm, which minimized matching errors for data collected on separate days.…”

“…In a study done by Rohner et al [104], a SenseFly eBee with an RGB camera flown in four day increments was able to capture velocity flows of a marine-terminating glacier in Greenland. Compared to satellite-derived velocity fields, the UAV was able to depict much smoother flows and resolve the acceleration towards the terminus, where speeds of 12 m/day were reached.…”

“…The position of the UAV had an advantage over the terrestrial method as the latter produced discontinuous results due to topographic-induced shadows. Rohner et al [104] considered the UAV data as in situ truth to validate satellite estimations, demonstrating a shift in the literature from a narrative of if UAVs can be useful or accurate, to assuming so.…”