Seasonal evolution of Narsap Sermia, SW Greenland, using time lapse imagery and high-resolution satellite radar data.

Greenlands tidewater glaciers (TWG) have been retreating since the mid-1990s, contributing to mass loss from the Greenland Ice Sheet and sea level rise. Optical and radar satellite imagery has been widely used to investigate TWG velocities and to determine the response of TWG calving behaviour to climate change. However, medium resolution data and multi-day revisit times make it difficult to determine short-term processes such as calving and detailed, shorter-term velocity changes that may condition this process. Here we present velocity and iceberg calving data for Narsap Sermia, SW Greenland, from time lapse imagery for the period July 2017 to June 2020 accompanied by ~1m spatial resolution satellite aperture radar (SAR) data for the period October 2019 to January 2021. The velocity data is compared to calving and proglacial plume data derived from hourly time-lapse images (n=13,513) for the period July 2017 to June 2020. Raw time-lapse images were orthorectified using the ImGRAFT package (Messerli & Grinsted, 2015) and based on a smoothed ArcticDEM tile from 2017 (RMSE=44.4px). The high temporal resolution of the imagery enabled the manual mapping of proglacial plume sizes from the orthorectified images and the recording of individual calving events. SAR satellite data was processed using the ISCE software (Rosen et al., 2012) to form time separated image pairs from which to obtain ice surface velocities (posting of ~16 metres) along azimuth and range directions. The velocity data shows annual spring acceleration followed by a summer slow-down to a distinct minimum and subsequent winter recovery for the years 2017 - 2019. However, velocities in winter 2020/21 exceed those of winter 2019/20 and those of spring acceleration in 2019, which has not been observed in previous years. The number of calving events is relatively constant on an annual basis, but the area of ice calved increases substantially between 2017 and 2020. Plume sizes also remain relatively consistent in size. However, later in the record plumes appear earlier in the year, coinciding with more days of open water at the ice front. By combining time lapse and high-resolution SAR imagery we are able to relate short-term changes in velocities to calving and climate forcings, thus enhancing our understanding of TWG behaviour under climate change.