Temporal Variability of Surface Reflectance Supersedes Spatial Resolution in Defining Greenland’s Bare-Ice Albedo

Ice surface albedo is a primary modulator of melt and runoff, yet our understanding of how reflectance varies over time across the Greenland Ice Sheet remains poor. This is due to a disconnect between point or transect scale albedo sampling and the coarser spatial, spectral and/or temporal resolutions of available satellite products. Here, we present time-series of bare-ice surface reflectance data that span a range of length scales, from the 500 m for Moderate Resolution Imaging Spectrometer’s MOD10A1 product, to 10 m for Sentinel-2 imagery, 0.1 m spot measurements from ground-based field spectrometry, and 2.5 cm from uncrewed aerial drone imagery. Our results reveal broad similarities in seasonal patterns in bare-ice reflectance, but further analysis identifies short-term dynamics in reflectance distribution that are unique to each dataset. Using these distributions, we demonstrate that areal mean reflectance is the primary control on local ablation rates, and that the spatial distribution of specific ice types and impurities is secondary. Given the rapid changes in mean reflectance observed in the datasets presented, we propose that albedo parameterizations can be improved by (i) quantitative assessment of the representativeness of time-averaged reflectance data products, and, (ii) using temporally-resolved functions to describe the variability in impurity distribution at daily time-scales. We conclude that the regional melt model performance may not be optimally improved by increased spatial resolution and the incorporation of sub-pixel heterogeneity, but instead, should focus on the temporal dynamics of bare-ice albedo

Recent Advances in Our Understanding of the Role of Meltwater in the Greenland Ice Sheet System

Nienow, Sole and Cowton’s Greenland research has been supported by a number of UK NERC research grants (NER/O/S/2003/00620; NE/F021399/1; NE/H024964/1; NE/K015249/1; NE/K014609/1) and Slater has been supported by a NERC PhD studentshipPurpose of the review:  This review discusses the role that meltwater plays within the Greenland ice sheet system. The ice sheet’s hydrology is important because it affects mass balance through its impact on meltwater runoff processes and ice dynamics. The review considers recent advances in our understanding of the storage and routing of water through the supraglacial, englacial, and subglacial components of the system and their implications for the ice sheet Recent findings:   There have been dramatic increases in surface meltwater generation and runoff since the early 1990s, both due to increased air temperatures and decreasing surface albedo. Processes in the subglacial drainage system have similarities to valley glaciers and in a warming climate, the efficiency of meltwater routing to the ice sheet margin is likely to increase. The behaviour of the subglacial drainage system appears to limit the impact of increased surface melt on annual rates of ice motion, in sections of the ice sheet that terminate on land, while the large volumes of meltwater routed subglacially deliver significant volumes of sediment and nutrients to downstream ecosystems. Summary:  Considerable advances have been made recently in our understanding of Greenland ice sheet hydrology and its wider influences. Nevertheless, critical gaps persist both in our understanding of hydrology-dynamics coupling, notably at tidewater glaciers, and in runoff processes which ensure that projecting Greenland’s future mass balance remains challenging.Publisher PDFPeer reviewe

Integrating historical, geomorphological and sedimentological insights to reconstruct past floods: Insights from Kea Point, Mt. Cook Village, Aotearoa New Zealand

Flood reconstruction is essential for establishing magnitude-frequency relationships and assessments of contemporary geohazards and risks. Traditionally, flood reconstructions rely upon the analysis of evidence acquired from a single discipline. This lack of integration limits the insights into a flood's source, pathway, and receptors (i.e. impacts). Here, our aim is to test the integration of qualitative historical documentary material with quantitative geomorphological and sedimentological evidence to reconstruct glacial lake outburst floods (GLOFs) in 1913 at Kea Point, Mount Cook National Park, Aotearoa New Zealand. Written documentary records show that, following heavy rainfall, GLOF events occurred in January and March, after the temporary impoundment of water between the glacier surface and lateral moraine. Peak flood discharge was estimated from slope-area and exposed boulder measurements as 316–1077 m3s−1 and 496–1622 m3s−1 respectively. Sedimentological information, combined with geomorphic mapping, a DEM derived from Structure from Motion (SfM) photogrammetry, and satellite imagery was used to describe the overall physical impact of the GLOF. Information from written documentary records, however, enabled a more detailed reconstruction of the timeline of the two floods and their impacts proximate to the original ‘Hermitage Hotel’, which was subsequently relocated. Our integrated approach exemplifies the informative level of multi-faceted detail that can be retrieved for historical flood events. We propose a framework for future studies that seek to reconstruct flood events and their source, pathway and receptors through combining evidence from historical documents/artefacts, sedimentological/geomorphological data, and integration with environmental monitoring/modelling outputs