Roughish ice, snow and turbulent heat fluxes on the Greenland ice sheet

Abstract

The mass loss of Earth’s land ice has caused 114 mm of global sea-level rise between 1901-2018, or more than half of the total measured sea-level rise (202 mm) in that period. The Greenland ice sheet and its peripheral glaciers contribute 40 mm to this sea-level rise, and are expected to continue to lose mass during the next century. Future projections of the mass loss of grounded ice, including the Greenland ice sheet require robust projections of both surface, basal and calving processes. Climate models are often used to calculate the surface mass balance of the ice sheet, defined as the precipitation minus the ablation at the surface. By far most of the ablation originates from the runoff of meltwater in the low-lying ablation zone, which covers only 10-16% of the ice sheet’s surface. In the ablation zone, surface melt is mainly driven by the absorption of solar radiation and by the sensible heat flux (SHF). The latter is defined as the vertical turbulent exchange of sensible heat between the surface and the overlying atmosphere. This turbulent heat flux explains most of the surface melt variability, and can become the major source of energy during extreme melt events. However, very few direct observations of turbulent fluxes exist on the ice sheet, which makes the simulation of these fluxes in climate models uncertain. This thesis aims to better quantify and therewith reduce the uncertainty of modelling the SHF across the Greenland ice sheet. In chapter 3, we address the issue of measuring the SHF on the ice sheet. A vertical propeller eddy covariance (VPEC) method is developed for this purpose. The large attenuation of the turbulent flux due to the large response times of propeller anemometers is accurately modelled, since the response times and the spectral characteristics of the turbulent fluxes are known. In chapter 4, we develop a novel method to map the surface aerodynamic roughness using either uncrewed aerial vehicles (UAV) or satellite laser altimetry (ICESat-2). A simplified model for obstacle height and drag is able to reproduce the in situ measurements. We develop a first map of surface aerodynamic roughness of the ice sheet which quantifies by how much the surface roughness decreases with elevation. In chapter 5, a model for the surface aerodynamic roughness is developed. A realistic description of the variation in the height of ice hummocks is found to yield a more accurate SHF, and therefore more accurate surface melt. In chapter 6, we build a database of surface energy balance fluxes from 25 automatic weather stations on the ice sheet, and we compare these measurements to the regional climate model RACMO2.3p2 with the updated roughness parameterizations from the previous chapter. We continue to monitor ice sheet surface processes with weather stations, eddy covariance and surface mass balance observations. Together with remote sensing observations, these can be used as benchmarks for the development of future models. The improvement of climate models allows for more accurate simulations of future ice sheet mass loss.