Mapping the aerodynamic roughness of the Greenland ice sheet surface using ICESat-2: Evaluation over the K-transect

Abstract

Abstract. The aerodynamic roughness of heat, moisture and momentum of a natural surface is an important parameter in atmospheric models, as it co-determines the intensity of turbulent transfer between the atmosphere and the surface. Unfortunately this parameter is often poorly known, especially in remote areas where neither high-resolution elevation models nor eddy-covariance measurements are available. In this study we adapt a bulk drag partitioning model to estimate the aerodynamic roughness length (z0m) such that it can be applied to 1D (i.e. unidirectional) elevation profiles, typically measured by laser altimeters. We apply the model to a rough ice surface on the K-transect (western Greenland ice sheet) using UAV photogrammetry, and evaluate the modelled roughness against in situ eddy-covariance observations. We then present a method to estimate the topography at 1 m horizontal resolution using the ICESat-2 satellite laser altimeter, and demonstrate the high precision of the satellite elevation profiles against UAV photogrammetry. The currently available satellite profiles are used to map the aerodynamic roughness during different time periods along the K-transect, that is compared to an extensive dataset of in situ observations. We find a considerable spatiotemporal variability in z0m, ranging between 10−4 m for a smooth snow surface over 10−1 m for rough crevassed areas, which confirms the need to incorporate a variable aerodynamic roughness in atmospheric models over ice sheets.

Impact of updated radiative transfer scheme in snow and ice in RACMO2.3p3 on the surface mass and energy budget of the Greenland ice sheet

“…Four modifications have been applied to the multilayer firn module, which we briefly address here and are discussed in more detail in Van Dalum et al (2020a). Firstly, Rp2 uses a prognostic fresh snow layer, which is effectively a sublayer of the uppermost snow layers.…”

“…In Rp3, superimposed ice is therefore treated as a snow layer, with a minimum grain radius of 0.720 mm for a layer density of 750 kg m −3 , and the grain radius increases linearly to the bare-ice value of 4.152 mm for a density of 917 kg m −3 . A more detailed model description, evaluation, and discussion of the snow and ice albedo product can be found in Van Dalum et al (2020a).…”

“…Statistically downscaling the SMB components to an even higher resolution of up to 1 km (Noël et al, 2019) solves many such issues. Still, the rough topography typically found close to the ice margin is often not modeled correctly and can lead to incorrect precipitation patterns, which illustrates the need to further improve SMB-related parameterizations (Van de Berg et al, 2020).…”

“…RACMO2 has been used extensively to model large-scale, ice-sheet-wide developments of the Greenland (Noël et al, 2018b;Noël et al, 2019) and Antarctic ice sheets (Van de Berg and Medley, 2016;Van Wessem et al, 2018) but also for glaciated areas on a smaller scale, like the glaciers and ice caps of the Canadian Arctic (Noël et al, 2018a). Furthermore, RACMO2 has been used to investigate physical processes like the snowmelt-albedo feedback (Jakobs et al, 2019) and föhn winds (Wiesenekker et al, 2018).…”

“…Recent improvements in snow albedo parameterizations and radiation transfer schemes now allow the implementation of a radiation penetration module in RACMO2 ( Van Dalum et al, 2019). The new version RACMO2.3p3, henceforth Rp3, incorporates a new snow and ice albedo and radiative transfer scheme, which includes internal heating by radiation penetration, and updates to the firn module (Van Dalum et al, 2020a).…”