Surface mass balance downscaling through elevation classes in an Earth system model: application to the Greenland ice sheet

Raymond Sellevold,Brice Noël,Miren Vizcaino,Leonardus Van Kampenhout,Jan T M Lenaerts,William H Lipscomb

doi:10.5194/tc-13-3193-2019

Abstract

Abstract. The modeling of ice sheets in Earth system models (ESMs) is an active area of research with applications to future sea level rise projections and paleoclimate studies. A major challenge for surface mass balance (SMB) modeling with ESMs arises from their coarse resolution. This paper evaluates the elevation class (EC) method as an SMB downscaling alternative to the dynamical downscaling of regional climate models. To this end, we compare EC-simulated elevation-dependent surface energy and mass balance gradients from the Community Earth System Model 1.0 (CESM1.0) with those from the regional climate model RACMO2.3. The EC implementation in CESM1.0 combines prognostic snow albedo, a multilayer snow model, and elevation corrections for two atmospheric forcing variables: temperature and humidity. Despite making no corrections for incoming radiation and precipitation, we find that the EC method in CESM1.0 yields similar SMB gradients to RACMO2.3, in part due to compensating biases in snowfall, surface melt, and refreezing gradients. We discuss the sensitivity of the results to the lapse rate used for the temperature correction. We also evaluate the impact of the EC method on the climate simulated by the ESM and find minor cooling over the Greenland ice sheet and Barents and Greenland seas, which compensates for a warm bias in the ESM due to topographic smoothing. Based on our diagnostic procedure to evaluate the EC method, we make several recommendations for future implementations.

Highlights

During the 20th century, the Arctic warmed much faster than the rest of the world (e.g., Serreze and Francis, 2006; Screen and Simmonds, 2010; Hartmann et al, 2013; Overland et al, 2018) due to shrinking sea ice cover (Serreze and Stroeve, 2015), associated positive albedo–temperature feedbacks (Pithan and Mauritsen, 2014), and increased moisture and heat transport from the midlatitudes (Screen et al, 2012)
We analyze the particular elevation class (EC) implementation in a specific Earth system models (ESMs) (CESM1.0), we aim to provide an evaluation and diagnostic framework to guide the future implementation of EC downscaling in other climate models, for offline surface mass balance (SMB) estimates and/or forcing of ice sheet models
While the EC method in CESM1.0 realistically simulates SMB gradients, we have shown here major deficiencies in the simulation of individual gradients of surface energy and mass balance components compared to the RACMO2.3 RCM

Summary

Introduction

During the 20th century, the Arctic warmed much faster than the rest of the world (e.g., Serreze and Francis, 2006; Screen and Simmonds, 2010; Hartmann et al, 2013; Overland et al, 2018) due to shrinking sea ice cover (Serreze and Stroeve, 2015), associated positive albedo–temperature feedbacks (Pithan and Mauritsen, 2014), and increased moisture and heat transport from the midlatitudes (Screen et al, 2012). Since the 1990s, the GrIS has lost mass at an accelerated rate (Shepherd et al, 2012; Kjeldsen et al, 2015; Hanna et al, 2013; Bamber et al, 2018; Mouginot et al, 2019) This mass loss is projected to be sustained and contribute a 0.04–0.21 m sea level rise by the end of the 21st century, depending on the climate scenario (Church et al, 2013). The range of uncertainty is due to uncertainties in climate scenarios, climate sensitivity, and simulated mass balance of the GrIS by ice sheet models (ISMs) This latter uncertainty is currently being targeted by the Ice Sheet Model Intercomparison for CMIP6 (ISMIP6; Nowicki et al, 2016), a major international effort to investigate future ice sheet evolution, constrain estimates of future global mean sea level, and explore ice sheet sensitivity to climate forcing

Objectives

Methods

Results

Discussion

Conclusion