Estimating Greenland surface melt is hampered by melt induced dampening of temperature variability

Uta Krebs-Kanzow,Gerrit Lohmann,Paul Gierz

doi:10.1017/jog.2018.10

Uta Krebs-Kanzow, Gerrit Lohmann + Show 1 more

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https://doi.org/10.1017/jog.2018.10

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Abstract

ABSTRACTThe positive degree-day (PDD) model provides a particularly simple approach to estimate surface melt from land ice based solely on air temperature. Here, we use a climate and snow pack simulation of the Greenland ice sheet (Modèle Atmosphérique Régional, MAR) as a reference, to analyze this scheme in three realizations that incorporate the sub-monthly temperature variability differently: (i) by local values, (ii) by local values that systematically overestimate the dampened variability associated with intense melting or (iii) by one constant value. Local calibrations reveal that incorporating local temperature variability, particularly resolving the dampened variability of melt areas, renders model parameters more temperature-dependent. This indicates that the negative feedback between surface melt and temperature variability introduces a non-linearity into the temperature – melt relation. To assess the skill of the individual realizations, we hindcast melt rates from MAR temperatures for each realization. For this purpose, we globally calibrate Greenland-wide, constant parameters. Realization (i) exhibits shortcomings in the spatial representation of surface melt unless temperature-dependent instead of constant parameters are calibrated. The other realizations perform comparatively well with constant parametrizations. The skill of the PDD model primarily depends, however, on the consistent calibration rather than on the specific representation of variability.

Highlights

The Greenland ice sheet (GrIS) is sensitive to a warming climate, as surface melting is a major element of its mass budget (Rignot and others, 2011)
In the Modèle Atmosphérique Régional (MAR) simulation we find, that above melting surfaces daily temperature amplitude declines from ∼5°C–1.5°C when monthly mean temperatures approach melting point from both directions (Fig. 1). σEFF can be understood as a rough estimate of an ‘effective’ temperature variability, freed from the dampening effect of phase transitions on the daily temperature amplitude
We find a strong sensitivity of degree-day factors (DDF) on the individual positive degree-day (PDD) approximation (Fig. 4)

Summary

Introduction

The Greenland ice sheet (GrIS) is sensitive to a warming climate, as surface melting is a major element of its mass budget (Rignot and others, 2011). To interpret historic and recent mass changes of the GrIS, it is necessary to quantify the components of the surface mass balance (SMB), which primarily are surface melt, accumulation and refreezing. A network of automatic mass-balance stations has been established to monitor the annual mass loss of the GrIS (Ahlstrom and others, 2008). Since 2002, the Gravity Recovery and Climate Experiment (GRACE) (Tapley and others, 2004) satellite mission allows to detect integral mass changes of the GrIS (Wouters and others, 2014), but additional data are necessary to separate and quantify the individual SMB components such as surface melt (Sasgen and others, 2012; Tedesco and Fettweis, 2012). High-resolution regional climate models such as the Regional Atmospheric Climate Model (RACMO) (Noel and others, 2015) or the Modèle Atmosphérique Régional (MAR) (Fettweis and others, 2017) allow for a separation of the SMB

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