Abstract
Abstract. We discuss Greenland Ice Sheet (GrIS) surface mass balance (SMB) differences between the updated polar version of the RACMO climate model (RACMO2.3) and the previous version (RACMO2.1). Among other revisions, the updated model includes an adjusted rainfall-to-snowfall conversion that produces exclusively snowfall under freezing conditions; this especially favours snowfall in summer. Summer snowfall in the ablation zone of the GrIS has a pronounced effect on melt rates, affecting modelled GrIS SMB in two ways. By covering relatively dark ice with highly reflective fresh snow, these summer snowfalls have the potential to locally reduce melt rates in the ablation zone of the GrIS through the snow-albedo-melt feedback. At larger scales, SMB changes are driven by differences in orographic precipitation following a shift in large-scale circulation, in combination with enhanced moisture to precipitation conversion for warm to moderately cold conditions. A detailed comparison of model output with observations from automatic weather stations, ice cores and ablation stakes shows that the model update generally improves the simulated SMB-elevation gradient as well as the representation of the surface energy balance, although significant biases remain.
Highlights
Since the mid-1990s, atmospheric and oceanic warming in the Arctic has led to accelerated Greenland ice sheet (GrIS) mass loss (Enderlin and Howat, 2013; Fettweis et al, 2013; Wouters et al, 2013)
Since surface melt over the GrIS is mainly driven by the absorption of shortwave radiation (Van den Broeke et al, 2008), surface albedo is a primary factor governing ice sheet surface mass balance (SMB) (Bougamont et al, 2005; Tedesco et al, 2011; Fitzgerald et al, 2012) and surface energy balance (SEB) (Tedesco et al, 2008; Van Angelen et al, 2012)
The RACMO2 physics package has recently been updated from cycle CY23r4 used in RACMO2.1 (White, 2001) to cycle CY33r1 in the current RACMO2.3 version (ECMWFIFS, 2008)
Summary
Since the mid-1990s, atmospheric and oceanic warming in the Arctic has led to accelerated Greenland ice sheet (GrIS) mass loss (Enderlin and Howat, 2013; Fettweis et al, 2013; Wouters et al, 2013). Fettweis et al (2005) analysed two heavy snowfall events in south-east Greenland at the end of July 1991, using MAR (Modèle Atmosphérique Régional) and AVHRR satellite imagery These events temporarily raised surface albedo, delaying the appearance of darker bare ice. Based on data from automatic weather stations (AWS), Van den Broeke et al (2011) showed that even minor summer snowfall events (< 5 mmWE) can considerably reduce surface melting. An accurate representation of (summer) snowfall events is essential to model the SMB of the GrIS (Fettweis et al, 2005; Van Angelen et al, 2012) This requires a high-resolution model to resolve the narrow ablation zone, and an explicit model of atmospheric and surface snow/ice physics.
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