Abstract
For the Greenland Ice Sheet (GrIS), large-scale melt area has increased in recent years and is detectable via remote sensing, but its relation to runoff is not known. Historical, modeled melt area and runoff from Modern-Era Retrospective Analysis for Research and Applications (MERRA-Replay), the Interim Re-Analysis of the European Centre for Medium Range Weather Forecasts (ERA-I), the Climate Forecast System Reanalysis (CFSR), the Modele Atmospherique Regional (MAR), and the Arctic System Reanalysis (ASR) are examined. These sources compare favorably with satellite-derived estimates of surface melt area for the period 2000-2012. Spatially, the models markedly disagree on the number of melt days in the interior of the southern part of the ice sheet, and on the extent of persistent melt areas in the northeastern GrIS. Temporally, the models agree on the mean seasonality of daily surface melt and on the timing of large-scale melt events in 2012. In contrast, the models disagree on the amount, seasonality, spatial distribution, and temporal variability of runoff. As compared to global reanalyses, time series from MAR indicate a lower correlation between runoff and melt area (r2 = 0.805). Runoff in MAR is much larger in the second half of the melt season for all drainage basins, while the ASR indicates larger runoff in the first half of the year. This difference in seasonality for the MAR and to an extent for the ASR provide a hysteresis in the relation between runoff and melt area, which is not found in the other models. The comparison points to a need for reliable observations of surface runoff.
Highlights
The Greenland Ice Sheet (GrIS) constitutes a vast reserve of freshwater that is intermittently discharged to the ocean
Runoff is defined as the surface net horizontal divergence of liquid water, none of the models examined here provide for a routing scheme in which runoff from a particular model grid box is advected to adjacent grid boxes (e.g., Liston and Mernild, 2012)
Several of the models indicate an underestimate in elevation along the steep ice sheet escarpment in the southeastern GrIS, and the difference plot for the European Centre for Medium Range Weather Forecasts (ERA-I) (Figure 1D) presents a pattern of negative and positive values parallel to the coast that is characteristic of the Gibbs phenomenon in spectral models (Hoskins, 1980)
Summary
The Greenland Ice Sheet (GrIS) constitutes a vast reserve of freshwater that is intermittently discharged to the ocean. Recent attention has been focused on GrIS surface hydrological processes as a result of enhanced, widespread melting that has been observed (e.g., Mernild et al, 2011) This melting was punctuated on 11-July, 2012 when almost the entirety of the ice sheet simultaneously experienced surface melt, including Summit (Nghiem et al, 2012; Box et al, 2013; Hall et al, 2013; Tedesco et al, 2013; Hanna et al, 2014; Shuman et al, 2014). Exceptional melt seasons have been documented in 2002, 2007, and 2010 (Steffen et al, 2004; Mote, 2007; Tedesco et al, 2008, 2011)
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