The Greenland Ice Sheet (GrIS) has been losing mass in recent decades, with an acceleration in mass loss since 2000. In this study, we apply a self‐organizing map classification to integrated vapor transport data from the ERA‐Interim reanalysis to determine if these GrIS mass loss trends are linked to increases in moisture transport to Greenland. We find that “moist” days (i.e., days featuring anomalously intense water vapor transport to Greenland) were significantly more common during 2000–2015 compared to 1979–1994. Furthermore, the two most intense GrIS melt seasons during the last 36 years were either preceded by a record percentage of moist winter days (2010) or occurred during a summer with a record frequency of moist days (2012). We hypothesize that moisture transport events alter the GrIS energy budget by increasing downwelling longwave radiation and turbulent fluxes of sensible and latent energy.

Abstract. Light transmission into bare glacial ice affects surface energy balance, biophotochemistry, and light detection and ranging (lidar) laser elevation measurements but has not previously been reported for the Greenland Ice Sheet. We present measurements of spectral transmittance at 350–900 nm in bare glacial ice collected at a field site in the western Greenland ablation zone (67.15∘ N, 50.02∘ W). Empirical irradiance attenuation coefficients at 350–750 nm are ∼ 0.9–8.0 m−1 for ice at 12–124 cm depth. The absorption minimum is at ∼ 390–397 nm, in agreement with snow transmission measurements in Antarctica and optical mapping of deep ice at the South Pole. From 350–530 nm, our empirical attenuation coefficients are nearly 1 order of magnitude larger than theoretical values for optically pure ice. The estimated absorption coefficient at 400 nm suggests the ice volume contained a light-absorbing particle concentration equivalent to ∼ 1–2 parts per billion (ppb) of black carbon, which is similar to pre-industrial values found in remote polar snow. The equivalent mineral dust concentration is ∼ 300–600 ppb, which is similar to values for Northern Hemisphere warm periods with low aeolian activity inferred from ice cores. For a layer of quasi-granular white ice (weathering crust) extending from the surface to ∼ 10 cm depth, attenuation coefficients are 1.5 to 4 times larger than for deeper bubbly ice. Owing to higher attenuation in this layer of near-surface granular ice, optical penetration depth at 532 nm is 14 cm (20 %) lower than asymptotic attenuation lengths for optically pure bubbly ice. In addition to the traditional concept of light scattering on air bubbles, our results imply that the granular near-surface ice microstructure of weathering crust is an important control on radiative transfer in bare ice on the Greenland Ice Sheet ablation zone, and we provide new values of flux attenuation, absorption, and scattering coefficients to support model development and validation.

Abstract. Light transmission into bare glacial ice affects surface energy balance, bio-photochemical cycling, and light detection and ranging (LiDAR) laser elevation measurements but has not previously been reported for the Greenland Ice Sheet. We present in-ice solar irradiance measured over the spectral range 350–900 nm and 12–77 cm depth collected at a site in the western Greenland ablation zone. The acquired spectral irradiance measurements are used to calculate flux attenuation coefficients using an exponential decay Bouguer law model and are compared to values calculated from two-stream radiative transfer theory. Relative to asymptotic two-stream theory, our empirical attenuation coefficients are up to one order of magnitude larger in the range 350–530 nm, suggesting light absorbing particles embedded in ice enhance visible light absorption at our field site. The empirical coefficients accurately describe light attenuation in the ice interior but underestimate light attenuation near the ice surface. Consequently, Bouguer’s law overestimates transmitted flux by up to 50 % depending on wavelength. Refraction is unlikely to explain the discrepancy. Instead, vertical variation in the ice microstructure and the concentration of light absorbing particles appears to enhance near-surface attenuation at our field site. The magnitude of this near-surface attenuation implies that optical penetration depth is lower by up to 19 cm (28 %) at wavelengths relevant to visible-wavelength lidar altimetry of ice surface elevation (e.g. 532 nm for the Ice, Cloud, and Land Elevation Satellite-2) than is suggested by e-folding depths inferred from two stream theory for optically pure glacier ice. This enhanced near-surface attenuation implies shallower light transmission and therefore lower subsurface light availability for subsurface radiative heating and bio-photochemical cycling. We recommend radiative transfer models applied to bare ice in the Greenland Ice Sheet ablation zone account for vertical variation in light attenuation due to the vertical distribution of light absorbing particles and ice microstructure, and we provide new values of flux attenuation, absorption, and scattering coefficients to support model validation and parameterization.

While supraglacial streams and meltwater ponds only cover 2% of the ice sheet surface in southwest Greenland, the combined effect of these low-albedo surfaces is responsible for 12% of the ice albedo variation, thereby disproportionately contributing to negative surface mass balances (Ryan et al., 2018). Modeling results show that the areal coverage of supraglacial surface water will increase with climate warming, especially

Abstract. Greenland ice sheet surface runoff is drained through supraglacial stream networks. This evacuation influences surface mass balance as well as ice dynamics. However, in situ observations of meltwater discharge through these stream networks are rare. In this study, we present 46 discrete discharge measurements and continuous water level measurements for 62 d spanning the majority of of the melt season (13 June to 13 August) in 2016 for a 0.6 km2 supraglacial stream catchment in southwest Greenland. The result is an unprecedentedly long record of supraglacial discharge that captures both diurnal variability and changes over the melt season. A comparison of surface energy fluxes to stream discharge reveals shortwave radiation as the primary driver of melting. However, during high-melt episodes, the contribution of shortwave radiation to melt energy is reduced by ∼40  % (from 1.13 to 0.73 proportion). Instead, the relative contribution of longwave radiation, sensible heat fluxes, and latent heat fluxes to overall melt increases by ∼24 %, 6 %, and 10 % (proportion increased from −0.32 to −0.08, 0.28 to 0.34, and −0.04 to 0.06) respectively. Our data also identify that the timing of daily maximum discharge during clear-sky days shifts from 16:00 local time (i.e., 2 h 45 min after solar noon) in late June to 14:00 in late July and then rapidly returns to 16:00 in early August. The change in the timing of daily maximum discharge could be attributed to the expansion and contraction of the stream network, caused by skin temperatures that likely fell below freezing at night. The abrupt shift, in early August, in the timing of daily maximum discharge coincides with a drop in air temperature, a drop in the amount of water temporarily stored in weathering crust, and a decreasing covariance between stream velocity and discharge. Further work is needed to investigate if these results can be transferable to larger catchments and uncover if rapid shifts in the timing of peak discharge are widespread across Greenland supraglacial streams and thus have an impact on meltwater delivery to the subglacial system and ice dynamics.