Neither dust nor black carbon causing apparent albedo decline in Greenland\u27s dry snow zone: Implications for MODIS C5 surface reflectance

Remote sensing observations suggest Greenland ice sheet (GrIS) albedo has declined since 2001, even in the dry snow zone. We seek to explain the apparent dry snow albedo decline. We analyze samples representing 2012–2014 snowfall across NW Greenland for black carbon and dust light-absorbing impurities (LAI) and model their impacts on snow albedo. Albedo reductions due to LAI are small, averaging 0.003, with episodic enhancements resulting in reductions of 0.01–0.02. No significant increase in black carbon or dust concentrations relative to recent decades is found. Enhanced deposition of LAI is not, therefore, causing significant dry snow albedo reduction or driving melt events. Analysis of Collection 5 Moderate Resolution Imaging Spectroradiometer (MODIS) surface reflectance data indicates that the decline and spectral shift in dry snow albedo contains important contributions from uncorrected Terra sensor degradation. Though discrepancies are mostly below the stated accuracy of MODIS products, they will require revisiting some prior conclusions with C6 data

Recent Accumulation Variability in Northwest Greenland from Ground-Penetrating Radar and Shallow Cores along the Greenland Inland Traverse

Accumulation is a key parameter governing the mass balance of the Greenland ice sheet. Several studies have documented the spatial variability of accumulation over wide spatial scales, primarily using point data, remote sensing or modeling. Direct measurements of spatially extensive, detailed profiles of accumulation in Greenland, however, are rare. We used 400 MHz ground-penetrating radar along the 1009 km route of the Greenland Inland Traverse from Thule to Summit during April and May of 2011, to image continuous internal reflecting horizons. We dated these horizons using ice-core chemistry at each end of the traverse. Using density profiles measured along the traverse, we determined the depth to the horizons and the corresponding water-equivalent accumulation rates. The measured accumulation rates vary from ~0.1 m w.e. a–1 in the interior to ~0.7 m w.e. a–1 near the coast, and correspond broadly with existing published model results, though there are some excursions. Comparison of our recent accumulation rates with those collected along a similar route in the 1950s shows a ~10% increase in accumulation rates over the past 52 years along most of the traverse route. This implies that the increased water vapor capacity of warmer air is increasing accumulation in the interior of Greenland

Lattice-Boltzmann modeling of the air permeability of polar firn

Recent advances in three‐dimensional (3D) imaging of snow and firn combined with numerical modeling of flow through complex geometries have greatly improved the ability to predict permeability values based on microstructure. In this work, we combined 3D reconstructions of polar firn microstructure obtained from microcomputed tomography (mCT) and a 3D lattice‐Boltzmann (LB) model of air flow. We compared the modeled results to measurements of permeability for polar firn with a wide range of grain and pore‐scale characteristics. The results show good agreement between permeability measurements and calculated permeability values from the LB model over a wide range of sample types. The LB model is better at predicting measured permeability values than traditional empirical equations for polar firn

Comparing MODIS daily snow albedo to spectral albedo field measurements in Central Greenland

The albedo of the Greenland ice sheet plays a key role in the energy balance and climate of the arctic. Change in snow albedo values associated with changing climate conditions can be monitored remotely from satellite platforms viewing the entire Greenland ice sheet, yet comparisons to high quality surface measurements are necessary to assess the accuracy of satellite measurements as new snow albedo algorithms are developed with higher spatial and temporal resolution. During May, June, and July 2011, we obtained daily measurements of spectral albedo at Summit, Greenland with an Analytical Spectral Devices (ASD) spectroradiometer, scanning at 350–2200 nm. We compare our spectral albedo field measurements to the Moderate Resolution Imaging Spectrometer (MODIS), using both the Version 005 Direct Broadcast daily albedo product and the recently developed Version 006 MCD43A daily albedo product. The spectral field measurements allow calculation of weighted integrals to compare to seven MODIS narrow bandwidths ranging the UV through Infrared, as well as a broadband integration to compare to the MODIS shortwave albedo. We additionally compare our field measurements to albedo measured at the Baseline Surface Radiation Network (BSRN) station at Summit. Using the MODIS Version 005 Direct Broadcast product, high-quality retrievals only, comparison to field measurements results in root mean square error (RMSE) of 0.033 for the MODIS shortwave product, and RMSE for the MODIS narrow bandwidths ranging 0.022–0.077. The new MODIS Version 006 product shows considerable improvement, with shortwave RMSE of 0.026, and narrow bandwidths ranging 0.020–0.048. These error values for the Version 006 albedo product show an improvement in reported error values from previous MODIS field validations in Greenland, which have been limited to broadband data from the Greenland Climate Network Automatic Weather Stations

Biomass Burning Emissions and Transport of Black Carbon (BC) to the Greenland Ice Sheet (GrIS) in 2013

International audienceThis study is the part of the SAGE project investigating the impact of light absorbing impurities (e.g., aerosols) on the Greenland Ice Sheet (GrIS). Previously ice-core snow samples collected on the GrIS indicated that black carbon (BC) concentrations were significantly enhanced, which could contribute to a decrease in albedo. Along with high levels of BC, the samples also showed significant amounts of ammonia, indicating the BC was sourced from biomass burning – likely from active forest fires in Eurasia and North America in July and August of 2013.In this study, we simulate the transport of potential smoke-filled air parcels using the NASA Langley Trajectory Model (LaTM), running in a backwards mode from selected ice-core sample sites on the GrIS from June 1st to August 31st 2013. The trajectory model is initialized for 24-hour sustained injection from each site, and air parcels are released from the surface to 2 km at 200m intervals. With the trajectory model outputs, we are able to identify trajectories that have coincidences with fires. As a case study, we focus on an event in early August 2013 when episodic enhancements in black carbon deposition are found in snow pit observations. We also utilize Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) data to verify smoke-aerosol signatures in boreal regions based on the NASA LaTM results from late July to early August. We ran backward and forward trajectories from the CALIOP aerosol signatures to verify coincidence with fire events and transport to the GrIS. We found large fires burning west side of the Hudson Bay in late July. CALIOP data captured thick smoke plumes on July 28th over that region and backward/forward trajectories and MODIS Terra/Aqua images support the transport of smoke from these fires to the GrIS