Lora S. Koenig

Polar Airborne Observations Fill Gap in Satellite Data

In October 2009, NASAs Ice, Cloud, and land Elevation Satellite (ICESat) stopped collecting science data. However, noting the progressive degradation of ICESats Geoscience Laser Altimeter System (GLAS), NASA had begun the previous year to plan a series of instrumented aircraft missions to fill the impending gap in satellite observations due to the loss of GLAS. Called Operation IceBridge, the project has been collecting data since March 2009 and will continue until the scheduled launch of ICESats replacement, ICESat-2, in about 2015. The primary goal of IceBridge is to use airborne laser altimetry to monitor rapidly changing areas of ice sheets, glaciers, and sea ice; a secondary goal is to acquire ice-penetrating-radar data to map the bedrock topography beneath the ice sheets. Data collected by IceBridge will yield a three-dimensional view of ice sheets and sea ice as they change. IceBridge will improve modeling efforts and knowledge of the contribution of the Greenland and Antarctic ice sheets to sea level rise. The project will also contribute to scientific understanding of changes in the extent and thickness of the sea ice cover as the planet warms. IceBridge flights maintain altimetry time series over outlet glaciers started by NASA in the early 1990s, expand airborne altimeter coverage to new areas, and add grids of ice-penetrating-radar data to better resolve bedrock topography.

Sensitivity and response of Bhutanese glaciers to atmospheric warming

] Glacierized change in the Himalayas affects river-discharge, hydro-energy and agricultural production, andGlacial Lake Outburst Flood potential, but its quantificationand extent of impacts remains highly uncertain. Here wepresent conservative, comprehensive and quantitative pre-dictions for glacier area and meltwater flux changes inBhutan, monsoonal Himalayas. In particular, we quantifythe uncertainties associated with the glacier area and melt-water flux changes due to uncertainty in climate data, acritical problem for much of High Asia. Based on a suiteof gridded climate data and a robust glacier melt model,our results show that glacier area and meltwater changeprojections can vary by an order of magnitude for differentclimate datasets. However, the most conservative resultsindicate that, even if climate were to remain at the present-day mean values, almost 10% of Bhutan’s glacierized areawould vanish and the meltwater flux would drop by as muchas 30%. Under the conservative scenario of an additional1 C regional warming, glacier retreat is going to continueuntil about 25% of Bhutan’s glacierized area will have dis-appeared and the annual meltwater flux, after an initialspike, would drop by as much as 65%.

A Satellite-Derived Climate-Quality Data Record of the Clear-Sky Surface Temperature of the Greenland Ice Sheet

AbstractThe authors have developed a climate-quality data record of the clear-sky surface temperature of the Greenland Ice Sheet using the Moderate-Resolution Imaging Spectroradiometer (MODIS) ice-surface temperature (IST) algorithm. Daily and monthly quality-controlled MODIS ISTs of the Greenland Ice Sheet beginning on 1 March 2000 and continuing through 31 December 2010 are presented at 6.25-km spatial resolution on a polar stereographic grid along with metadata to permit detailed accuracy assessment. The ultimate goal is to develop a climate data record (CDR) that starts in 1981 with the Advanced Very High Resolution Radiometer (AVHRR) Polar Pathfinder (APP) dataset and continues with MODIS data from 2000 to the present, and into the Visible Infrared Imager Radiometer Suite (VIIRS) era (the first VIIRS instrument was launched in October 2011). Differences in the APP and MODIS cloud masks have thus far precluded merging the APP and MODIS IST records, though this will be revisited after the APP dataset h...

A Sea-Ice Lead Detection Algorithm for Use With High-Resolution Airborne Visible Imagery

The detection of leads, or cracks, in sea ice is critical for the derivation of sea-ice freeboard from altimetric measurements of sea-ice elevation. We present an approach for lead detection in sea ice using high-resolution visible imagery from airborne platforms. We develop a new algorithm, i.e., the sea-ice lead detection algorithm using minimal signal (SILDAMS), that detects clouds, extracts leads, and classifies ice types within leads from airborne visible imagery. Cloud detection is based on an assessment of local variances of pixel brightness across image scenes and where available coincident altimetric measurements are used to confirm suspected cloudy scenes. The lead extraction step computes affine time-frequency distributions (minimal signal) for the Red, Green, and Blue channels of each image. The transformed outputs are combined to take advantage of three channels simultaneously. Finally, lead pixel geolocations are extracted using a set of uniform thresholds for ice typing (including open water, thin ice, and gray ice) within leads along each flight line. SILDAMS was tested using data from the Digital Mapping System (DMS). DMS digital photographs represent the highest resolution ( ≈10 cm) visible imagery available over sea ice and were collected during NASA Operation IceBridge sea-ice flights in the Antarctic and the Arctic in 2009 and 2010, respectively. We demonstrate that SILDAMS has a high lead detection capability of 99%.

Comparison of satellite, thermochron and air temperatures at Summit, Greenland, during the winter of 2008/09

Current trends show a rise in Arctic surface and air temperatures, including over the Greenland ice sheet where rising temperatures will contribute to increased sea-level rise through increased melt. We aim to establish the uncertainties in using satellite-derived surface temperature for measuring Arctic surface temperature, as satellite data are increasingly being used to assess temperature trends. To accomplish this, satellite-derived surface temperature, or land-surface temperature (LST), must be validated and limitations of the satellite data must be assessed quantitatively. During the 2008/ 09 boreal winter at Summit, Greenland, we employed data from standard US National Oceanic and Atmospheric Administration (NOAA) air-temperature instruments, button-sized temperature sensors called thermochrons and the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite instrument to (1) assess the accuracy and utility of thermochrons in an ice-sheet environment and (2) compare MODIS-derived LSTs with thermochron-derived surface and air temperatures. The thermochron-derived air temperatures were very accurate, within 0.1 ± 0.3°C of the NOAA-derived air temperature, but thermochron-derived surface temperatures were ∼3°C higher than MODIS-derived LSTs. Though surface temperature is largely determined by air temperature, these variables can differ significantly. Furthermore, we show that the winter-time mean air temperature, adjusted to surface temperature, was ∼11°C higher than the winter-time mean MODIS-derived LST. This marked difference occurs largely because satellite-derived LSTs cannot be measured through cloud cover, so caution must be exercised in using time series of satellite LST data to study seasonal temperature trends.

Spatial extent and temporal variability of Greenland firn aquifers detected by ground and airborne radars

We document the existence of widespread firn aquifers in an elevation range of ~1200–2000 m, in the high snow-accumulation regions of the Greenland ice sheet. We use NASA Operation IceBridge accumulation radar data from five campaigns (2010–2014) to estimate a firn-aquifer total extent of 21,900 km2. We investigate two locations in Southeast Greenland, where repeated radar profiles allow mapping of aquifer-extent and water table variations. In the upper part of Helheim Glacier the water table rises in spring following above-average summer melt, showing the direct firn-aquifer response to surface meltwater production changes. After spring 2012, a drainage of the firn-aquifer lower margin (5 km) is inferred from both 750 MHz accumulation radar and 195 MHz multicoherent radar depth sounder data. For 2011–2014, we use a ground-penetrating radar profile located at our Ridgeline field site and find a spatially stable aquifer with a water table fluctuating less than 2.5 m vertically. When combining radar data with surface topography, we find that the upper elevation edge of firn aquifers is located directly downstream of locally high surface slopes. Using a steady state 2-D groundwater flow model, water is simulated to flow laterally in an unconfined aquifer, topographically driven by ice sheet surface undulations until the water encounters crevasses. Simulations suggest that local flow cells form within the Helheim aquifer, allowing water to discharge in the firn at the steep-to-flat transitions of surface topography. Supported by visible imagery, we infer that water drains into crevasses, but its volume and rate remain unconstrained.

Operation icebridge: Using instrumented aircraft to bridge the observational gap between icesat and icesat-2

Operation IceBridge, a six-year NASA mission, is the largest airborne survey of Earths polar ice ever flown. Data collected during IceBridge will help scientists bridge the gap in polar observations between NASAs Ice, Cloud and Land Elevation Satellite (ICESat), in orbit from 2003 to 2009, and ICESat-2, planned for launch in late 2015, making IceBridge critical for ensuring a continuous series of observations. Operation IceBridge is using airborne instruments to map Arctic and Antarctic areas once a year, building on two decades of repeat airborne measurements of rapidly changing areas in the Arctic. Operation IceBridge is also producing critical data that cannot be measured from space such as ice thickness measurements. The first Operation IceBridge flights were conducted in boreal spring 2009 over Greenland and the boreal fall 2009 over Antarctica. Other smaller airborne surveys around the world are also part of NASAs Operation IceBridge campaign.

A Characterization of Greenland Ice Sheet Surface Melt and Runoff in Contemporary Reanalyses and a Regional Climate Model

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.

Hydraulic Conductivity of a Firn Aquifer in Southeast Greenland

Some regions of the Greenland ice sheet, where snow accumulation and melt rates are high, currently retain substantial volumes of liquid water within the firn pore space throughout the year. These firn aquifers, found between ~10-30 m below the snow surface, may significantly affect sea level rise by storing or draining surface meltwater. The hydraulic gradient and the hydraulic conductivity control flow of meltwater through the firn. Here we describe the hydraulic conductivity of the firn aquifer estimated from slug tests and aquifer tests at six sites located upstream of Helheim Glacier in southeastern Greenland. We conducted slug tests using a novel instrument, a piezometer with a heated tip that melts itself into the ice sheet. Hydraulic conductivity ranges between 2.5x10-5 and 1.1x10-3 m/s. The geometric mean of hydraulic conductivity of the aquifer is 2.7x10-4 m/s with a geometric standard deviation of 1.4 from both depth specific slug tests (analyzed using the Hvorslev method) and aquifer tests during the recovery period. Hydraulic conductivity is relatively consistent between boreholes and only decreases slightly with depth. The hydraulic conductivity of the firn aquifer is crucial for determining flow rates and patterns within the aquifer, which inform hydrologic models of the aquifer, its relation to the broader glacial hydrologic system, and its effect on sea level rise.

Investigation of Firn Aquifer Structure in Southeastern Greenland Using Active Source Seismology

In spring of 2011, a perennial storage of water was observed in the firn of the southeastern Greenland ice sheet, a region of both high snow accumulation and high melt. This aquifer is created through percolation of surface meltwater downward through the firn, saturating the pore space above the ice-firn transition. The aquifer may play a significant role in sea level rise through storage or draining freshwater into the ocean. We carried out a series of active source seismic experiments using continuously refracted P-waves and inverted the first P-arrivals using a transdimensional Bayesian approach where the depth, velocity, and number of layers are allowed to vary to identify the seismic velocities associated with the base of the aquifer. When our seismic approach is combined with a radar sounding of the water table situated at the top of the firn aquifer, we are able to quantify the volume of water present. In our study region, the base of the aquifer lies on average 27.7±2.9 m beneath the surface, with an average thickness of 11.5±5.5 m. Using a Wyllie average for porosity, we found the aquifer has an average water content of 16±8%, with considerable variation in water storage capacity along the studied east-west flow line, 40 km upstream of the Helheim glacier terminus. Between 2015 and 2016, we observed a 1-2 km uphill expansion of the aquifer system, with a site dry in summer 2015 exhibiting a water content of 530 kg m-2 in summer 2016. We estimate the volume of water stored in the aquifer across the entire region upstream of Helheim glacier to be 4.7±3.1 Gt, approximately 3% of the total water stored in firn aquifers across the Greenland ice sheet. Elucidating the volume of water stored within these recently discovered aquifers is vital for determining the hydrological structure and stability of the southeastern Greenland ice sheet.