Search

The National Aeronautics and Space Administration (NASA)’s Operation IceBridge (OIB) was a 13-year (2009–2021) airborne mission to survey land and sea ice across the Arctic, Antarctic, and Alaska. Here, we review OIB’s goals, instruments, campaigns, key scientific results, and implications for future investigations of the cryosphere. OIB’s primary goal was to use airborne laser altimetry to bridge the gap in fine-resolution elevation measurements of ice from space between the conclusion of NASA’s Ice, Cloud, and land Elevation Satellite (ICESat; 2003–2009) and its follow-on, ICESat-2 (launched 2018). Additional scientific requirements were intended to contextualize observed elevation changes using a multisensor suite of radar sounders, gravimeters, magnetometers, and cameras. Using 15 different aircraft, OIB conducted 968 science flights, of which 42% were repeat surveys of land ice, 42% were surveys of previously unmapped terrain across the Greenland and Antarctic ice sheets, Arctic ice caps, and Alaskan glaciers, and 16% were surveys of sea ice. The combination of an expansive instrument suite and breadth of surveys enabled numerous fundamental advances in our understanding of the Earth’s cryosphere. For land ice, OIB dramatically improved knowledge of interannual outlet-glacier variability, ice-sheet, and outlet-glacier thicknesses, snowfall rates on ice sheets, fjord and sub-ice-shelf bathymetry, and ice-sheet hydrology. Unanticipated discoveries included a reliable method for constraining the thickness within difficult-to-sound incised troughs beneath ice sheets, the extent of the firn aquifer within the Greenland Ice Sheet, the vulnerability of many Greenland and Antarctic outlet glaciers to ocean-driven melting at their grounding zones, and the dominance of surface-melt-driven mass loss of Alaskan glaciers. For sea ice, OIB significantly advanced our understanding of spatiotemporal variability in sea ice freeboard and its snow cover, especially through combined analysis of fine-resolution altimetry, visible imagery, and snow radar measurements of the overlying snow thickness. Such analyses led to the unanticipated discovery of an interdecadal decrease in snow thickness on Arctic sea ice and numerous opportunities to validate sea ice freeboards from satellite radar altimetry. While many of its data sets have yet to be fully explored, OIB’s scientific legacy has already demonstrated the value of sustained investment in reliable airborne platforms, airborne instrument development, interagency and international collaboration, and open and rapid data access to advance our understanding of Earth’s remote polar regions and their role in the Earth system.

Abstract Background The All India Coordinated Rice Improvement Project (AICRIP) organizes multi-location tests (METs) with new genotypes from breeding programs. The best performing genotypes in METs for 3–4 years are identified and notified as commercial cultivars by the Government of India (GOI) to authorize breeder seed (BS) production. Methods We created a database and analyzed data on BS production for 24 years (1995–96 to 2018–19) in 475 inbred and 22 F1 hybrid cultivars. Estimates were made to rank cultivars on the proportional contribution of a cultivar, quantity produced and a new BS index. Correlation and regression analyses were used to find the relationships between BS, certified quality seed distributed (CQSD), and milled rice production. We compared cultivars in BS production chain with those grown in farmers’ fields across the country as identified in the production-oriented surveys. Results The top ten inbred cultivars identified are Jaya (notified by GOI in 1969), Swarna, Kranthi, IR 36, Sarjoo 52, Samba Mahsuri and Pusa Basmati 1 (notified in 1980–1989), and IR 64, Vijetha and Cottondora Sannalu (notified in 1991–2000). BS production in hybrid F1 was insignificant. We detected a Pearson correlation (r = 0.806, P 73 kg rice per capita per year (~ 40% of food grains). The process of notification, receipt of indent for BS from states, allotment and BS production, and de-notification of a cultivar by GOI need a review to ensure profits to farmers. As the 1000-seed weight varies in cultivars, it must be notified by GOI to adjust seed rate and maintain the recommended plant population at the farms. There is scope to increase the priceless BS production in cultivars to raise the country’s production further, facilitate export and ensure profits to all stakeholders.

The basal thermal state of an ice sheet (frozen or thawed) is an important control upon its evolution, dynamics and response to external forcings. However, this state can only be observed directly within sparse boreholes or inferred conclusively from the presence of subglacial lakes. Here we synthesize spatially extensive inferences of the basal thermal state of the Greenland Ice Sheet to better constrain this state. Existing inferences include outputs from the eight thermomechanical ice-flow models included in the SeaRISE effort. New remote-sensing inferences of the basal thermal state are derived from Holocene radiostratigraphy, modern surface velocity and MODIS imagery. Both thermomechanical modeling and remote inferences generally agree that the Northeast Greenland Ice Stream and large portions of the southwestern ice-drainage systems are thawed at the bed, whereas the bed beneath the central ice divides, particularly their west-facing slopes, is frozen. Elsewhere, there is poor agreement regarding the basal thermal state. Both models and remote inferences rarely represent the borehole-observed basal thermal state accurately near NorthGRIP and DYE-3. This synthesis identifies a large portion of the Greenland Ice Sheet (about one third by area) where additional observations would most improve knowledge of its overall basal thermal state.

Recent peripheral thinning of the Greenland Ice Sheet is partly offset by interior thickening and is overprinted on its poorly constrained Holocene evolution. On the basis of the ice sheet's radiostratigraphy, ice flow in its interior is slower now than the average speed over the past nine millennia. Generally higher Holocene accumulation rates relative to modern estimates can only partially explain this millennial-scale deceleration. The ice sheet's dynamic response to the decreasing proportion of softer ice from the last glacial period and the deglacial collapse of the ice bridge across Nares Strait also contributed to this pattern. Thus, recent interior thickening of the Greenland Ice Sheet is partly an ongoing dynamic response to the last deglaciation that is large enough to affect interpretation of its mass balance from altimetry.

The flow of ice is temperature-dependent, but direct measurements of englacial temperature are sparse. The dielectric attenuation of radio waves through ice is also temperature-dependent, and radar sounding of ice sheets is sensitive to this attenuation. Here we estimate depth-averaged radar-attenuation rates within the Greenland Ice Sheet from airborne radar-sounding data and its associated radiostratigraphy. Using existing empirical relationships between temperature, chemistry, and radar attenuation, we then infer the depth-averaged englacial temperature. The dated radiostratigraphy permits a correction for the confounding effect of spatially varying ice chemistry. Where radar transects intersect boreholes, radar-inferred temperature is consistently higher than that measured directly. We attribute this discrepancy to the poorly recognized frequency dependence of the radar-attenuation rate and correct for this effect empirically, resulting in a robust relationship between radar-inferred and borehole-measured depth-averaged temperature. Radar-inferred englacial temperature is often lower than modern surface temperature and that of a steady state ice-sheet model, particularly in southern Greenland. This pattern suggests that past changes in surface boundary conditions (temperature and accumulation rate) affect the ice sheet's present temperature structure over a much larger area than previously recognized. This radar-inferred temperature structure provides a new constraint for thermomechanical models of the Greenland Ice Sheet.

Several decades of ice-penetrating radar surveys of the Greenland and Antarctic ice sheets have observed numerous widespread internal reflections. Analysis of this radiostratigraphy has produced valuable insights into ice sheet dynamics and motivates additional mapping of these reflections. Here we present a comprehensive deep radiostratigraphy of the Greenland Ice Sheet from airborne deep ice-penetrating radar data collected over Greenland by The University of Kansas between 1993 and 2013. To map this radiostratigraphy efficiently, we developed new techniques for predicting reflection slope from the phase recorded by coherent radars. When integrated along track, these slope fields predict the radiostratigraphy and simplify semiautomatic reflection tracing. Core-intersecting reflections were dated using synchronized depth-age relationships for six deep ice cores. Additional reflections were dated by matching reflections between transects and by extending reflection-inferred depth-age relationships using the local effective vertical strain rate. The oldest reflections, dating to the Eemian period, are found mostly in the northern part of the ice sheet. Within the onset regions of several fast-flowing outlet glaciers and ice streams, reflections typically do not conform to the bed topography. Disrupted radiostratigraphy is also observed in a region north of the Northeast Greenland Ice Stream that is not presently flowing rapidly. Dated reflections are used to generate a gridded age volume for most of the ice sheet and also to determine the depths of key climate transitions that were not observed directly. This radiostratigraphy provides a new constraint on the dynamics and history of the Greenland Ice Sheet.Key pointsPhase information predicts reflection slope and simplifies reflection tracingReflections can be dated away from ice cores using a simple ice flow modelRadiostratigraphy is often disrupted near the onset of fast ice flow.

©2015. The Authors. Several decades of ice-penetrating radar surveys of the Greenland and Antarctic ice sheets have observed numerous widespread internal reflections. Analysis of this radiostratigraphy has produced valuable insights into ice sheet dynamics and motivates additional mapping of these reflections. Here we present a comprehensive deep radiostratigraphy of the Greenland Ice Sheet from airborne deep ice-penetrating radar data collected over Greenland by The University of Kansas between 1993 and 2013. To map this radiostratigraphy efficiently, we developed new techniques for predicting reflection slope from the phase recorded by coherent radars. When integrated along track, these slope fields predict the radiostratigraphy and simplify semiautomatic reflection tracing. Core-intersecting reflections were dated using synchronized depth-age relationships for six deep ice cores. Additional reflections were dated by matching reflections between transects and by extending reflection-inferred depth-age relationships using the local effective vertical strain rate. The oldest reflections, dating to the Eemian period, are found mostly in the northern part of the ice sheet. Within the onset regions of several fast-flowing outlet glaciers and ice streams, reflections typically do not conform to the bed topography. Disrupted radiostratigraphy is also observed in a region north of the Northeast Greenland Ice Stream that is not presently flowing rapidly. Dated reflections are used to generate a gridded age volume for most of the ice sheet and also to determine the depths of key climate transitions that were not observed directly. This radiostratigraphy provides a new constraint on the dynamics and history of the Greenland Ice Sheet.

A significant amount of the measured coastal thinning of the Greenland ice sheet may be due to recent acceleration of outlet glaciers. Using remote sensing, we measured two major periods of speedup on Helheim Glacier between 2000 and 2005 that increased peak speeds from approximately 8 to 11 km/yr. These speedups coincided with rapid retreats of the calving front, totaling over 7.5 km. The glacier also thinned by over 40 m from 2001 to 2003. Retreat of the ice front appears to decrease resistance to flow and concentrates the gravitational driving force over a smaller area. Farther up-glacier, acceleration may be a delayed response to surface draw-down and steepening of the glacier's main trunk. If the 2005 speedup also produces strong thinning, then much of the glacier's main trunk may un-ground, leading to further retreat.

Satellite radar interferometry data from 1995 to 2004, and airborne ice thickness data from 2002, reveal that the glaciers flowing into former Wordie Ice Shelf, West Antarctic Peninsula, discharge 6.8 ± 0.3 km3/yr of ice, which is 84 ± 30 percent larger than a snow accumulation of 3.7 ± 0.8 km3/yr over a 6,300 km2 drainage basin. Airborne and ICESat laser altimetry elevation data reveal glacier thinning at rates up to 2 m/yr. Fifty km from its ice front, Fleming Glacier flows 50 percent faster than it did in 1974 prior to the main collapse of Wordie Ice Shelf. We conclude that the glaciers accelerated following ice shelf removal, and have been thinning and losing mass to the ocean over the last decade. This and other observations suggest that the mass loss from the northern part of the Peninsula is not negligible at present. Copyright 2005 by the American Geophysical Union.

Interferometric synthetic aperture radar (InSAR) data from the ERS-1 and ERS-2 satellites are used to measure the surface velocity, topography, and grounding line position of the major outlet glaciers in the northern sector of the Greenland ice sheet. The mass output of the glaciers at and above the grounding line is determined and compared with the mass input. We find that the grounding line output is approximately in balance with the input, except for the three largest glaciers for which the mass loss is 4±3 km3 ice year-1 or 11±8% of the mass input. Along the coast we detect a systematic retreat of the grounding lines between 1992 and 1996 with InSAR, which implies that the outlet glaciers are thinning. The inferred coastal thinning is too large to be explained by a few warm summers. Glacier thinning must be of dynamic origin, that is, caused by spatial and temporal changes in ice velocity. Iceberg production from the glaciers is uncharacteristically low. It accounts for only 8% of the ice discharge to the ocean. About 55% of the ice is lost through basal melting (5-8 m ice year-1 on average) from the underside of the floating glacier tongues that are in contact with warm ocean waters. Mass losses are highest in the first 10 km of floating ice, where ice reaches the greatest depths and basal melting is 3 times larger than on average. Only a small increase in basal melting would suffice to disintegrate the floating glacier tongues. Copyright 2001 by the American Geophysical Union.

Synthetic-aperture radar interferometry data and airborne ice-sounding radar (ISR) data are employed to obtain modern estimates of the inland ice production from Nioghalvfjerdsbrae (NB) and Zachariae Isstrom (ZI), the two largest glaciers draining the northeast sector of the Greenland ice sheet. Ice fluxes are measured at the grounding line (14.2 ± 1 km3 ice a-1 for NB and 10.8 ± 1 km3 ice a-1 for ZI) with an ice thickness deduced from ice-shelf hydrostatic equilibrium, and along an ISR profile collected upstream of the grounding line (14.3 ± 0.7 km3 ice a-1 for NB and 11.6 ± 0.6 km3 ice a-1 for ZI). Balance fluxes calculated from a map of snow accumulation and model predictions of surface melt are 11.9 ± 2 km3 ice a-1 for NB and 10.0 ± 2 km3 ice a-1 for ZI at the grounding line, and 12.2 and 10.3 km3 ice a-1, respectively, at the ISR line. The two glaciers therefore exhibit a negative mass balance equivalent to 14% of their balance flux, with a ±12% uncertainty. Independently, we detect a retreat of the grounding line of NB between 1992 and 1996 which is larger at the glacier center (920 ± 250 m) than on the sides (240 ± 50 m). The corresponding ice-thinning rates (2±1 m a-1 at the glacier center and 0.6±0.3 m a-1 on the sides) are too large to be accommodated by temporal changes in ablation or accumulation, and must be due to dynamic thinning.

Flow-field interactions are studied in a high-through-flow, axial-flow transonic compressor using Digital Particle Image Velocimetry (DPIV). Measurement of instantaneous velocities in two-dimensional (2D) planes in the main flow direction allows characterization of the unsteadiness of spatial structures from an upstream blade row and their interaction with the downstream rotor. The measurement system is specially designed for a large transonic environment, which introduces conditions that differ from those generally encountered by traditional DPIV systems. Viewing windows on the compressor housing are used to allow optical access, and the design of a special optical probe permits laser-sheet delivery through one of the wake generators (WG). The system is synchronized with the blade passage and is remotely monitored and controlled. Through flow visualization and instantaneous and ensemble-averaged quantities, it clearly captures the interactions of the wake with the potential field of the rotor leading edge (LE) and its bow shock, vortex shedding, vortex-blade synchronization, wake chopping, and boundary-layer flow at the housing for several configurations. [DOI: 10.1115/1.1429638]