Ice Bridge Level-1 Science Requirements
Draft
December 6, 2010
Intended as an Appendix to the Project Plan
1. IceBridge Program Overview and the IceBridge Science Team
NASA has established the Operation IceBridge program (OIB) which is mandated to fulfill the
following observational goals:
P1 Make airborne altimetry measurements over the ice sheets and sea ice to extend and improve the
record of observations begun by ICESat.
P2 Link the measurements made by historical airborne laser altimeters, ICESat, ICESat-2, and
CryoSat-2 to allow accurate comparison and production of a long-term, ice altimetry record.
P3 Monitor key, rapidly changing areas of ice in the Arctic and Antarctic to maintain a long term
observation record.
P4 Provide key observational data to improve our understanding of ice dynamics, and better
constrain predictive models of sea level rise and sea ice cover conditions.
In additional, OIB has the following technical goal:
P5 Adapt existing instruments for airborne remote sensing of ice by unmanned aerial
systems such as NASA’s Global Hawk.
The IceBridge Project is directed from NASA’s Goddard Space Flight Center. There are
separate management functions for IceBridge Instruments, logistics, data management, and
science. The 6 programmatic goals listed above provide general scientific direction for the
project. Specific direction is provided by the IceBridge Science team (IST). The team has three
tasks mandated by the Program (see OIB Science Team Terms of Reference Document, 2010):



Final development of the IceBridge Science Definition Document and Level-1 Scientific
Requirements Document;
Evaluation of the IceBridge mission designs in achieving the goals defined by the
Science Definition Document and Level-1 Scientific Requirements Document as
requested by the NASA Program Scientist; and
Support to the IceBridge Program Scientist and Project Scientist in the development of
the required analyses, documentation, and reporting during the IceBridge mission.
In addition, the science team, in collaboration with the instrument teams, will ensure the fidelity
of the data products delivered to the public. This includes thorough documentation and access to
level 1 data and corrections (e.g. geophysical corrections, trajectory, orientation, ranging) as to
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provide the means for future investigation and improvement of the instrument level 2 products
(e.g. footprint surface elevation).
This document addresses the first science team task to establish Level-1 science requirements.
The science team adopted the following strategy for completing this task. First, the team
articulated broad scientific goals or themes addressing Greenland and Antarctica ice sheets and
sea ice and also glaciers in Alaska and ice caps in the Canadian Arctic. The goals then flowed
into a set of more specific questions that could be addressed with the IceBridge data suite. The
science questions were then linked to a set of observational goals, which are themselves driven
by a set of specific measurement requirements. Science requirements (both measurement
accuracy and geography requirements) are detailed in Section 5 of this report. Justification for
many of the requirements parameters are reviewed in Appendix A.
Note that the science themes, questions and measurement plans are important, ambitious and
wide ranging. Consequently, the science community beyond just the IST is envisioned as active
participants using the IceBridge data suite to address these questions. Two important functions
of the IST are to engage the external community when developing data acquisition plans and to
assure that the data set is as complete and as accurate as possible in order to facilitate broad and
aggressive use by the science community. The team must walk the boundary between making
recommendations on prudent and somewhat cautious use of the resources while at the same time
anticipating the likelihood of new scientific investigations relying on a rich and cutting edge data
set.
2. IceBridge Science Goals
At the highest level, the IceBridge data set will help address the following science goals
(numerals in parentheses refer to Program Objectives of section 1.0).
Goal 1. Document volume changes over the accessible domain of the Greenland and Antarctica ice
sheets between Icesat-1 and Icesat-2. A particular focus will be to document rapid changes.
Icebridge will answer: how have the ice sheet volumes (areas accessible by a/c) changed during
these 5 years? (P1,P2))
Goal 2. Document the thickness of glacier beds and other geophysical properties to better
interpret volume changes measured with laser altimetry and to enable more realistic
simulations of ice sheet flow with numerical models. IceBridge will answer: how are the ice
sheets likely to change in the future? (P3,P4)
Goal 3. Document the spatial and interannual changes in the mean sea ice thickness and the
thickness distribution in the Arctic and Southern Oceans between ICESat-1 and ICESat-2, in
support of climatological analyses and assessments.
Goal 4. Improve sea ice thickness retrieval algorithms by advancing technologies for measuring
sea ice surface elevation, freeboard, and snow depth distributions on sea ice.
Note that IceBridge data cannot be used solely to tackle these important questions. Other data
(for example, ice sheet surface velocity, sea ice deformation and motion data) must come from
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other sources. However, IceBridge surface elevation, surface elevation change, and ice thickness
data are essential ingredients needed to resolve these questions. Moreover, some of the
measurements (swath altimetry and ice sounding radar) are only implementable on airborne
platforms at present.
3. Science Questions
Several specific science questions flow from the IceBridge science goals. This increasing level
science specificity will be used to establish quantitative measurement requirements. Shown in
parentheses is the traceability from the science goal to each question.
Ice Sheets
IQ1. Where are glaciers continuing to thin and where may they be slowing/ thickening
(G1)
IQ2. What are the major forces and mechanisms causing the ice sheets to lose mass and
change velocity, and how are these processes changing over time? (G2)
• How do the ice sheet/glacier surface topography, bed topography, bed geology,
ice shelves/tongues, and grounding line configurations effect ice dynamics?
• How far inland are the effects of coastal thinning transmitted and by what
physical processes?
• How far downstream do changing processes near the ice divide (changes in
snow accumulation, divide migration) effect glacier flow
• What is the important scale for measuring geophysical parameters so as to
substantially improve modeling fidelity?
• Where is the subglacial water produced and where is it going?
IQ3. How do ocean, sea ice, ice sheet interactions influence ice sheet behavior (G2)
• How does the bathymetry beneath Arctic fjords and Antarctic ice shelves
influence ocean/ice sheet interactions and ice sheet/glacier flow dynamics?
IQ4. What are yearly snow accumulation/melt rates over the ice sheets? (G1)
• How do changing accumulation rates (and hence near surface densities and firn
structure) impact altimetry measurements
• What are the surface-melt flow-patterns and how do they change with time?
Sea Ice
SQ1.
How are the physical characteristics of the sea ice covers changing (e.g. strength,
thickness, snow depth, sea ice age)
SQ2. What level of accuracy in ice thickness observations is desirable for climate forecast?
Operational forecast? for climate/operational?
SQ3.
What is the optimal configuration of instruments to remotely measure:
– Sea ice freeboard, as a function of sea ice elevation and sea surface elevation (via
leads)
– Snow depth
– Sea ice thickness (as derived from sea ice freeboard and snow depth)
– Surface roughness (distinction of ice types, changes in ridge characteristics due
to ice dynamics, relating melt point coverage to snow depth)
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–
Change in floe size and lead width distribution (affects ice albedo feedback)
SQ4.
What is the optimal configuration of an Arctic sea ice observing network?
Antarctic?
• How can data from in situ, airborne, submarine and satellite platforms, each with
a unique spatial and temporal signature, be effectively combined?
• Are there locations that should be specifically monitored to aid in the observation
and prediction of ice volume (e.g. monitoring sea ice flux through Fram Strait)?
SQ5.
How does snow depth impact melt pond formation? What is the relationship
between between snow and ice roughness?
SQ6.
How is momentum being transferred between the ice and atmosphere? Ice and
ocean?
4. IceBridge Dataset Requirements
The IceBridge data set will have the following attributes based on the programmatic goals
(section 1.0)
DR1.
DR2.
DR1.
DR2.
DR3.
DR4.
DR5.
DR6.
Provide a dataset for cross-calibration and validation of ice-sheet elevations from
satellite lidars (ICESat-1, ICESat-2, DesDynI-Lidar) and radars (CryoSat-2 and
Envisat). (1,2,3)
Provide a dataset for improving and linking ICESat and ICESat-2 the ice-sheet
elevation time series, including better characterization of ICESat-1 errors. (1,2)
Provide a data sets for investigating critical ice sheet processes (3,4)
Provide a dataset for improving and comparing numerical models of ice-sheet
dynamics, especially precise maps of the bed beneath glaciers and coarse maps of
the sea bed beneath ice shelves. (1,2,3,4)
Provide a dataset for improving instrument simulation and performance analysis
in support of future missions, such as ICESat-2 and DesDynI-Lidar. (1,3,4,5)
Collaborate with field programs that will enhance interpretation of ice bridge data.
(4,5)
Provide a timely, well documented dataset for easy use by the science community.
The data set should complement the ongoing and planned programs of
international partners.
Programmatic goal P5 (UAV and advanced aircraft) does not trace into these data set
requirements nor the baseline science requirements (section 5). However the issue of advanced
systems and sensors is partly addressed in the projected science requirements.
5.0 IceBridge Science Requirements
As summarized in the body of the Project Plan, the IceBridge science requirements must satisfy
NASA’s established programmatic goals to provide for measurement continuity between ICESat
and ICESat-2, measurement comparison and continuity between ICESat/Cryosat-2/ICESat-2 to
create a decade long change record of ice sheet and sea ice characteristics, monitor rapidly
changing areas of the arctic and Antarctic, improve understanding of ice dynamics, and to
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provide data necessary to improve predictive models. Specific science requirements (both
measurement and geographic constraints are presented in this section. Measurement accuracy
and geographic requirements are culled from the literature and are also based on the
measurement parameter analyses presented in section 6...
Table W summarizes the threshold requirements that OIB must satisfy. The table combines both
terrestrial and marine ice requirements and all have equal priority. Table X summarizes the prioritized
baseline line science requirements that must be achieved by a multiyear, airborne measurement program
designed to addresses the above objectives and reach the major scientific goals. To that end, the list is
composed of relatively well established, essential parameters such as repeat measurement of ice surface
topography, ice elevation change, ice thickness, glacier bed topography, snow thickness on sea ice, a first
order description of bathymetry in front of tidewater glaciers and underneath ice shelves. There are also
projected requirements in table Y that include a set of important parameters that could reasonably be but
are not yet realized on an operational basis because of insufficient data to develop a vetted, standardized
measurement methodology (for example, measuring the changing distribution of subglacial water; large
scale measurements of surface accumulation rate; subglacial geothermal heat flux). Similarly, the science
requirements include geographic objectives that are demonstrably in reach of manned aircraft in the time
frames consistent with previous airborne programs in the polar regions. There are also spatial and
temporal requirements that are highly desirable but which likely would require different platforms and
operational strategies to achieve. Quoted measurement accuracies represent uncertainties of 1 standard
deviation about the mean.
The science requirements also draw on publications that summarize community consensus on important
variables and their measurement sensitivities (ISMASS, 2004; NRC, 2007, IGOS, 2007, ISMASS, 2010).
The scientific basis for these requirements is presented in section 6.
Table W Operation IceBridge Threshold Science Requirements
T1. Measure annual changes in glacier, ice cap and ice sheet surface elevation sufficiently accurate to
detect 0.15 m changes in uncrevassed and 1.0 m changes in crevassed regions along sampled profiles
over distances of 500 m.
T2. Make sea ice surface elevation measurements with a shot-to-shot accuracy of 5 cm, assuming
uncorrelated errors.
T3. Make sea ice elevation measurements of both the air-snow and the snow-ice interfaces to an
uncertainty of 3 cm, which enable the determination of snow depth to an uncertainty of 5 cm.
T4. Acquire annually, near-contemporaneous, spatially coincident ice elevation data with ESA’s Cryosat
for underpasses in the Arctic and Antarctica. Coordinate with ESA in situ validation campaigns as
possible.
T5. Conduct one campaign in the Arctic and one campaign in the Antarctic each year
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Table X Operation IceBridge Baseline Science Requirements
Table X.1 Baseline Science Requirements for Ice Sheets
IS1.
Measure surface elevation with a vertical accuracy of 0.5 m or better.
IS2.
Measure annual changes in ice sheet surface elevation sufficiently accurate to detect 0.15 m
changes in uncrevassed and 1.0 m changes in crevassed regions along sampled profiles over distances
of 500 m.
IS3.
Measure ice thickness with an accuracy of 50 m or 10% of the ice thickness, whichever is greater.
IS4.
Measure free air gravity anomalies to an accuracy of 0.5 mGal and at the shortest length scale
allowed by the aircraft
IS5.
Acquire annually, near-contemporaneous ice elevation data with ESA’s Cryosat for underpasses
across Greenland and Antarctica. Flight segment should span ESA SARIN and LRS mode
boundaries. Coordinate with ESA in situ validation campaigns as possible.
IS6.
Remeasure annually Antarctic and Greenland surface elevation along established airborne
altimeter and ICESat underflight lines that extend from near the glacier margin to near the ice divide.
IS7.
Collect elevation data so that the combined ICESAT-1-OIB sampling provides an elevation
measurement within 10-km for 90% of the area within 100-km of the edge of the continuous
Greenland Ice Sheet, as well the Antarctic Amundsen Sea Coast and Peninsula.
IS8.
Measure ice thickness, gravity, surface, and bed elevation along central flowlines of the outlet
glaciers in Greenland with terminus widths of 2 km or greater1. Measurements should extend at least
1.5 times farther than predicted outlet glacier valley dimensions. Repeat surface elevation
measurements as practical.
IS9.
Measure once, ice thickness, surface, and bed elevation across-flow transects at 3- and 8-km
upstream of the terminus for each glacier in (8). Repeat surface elevation measurements as practical.
IS10. Measure once Greenland ice sheet elevation and ice thickness about four, nearly continuous close
loops approximately about the 1000, 2000, and 2500 ice sheet elevation contours.
IS11. Measure ice thickness, elevation, gravity and magnetic anomalies over 10 Greenland glaciers2
and 15 Antarctic glaciers3 that are rapidly changing now or are likely to change in the next 10 years.
Coverage should extend from the terminus to the elevation where velocities are about 50 m/yr. Over
the fast flowing deep troughs, the grids must have 5-km spacing or better, with 10-km or better
spacing on the surrounding regions of the lower catchment. Cycle through the glacier list for the
duration of IceBridge.
IS12. Measure once ice thickness, surface elevation, gravity anomalies within 3 km of the Antarctic Ice
Sheet Grounding line and along a second line located 10 km upstream of the grounding line.
IS13. Measure once surface elevation, ice thickness and seabed bathymetry beneath selected Antarctic
Ice Shelves4, along Greenland Fjords5 where we will collect a line along the center of the fjords and
three lines across (one at the sill, one near the middle, and one near the glacier front) and beneath
Greenland Ice Tongues6.
IS14. Measure changing distribution of subglacial water over regions of rapidly flowing ice and
distribution of subglacial water over interior cold ice.
IS15. Acquire submeter resolution, stereo color imagery covering laser altimetry swaths
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Table X.2 Baseline Glaciers and Ice Caps Requirement
IC1.
Annually to semi-annually collect LiDAR swath data along the centerlines of major
Gulf of Alaska glacier and icefield systems, repeating previous IceSAT measurements and airborne
laser altimetry centerline profiles7.
IC2.
Make annual repeat measurement of surface elevation on select Alaskan Glaciers
IC3.
Make ice elevation, ice thickness and gravity measurements on Canadian Ice Caps at least twice
during the IceBridge program. Coverage should be based on previous airborne campaigns and
leverage against ESA supported in situ Cryosat validation activities.
IC4.
Make ice elevation, ice thickness and gravity measurements on selected ice caps and alpine
glaciers around the Greenland Ice Sheet. Repeat the elevation measurements at least once during the
IceBridge program8.
Table X.3 Baseline Requirement for Sea Ice
SI1. Make surface elevation measurements with a shot-to-shot accuracy of 5 cm, assuming
uncorrelated errors.
SI2. Make elevation measurements of both the air-snow and the snow-ice interfaces to an
uncertainty of 3 cm, which enable the determination of snow depth to an uncertainty of 5 cm.
SI3. Provide annual acquisitions of sea ice surface elevation during the late winters of the
Arctic and Southern Oceans along near-exact repeat tracks, to within 500 m of the previous
years’ flight tracks, in regions of the ice pack that are undergoing rapid change. Flight lines
shall be designed to ensure measurements are acquired across a range of ice types including
seasonal (first-year) and perennial (multiyear) sea ice to include, as a minimum:
Arctic
a. At least two transects to capture the thickness gradient across the perennial and
seasonal ice covers between Greenland, the central Arctic, and the Alaskan Coast.
b. Perennial sea ice pack from the coasts of Ellesmere Island and Greenland north to the
pole, and westward across the northern Beaufort Sea.
c. Sea ice across the Fram Strait and Nares Strait flux gates.
d. Sea ice covers of the Eastern Arctic North of the Fram Strait.
Antarctic
a. Weddell Sea ice between the tip of the Antarctic Peninsula and Cape Norvegia.
b. Mixed ice cover in the western Weddell between the tip of Antarctic Peninsula and
Ronne Ice Shelf.
c. The ice pack of the Bellingshausen and Amundsen Seas.
SI4. Include flight tracks for sampling the ground tracks of satellite lidars (ICESat-1 and
ICESat-2) and radars (CryoSat-2 and Envisat) and, in the case of CryoSat-2, both IceBridge
and CryoSat-2 ground tracks should be temporally and spatially coincident whenever
possible. At least one ground track of each satellite should be sampled per campaign.
SI5. Conduct sea ice flights as early as possible in the flight sequence of each campaign,
preferably prior to melt onset.
SI6. Collect coincident natural color visible imagery of sea ice conditions at a spatial
resolution of at least 20 cm per pixel to enable direct interpretation of the altimetric data.
SI7. Conduct sea ice flights primarily in cloud-free conditions, and data shall be retained
under all atmospheric conditions, such that a flag shall be included to indicate degradation or
loss of data due to clouds.
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SI8. Make full gravity vector measurements on all low-elevation (< 1000 m) flights over sea
ice, to enable the determination of short wavelength (order 10 to 100 km) geoid fluctuations
along the flight track to a precision of 2 cm.
SI9. Actively seek out and coordinate with field campaigns that are consistent with and
advance IceBridge project objectives to extend and improve the record of observations begun
by ICESat related to sea ice thickness and snow depth retrievals.
SI10. Make available to the community instrument data on sea ice surface elevation and snow
depth within 3 months of acquisition, and derived products (via NSIDC) within 6 months of
data acquisition.
Table Y Projected Science Requirements on Future IceBridge Development
1.
2.
3.
4.
5.
Table Y.1 Projected Ice Sheet Science Requirements on Future IceBridge Development
Measure surface snow accumulation rate with an accuracy of 4 cm/yr averaged over 25 km square
areas in dry snow regions with annual accumulation in excess of 10 cm/yr.
Measure the distribution and changing distribution of subglacial water over 5 km square areas.
Seasonally remeasure surface elevation on select Greenland and Antarctic Glaciers using UAVs.
Estimate relative spatial changes in subglacial geothermal heat flux using ice thickness, gravity and
magnetic data.
Remeasure ice sheet surface elevation at the locations of ICESat detected subglaical lakes located
beneath West Antarctic Ice Streams and outlet glaciers draining into the Ross Ice Shelf. Measure
with accuracy of 10 cm and at least once during the IceBridge mission using UAVs.
Measure free air gravity anomalies to an accuracy of 0.5 mGal and a wavelength of 2.5km
6.
7. Acquire photogrammetrically calibrated, stereo, color imagery covering laser altimetry
swaths and adjacent areas for creating DEMs and orthophotographs with submeter resolution
and accuracy.
Table Y.2 Projected Sea Ice Science Requirements on Future IceBridge Development
1. Improve sea ice baseline requirement 1 to make surface elevation measurements with a shotto-shot accuracy of 3 cm (versus 5 cm), assuming uncorrelated errors.
2. Extend sea ice baseline requirement 3 to other regions of the Arctic and Southern Oceans:
Arctic(to better constrain estimates of sea ice volume change)
a. North Pole region
b. Southern Beaufort Sea, west of Banks Island
c. Sea ice along the east coast of Greenland
d. Southern Chukchi Sea north of Bering Strait
e. Davis Strait
f. Lancaster Sound and other parts of the Canadian Archipelago
Antarctic(to better understand the process of sea ice formation and snow accumulation)
a. Ross Sea
b. Surveys of areas of polynya formation, over and downwind of the polynya
c. Surveys of areas where katabatic winds may deposit abundant snow.
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3. Collection thermal images for a swath that, as a minimum, covers the LVIS data swath with a
resolution of 0.5 m or better, and are calibrated to brightness temperature with an accuracy of
0.1K.
4. Improve sea ice baseline requirement 6 to collect coincident digital stereo imagery of sea ice
conditions at a spatial resolution of at least 10 cm (versus 20 cm) per pixel, at a vertical
resolution of 20 cm, to enable direct interpretation of the altimetric data and provide a
complimentary surface elevation product.
5. IceBridge shall support the validation of operational sea ice analysis and forecast products by
providing estimates of sea ice freeboard within 1 week of data acquisition, and estimates of
sea ice thickness within 2 weeks of data acquisition.
Footnotes on the Requirements
1. For a list of Greenland outlet glaciers see Moon and Joughin (2007)
2. The targeted list of Greenland glaciers includes but is not exclusive to: Petermann, Humboldt, 79
North, Zaceriea (NE IceStream), Store, Rinks, Jacobshavn, Eqalorutsit kangigdlit sermiat,
Nordboggletscher, Helheim, Daugaard-Jensen
3. The list of Antarctic Glaciers includes but is not exclusive to: Pine Island, Thwaite, Crane, Rutford,
Lambert, Toten, Mertz, Shirase, Recovery, Jutulstraumen, David, Byrd, Nimrod, WAIS Ice Streams.
4. The list of ice shelves includes but is not exclusive to: Getz, Dotson, Crosson, Thwaites, Pig,
Cosgrove, Abbot, George VI, Larsen C, Venable, Cook, Moscow, Totten, Riiser Larsen, Fimbul, West,
Shackleton.
5. The list of fjords includes but is not exclusive to: Nordre Sermilik, Kangiata (Nuuk), jakobshavn,
Torssukataq, Ummanaq (3 fjords), Upernavik, and the mini fjords in Melville bay should be
pursued based on the 2010 results, Ingelfield Bredening (north thule) and Humboldt Heimdal Fj.,
Bernstorft Fj., Gylden love fj., Helheim, Kangerlugssuaq, Vestfjord, Daugaard Jensen, Keyser
Franz Joseph Fj., Borfjorden (Storstrommen).
6. The list of ice tongues includes but is not exclusive to: Peterman, 79 north, and Zacheriae Glaciers.
7. Targeted glaciers include but ar not exclusive to the Columbia-Tazlina system, the Bering-Bagley
system, the Seward-Malaspina system, the Yathse, Guyot, Tyndall and Tsaa tidewater glaciers in Icy Bay,
the Yakutat Icefield, Glacier Bay's Grand Plateau, Fairweather, Grand Pacific, Margerie, Brady, Carroll,
and Muir glaciers, and finally the Stikine, Juneau, Nabesna and Harding Icefields.
8. Coverage will be selected to be representative for varying climate zones and priority will be given to
ice caps and alpine adjacent to rapidly changing ice sheet regions. Suggested regions: Sukkertoppen
Iskappe, Disko Island, North Ice Cap, Kronprins Christian Land, Renland Iskappe.
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6. Scientific Basis for IceBridge Science Requirements
The purpose of this section is to establish the science basis for IceBridge baseline requirements
on ice sheet, glacier, and sea ice measurements. Other useful tabulations of cyrospheric
measurement requirements can be found in the literature (e.g. IGOS, 2007; ISMASS, 2004,
2010) and we have relied on the literature to further support our requirement specifications. We
do not separately address the threshold requirements as these constitute the essential subset of the
baseline requirements.
6.1. Ice Sheets, Ice Caps, Glaciers Baseline Justification
(IS1) Elevation requirements are estimated based on ice sheet driving stress (d)
𝜏𝑑 = 𝜌𝑔𝐻
𝜕ℎ
𝜕𝑥
Here, 
topography. Driving stress on West Antarctic Ice streams is approximately 10 kPa. Using that
number as a required measurement accuracy, and selecting 1000 m of ice as a reasonable average
value, the required accuracy on driving stress is written as
2
2
1/2
𝜕ℎ
𝜕ℎ
∆𝜏𝑑 = 10 𝑘𝑃𝑎 = {(𝜌𝑔𝐻 (∆ )) + (𝜌𝑔 (∆𝐻)) }
𝜕𝑥
𝜕𝑥
Assuming the total driving stress uncertainty is distributed equally between the two terms, the
required surface slope accuracy is 0.06 degrees. Taking the surface slope (S) as
𝑆=
𝜕ℎ ℎ0 − ℎ1
=
𝜕𝑥 𝑥𝑜 − 𝑥1
and assuming negligible error in the longitudinal coordinate, the uncertainty in S is
∆𝑆 =
√2 ∗ ∆ℎ
𝑥𝑜 − 𝑥1
From here we can estimate either the requirement on surface elevation accuracy or the required
sample spacing. For the 0.06 degree slope accuracy requirements and choosing the instrumental
surface elevation error to be 0.5 m, the required surface elevation sample spacing is about 700 m
(or about an ice thickness for this case) in the along flow direction. Alternatively, the sample
spacing could be chosen and the elevation accuracy estimated. So if we set the sample interval at
1 km, the requirement on elevation accuracy is 0.75 m. We retain the 0.5 m requirement based
on the IGOS recommendation of 0.5 cm at 1 km sampling intervals (IGOS, 2007, p. 86).
IS2) Ice elevation change requirements follow from an assessment of natural variabilities
separate form average annual mass loss, which occurs on scales of months to years with
magnitudes of cm/yr to ~10 m/yr. The near surface density changes with temperature,
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accumulation rate and month (equivalent seasonal changes in surface elevation are on the order
of 20 cm). The net surface accumulation can change daily. The surface roughness can change
diurnally. Surface roughness influences the instantaneous mean-elevation accuracy of a small
area (over an instrument footprint where roughness is typically 5-10 cm rms on the interior ice
sheet). For these reason, local surface elevation change knowledge is required to about 15 cm.
To further refine our estimates of surface elevation change, we require having many, near
simultaneous, independent observations to remove natural random fluctuations. A meaningful
spatial averaging dimension of independent observations depends on the terrain but is between 1
and 5 km sq areas for the perimeter areas.
The requirements on spatial repeatability accuracy are as follows. For regions where 2dimensional data are acquired (1x1 km), the exact distribution of samples within the area is not
important. For places where we simply have at least 500 m long tracks of narrow swaths (ATM
for example), we need to have the swaths overlap to say 50% in order to reduce slope induced
errors. ATM has achieved this overlap goal.
IS3) The depth integrated continuity equation informs on the required ice thickness
requirements. Writing the equation as a spatially averaged volume-balance over an area A that
extends from near the grounding line of a basin to the bounding ice divides, the spatially
averaged change in ice thickness, H, (or elevation h on a static crust) is expressed in terms of the
output Qo flux, the basin averaged accumulation rate a and the basin averaged basal melt rate b
𝑑ℎ
−(𝑄𝑜 )
=
+ 𝑎̇ + 𝑏̇ (1)
𝑑𝑡
𝐴
The flux across an output gate is given as
𝑄𝑜 = 𝛼𝑉𝐻𝑆
Here V is the speed orthogonal across the outward boundary S of the basin. Vertical variation in
the speed is accounted for by a weighting parameter .
Assuming the errors on each measurement parameter are normally distributed, the variance of
the flux is
(𝛿𝑄𝑜 )2 = (𝛼𝑉𝑆𝛿𝐻)2 + (𝛼𝐻𝑋𝛿𝑉)2 + +(𝛼𝐻𝑉𝛿𝑆)2 + (𝐻𝑉𝑆𝛿𝛼)2 (2)
Accounting for errors in the interface balance terms, the error on the equivalent, basin-averaged
thickening rate is
2
𝑑ℎ
1
𝑄𝑜
2
2
2
2
𝛿
= [( 2 ((𝛼𝑉𝑆𝛿𝐻) + (𝛼𝐻𝑋𝛿𝑉) + +(𝛼𝐻𝑉𝛿𝑆) + (𝐻𝑉𝑆𝛿𝛼) )) + ( 2 𝛿𝐴) + 𝛿𝑎̇ 2
𝑑𝑡
𝐴
𝐴
1
+ 𝛿𝑏̇ 2 ] 2 (3)
The equation illustrates that there are 7 distinct error terms contributing to the error in ice sheet
thickening rate. Taking the error on elevation changes as 15 cm/yr and given that the length of
the outflux gate S, the depth-averaged-velocity tuning parameter (), and the velocity V, and the
basin area A can be accurately measured or reasonably modeled, the allowable error on the
remaining terms probably can be relaxed to (15/(3)1/2) cm/yr or about 9 cm/yr ice equivalent.
This now allows an estimate on the required thickness accuracy from
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𝛼𝑉𝑆𝛿𝐻 2
(9 𝑐𝑚/𝑦𝑟) = (
)
𝐴
2
Taking V to be 1000 m/yr, S to be 1200 km,  is unity, and A is 10^6 km (roughly the
dimensions of the Amery drainage and around the grounding line) the average error in ice
thickness needs to be about 75 m. Given that this is one of if not the largest glacier drainage in
the world, it seems reasonable to reduce the ice thickness error to 50 m on average for a basin
half this size.
Ice sounding radars are capable of achieving these levels of accuracy as illustrated by following
Skolnick (1962, p 464) who adopts the following relations for estimating travel time accuracy on
radar measurements in the presence of noise
∆𝑇 =
𝑡𝑟
𝑆
(𝑁)
Here T is the echo arrival time uncertainty, tr is the echo rise time, S is the signal strength and N
is the noise (clutter) level. Conservatively taking the rise time as proportional to the inverse
bandwidth (bw)
1
( )
𝑏𝑤
∆𝑇 =
𝑆
(𝑁)
And the ice thickness error is
1
𝑉
𝑉 (𝑏𝑤)
∆𝐼𝑇 = ∆𝑇 =
2
2 (𝑆)
𝑁
UHF and VHF radars are presently operating at bandwidths of about 20 MHz. For 0 dB SNR,
the echo time error results in about 4 m error in ice thickness. Allowing for small errors in wave
speed (well documented in both the laboratory and in the field), an ice thickness uncertainty of
50 m is well realizable with existing radar systems. The two confounding factors which reduce
SNR are surface clutter and temperature-dependent absorption. Both processes are important in
fast flowing outlet glaciers. This is illustrated in the coverage map showing where ice thickness
data have been successfully acquired by the University of Kansas (Figure 1 ). Consequently we
impose a less restrictive requirement of 10% of the ice thickness for outlet glaciers. Note also
that warmer, more absorptive ice in southern Greenland also has restricted ice thickness coverage
(figure 2)
12
Figure 1. Left: Ice thickness coverage (red) and all flight lines (red and blue) from 1993-2009
from the University of Kansas. Right: 1993-2009 ice thickness data collected by the University
of Kansas over Jacobshavn Glacier. The red points represent places where automatically picked
ice thickness data are available. Thickness data coverage can be expanded (especially along the
length of the glacier channel) by using advanced signal processing to enhance bed echoes and
manual picking techniques, as has been demonstrated on other glaciers . Consequently, these
maps are a conservative estimate of coverage. (Figure compiled by A. Hock and K. Jezek)
13
Figure 2. Basal topography and ice thickness data coverage (IS4). Figure prepared by E.
Rignot.
IS4) GRAVITY ACCURACY JUSTIFICATION TBD ROBIN
IS5) Near contemporaneous ice elevation data should be acquired during overflights of Cryosat2. The primary goal of the data acquisitions is to intercompare laser and radar altimeter data and
to improve the ice elevation change record that will eventually span ICESat-1 to Cryosat-2 to
ICESat-2. At least one OIB ice sheet underflight should occur during each Arctic and Antarctic
deployment. The underflights should span a distance starting seaward of the ice sheet when
Cryosat-2 is operated in the SARIn mode. The underflight should continue at least 100 km past
the point at which the satellite is operating in low-resolution mode. Locations selected for
underflights should maximize the range of ice sheet surface slopes and glacier regimes to
14
facilitate the best comparison of lidars and Cryosat radars for later, long term development of
elevation change records. Figure is an example of a Cryosat underflight across the complex
topography of Pine Island and Thwaites Glaciers and illustrates the boundaries between Cryosat
operating modes.
Figure 3. Flight lines designed to coincide with Cryosat orbit tracks. Yellow lines indicate
boundaries between Cryosat operating modes (figure provided by K. Jezek)
IS6) Icebridge supports the ICESat 1/2 measurement continuity and enhances the record begun
with earlier airborne measurements by select monitoring of ice sheet elevation along ICESat and
airborne altimeter tracks over areas of rapid change and potentially long term change (Figure 4).
There are two separate but complementary strategies for fulfilling mission continuity goals.
First, there is a requirement many tracks and terrain by flying across the ICESat orbit tracks.
This will provide many opportunities for cross over elevation estimates and will mitigate some of
the positional uncertainty between aircraft, tracks locations, ICESat orbits, and ICESat reference
orbits. (See IS10). Where there is a good confluence between ICESat orbits and the orientation
of glacier regime, there is a requirement to fly along the orbit track for continuing the record and
tie through cross-overs to ICESat. In the long term, accurate knowledge about ICESat-2 orbits
will not be known until shortly before or more likely after launch. So, ICESat-2 OIB planning is
best deferred until late in the program. In the short term, OIB can cross calibrate with the
ICESat-2 simulator (MABEL) to be flown on an ER2. First MABEL flights will be in April
2011. The second deployment is April 2012 (sea ice focus out of Alaska). Also, OIB can
contribute to planning for ICESat-2 through assessing algorithm performance error sources and
uncertainties. An approach is to coordinate MABEL and OIB coincident data.
15
Figure 4. Temporal evolution of ice sheet elevation change in Greenland from ICESat and ATM
data. Major changes are over the large outlet glaciers (Jak, Kanger and Helheim,) and over the
marine terminating glaciers of NW Greenland. Increasing thinning is indicated in N Greenland,
spreading over higher elevations on the NE ice stream. Maps provided by B. Csatho.
Figure 7. GRACE-derived, Antarctic Ice Sheet annual balance showing substantial mass loss
along the western and southeastern flanks of the ice sheet (map provided by S. Luthcke)
16
IS7) OIB surface elevation measurements improve the ICESat data set by refining off-track
slopes and ICESat-derived digital elevation models used for ice dynamics modeling. Because of
natural dimensions of Greenland coastal glaciers, ICESAT-1 tracks under-sample many narrow
outlets near the coast where discharge and dynamics thinning rates are high (Figure 5). At more
southerly latitudes where orbit tracks dirverge, the orbit geometry limits knowledge of cross
track slope corrections to ICESat data. Finally, cloud cover obscured observations over
Greenland (figure 6). IceBridge will acquire a reference elevation set for coastal portions of
Greenland and selected areas of Antarctica. The data will improve upon ICESAT-1 sampling for
long term (>5 yr) elevation change estimates once ICESAT-2 is launched. This bridge data set
will allow ICESAT-2 to begin making long-term (~5 year) measurements of thinning rates in the
first year at a much improved spatial resolution.
Figure 5. ICESat-1 track coverage illustrates the undersampling of Greenland outlet glaciers
(figure provided by I Joughin).
17
Figure 6. Coverage gaps in the ICESat record (Figure provided by B. Smith).
IS8) Ice discharged from Greenland flows primarily through outlet glaciers. These glaciers are
now known to be underlain by erosional troughs, the dimensions of which are important controls
on ice flow. Once a glacier retreats out of its trough to bed elevations at or near sea level, it is no
longer subject to tidewater instabilities, likely limiting its rate of retreat. Thus, knowing how far
inland the troughs extend allows a first-order estimate of the region where outlet glacier
dynamics may dominate thinning. Elevations along the central trunks will provide a reference
data set for comparison with future altimetry measurements (ICESAT-2) on the portions of the
glacier subject to the greatest dynamic thinning. , determining how far inland deep fjords
controlling ice discharge extent, baseline elevation measurement for determining subsequent
elevation from OIB, ICESAT-2 and other altimeters, and providing data for flowline models.
The extent to which subglacial troughs extend upstream and an associated constraint on
18
measurement coverage can be estimated by geomorphologic relationships shown in figure.
Some confidence in the figure is gained by noting that the Jacobshavn trough depth (1.1 km) and
a drainage basin are of 92000 sq km is consistent with the geomorphology data.
Figure 7 Relationships between ice drainage basin area and outlet valley size dimensions of
valley depth and valley length for various deglaciated terrains (from Brooks 2 )
IS9) IAN?? While net discharge from Greenland has been monitored successfully with InSAR
at flux gates across the major outlets, there are substantial uncertainties due to a lack of measured
bed elevation, thickness, and thickness change near the grounding lines. Ice Bridge will remove
this uncertainty, and the collection of a second, more inland, gate will allows for several km of
future retreat.
IS10) To, increase the number of ICESat orbits sampled by OIB and to facilitate flux
measurement for comparison with altimeter derived estimates of ice thickness change, OIB shall
collect elevation and ice thickness data along continuous lines parallel to 3 closed elevation
contours (1000, 200 and 2500 m). Measurements at 2000 m will also extend the record begun in
1995 when a series of surface and airborne campaigns measured ice elevation and ice thickness.
19
B
Figure. 1000, 2000 and 2500 m contours about the Greenland Ice Sheet (left). Contours in the
vicinity of Jacobshavn Glacier and ICESat reference orbits (right).
IS11) . The inability to place an upper bound on sea level change in the last IPCC reports stems
largely from the uncertainty in our knowledge of glacier dynamics, the physics of which are not
included in current whole ice-sheet models. OIB will acquire a comprehensive set of
measurements for process modeling studies aimed at understanding outlet glacier dynamics. The
data set will allow the development of process-level model experiments to determine the physics
that govern fast flow and to develop the parameterizations needed for larger-scale predictive
models. A representative set of glaciers should be chosen, including Jakobshavn, Kanger,
Helheim, a northern glacier, and 2 or 3 glaciers each from the currently changing regions in the
northwest and southeast. Measurements will extend to interior regions where surface velocities
decrease to 50 m/yr. Coverage requirements are justified in the following sections.
20
Figure 8.
velocity
thresholds on the Greenland Ice Sheet. (Figure provided by E. Rignot).
Surface
Detailed bed mapping in selected region.
The level to which subglacial properties affect ice dynamics and the level of details required to
capture these effects remains an open question. As illustrated in Figure 1, the details in the
bedrock topography in the Jakobshaven region influence the friction and driving stress resulting
from inversion methods, and therefore the basal condition to be applied in any numerical model.
The driving stress depends on the ice thickness and to compensate for the smaller thicknesses
obtained with the 5km resolution models, the model need to drop the basal drag in order to fit the
surface velocities. This example illustrates that any inversion made on the wrong bed will result
in the wrong basal boundary condition and any prognostic simulations resulting from this dataset
should be view with caution.
IOB has the potential to test the level of knowledge needed from the basal topography, and an
answer to this question requires a bedrock known at a very fine resolution. Further refinement to
the 2km grid flown in 2010 of the land terminating Russell glacier (Greenland) is a high priority
and should ideally be complemented by a similar grid in a fjord region and an Antarctic
grounding zone region.
21
Figure 8: Basal topography (top), driving stress (middle), and basal velocity (bottom) in the
Jakobshaven region from 3 distinct bedrocks applied to the ISSM model. Bedrock used are the
5km Bamber dataset (left), the 5km (middle) and the 1km (right) SeaRISE datasets that
incorporates the fine scale Cresis data. Figure courtesy of Mathieu Morlinghem and Helene
Seroussi.
Whole ice sheet and regional models.
To improve prognostic predictions of future sea-level contribution from the ice Greenland and
Antarctic ice sheets obtained with whole ice sheet models, IOB is required to continue
measurements that fill the gap in our current knowledge in bedrock elevations over coastal
regions. IOB is required to make bedrock measurements on a 5km grid over areas of slow flow,
and on a grid of the order of at least the ice thickness for regions of fast flows, or regions of
22
complex stress regime (eg: grounding line or shear margins). Priority should be given to regions
where it is suspected that canyons might be present, or regions with bedrock below sea-level that
could connect to the interior of the ice sheet (ie: high risk regions for ice loss).
The motivation for the strong focus in coastal regions compared to interior regions is due to
Stone et al. (2010), where a good agreement in ice thickness is obtained in the interior of the
Greenland ice sheet between the Bamber and Letreguilly dataset even thought the bedrock are
significantly different in the two simulations (Figure 2).
Figure 9: The ratio of the difference in ice thickness (a) and bedrock topography (b) between the
Bamber and Letreguilly datasets expressed as a percentage (z_bamber –
z_letreguilly/z_letreguilly). Figure adapted from Stone et al. (2010).
The motivation for more refined bedrock measurement in regions of fast or complex flow is
based on experience with the SeaRISE simulations, where a 5km bedrock in the Jakoshaven
region produced from the fine scale Cresis measurements (see Figure 1 “Glen 5km”) is necessary
in order to reproduce a surface velocity similar to the observed velocity. These regions are also
regions where the coarse resolution of whole ice sheet models often prevents them from reliable
diagnostic or prognostic simulations, such that regional models have an important role (ex:
Fastook, AGU talk 2010?).
Flow band models.
Continue the time series annually in a few selected regions with existing long term
measurements (ex: Helheim Glacier or Jakobshavn Isbrea). These regions allow a study of the
dynamics of glacier retreat and inversion of surface properties and elevation could provide
insight on whether the basal condition are changing in time, or to test our current understanding
of the underlying physics at play.
23
IS12) For Antarctica, important mass balance and dynamical changes are associated with
processes occurring at the grounding line – the boundary where the inland ice sheet begins to
float on the ocean - or ice sheet land terminus (ISMASS, 2003). (Figure 8). Therefore OIB shall
make measurements of surface elevation, ice thickness and gravity along the grounding line and
along a parallel track approximately 10 km upstream of the grounding line. The data will be
crucial for ice flux measurements and to investigate dynamical processes at the grounding line.
The two tracks are required partly because grounding likely occurs over a zone and partly to
increase information available for ice dynamics models (e.g. surface and basal topography
gradients). Success will constitute the first, complete circum-ice-sheet measurements of ice
thickness and basal topography. Combined with surface velocity data acquired using spaceborne
synthetic aperture radar data, the data will enable the most accurate estimate of the total volume
flux of ice being discharged from the ice sheet in the vicinity of the ice margin or grounding line.
Differencing the ice flux across our flight path with the integrated accumulation rate over the
interior surface will provide an independent estimate of total ice sheet volume change.
Figure 10. Radarsat Antarctic Mapping Project (RAMP) coast line (green), ASAID grounding
line (yellow courtesy of R. Bindschadler) overlain on RAMP coherence mosaic. The Riiser24
Larsen Ice Shelf grounding line is distinguishable in the coherence data by the dark band
separating the smooth ice shelf from the textured interior ice sheet. The ASAID grounding line
and the coherence band generally agree to about 3 km in this area. Map provided by K. Jezek.
IS13) ERIC R. A first order characterization of the depth of the sea floor in front of
glaciers, beneath ice shelves, and at the mouth of glacial fjords and embayments in all
approachable critical areas to help constrain how ocean heat fluxes can reach grounding
lines.
IS14) Subglacial Water, Mark Robin, BeaTBD ???
IS15) Visible imagery BEA???
6.2. Ice Sheet Projected Requirements
(1) Accumulation rate estimates are essential to flux gate estimates of mass balance, but the
extent to which these measurement can be applied and the need for additional in situ data is
required are poorly characterized. Accumulation rates contribute to the uncertainty in flux gate,
gravity, and altimetry measurements, making the ability measure them highly desirable. While
the ability to determine such rates is currently experimental, the collection of the radar data by
OIB will provide an extremely important data set for developing and validating the methods for
accumulation retrieval.
Equation 1 under (IS1) illustrates that there are 7 distinct error terms contributing to the error in
ice sheet thickening rate. Taking 15 cm/yr as the basin-averaged thickening rate accuracy, then
each of the error terms on the right side of equation should be on the order of (15/(7)1/2) cm/yr of
ice equivalent or about 6 cm/yr ice equivalent. This immediately gives an approximate bound on
the required accuracy for the surface and basal mass balance (6 cm/yr ice equivalent). At subbasin scales, the accuracy requirement is 4 cm/yr ice equivalent.
2) Subglacial water (MARK, ROBIN)
(3) Annual sampling is adequate but seasonal sampling would improve understanding of the
stochastic processes that are superimposed the long term thickness change signal. Spatial and
temporal sampling in the interior of Antarctica is more difficult because although accumulation
and accumulation driven changes in density are less, dynamic thinning is also much less.
Roughness is similar. A solution is to average over much larger areas than near the coast
(100x100 km or more is the Cryosat estimate) and over longer periods of time than just one year
(3 years or more). Note that while thinning at a point in the interior is less than at a point on the
coast, the area of the interior is huge so a little thinning can result in a big overall mass change.
4) Geothermal Heat (Bea ROBIN OTHERS??)
25
5) Subglacial lakes (KCJ)
6) Enhanced free air gravity (ROBIN, ERIC R.)
7) Photogrametry (BEA)
6.3. References
Brooks
Fahnestock, M., Waleed Abdalati, Ian Joughin, John Brozena, and Prasad Gogineni (14
December 2001) High Geothermal Heat Flow, Basal Melt, and the Origin of Rapid Ice Flow in
Central Greenland, Science 294 (5550), 2338. [DOI: 10.1126/science.1065370]
Fricker, H. A., Scambos, T., Bindschadler, R. & Padman, L. (2007), An active subglacial water
system in West Antarctica mapped from space. Science 315, 1544-1548.
Gray, L., I. Joughin, S. Tulaczyk, V.B. Spikes, R. Bindschadler, and K. Jezek, 2005: Evidence
for subglacial water transport in the West Antarctic Ice Sheet though three-dimensional satellite
radar interferometry. Geophys. Res. Lett., 32,L03501, doi:10.1029/2004GL021387.
IGOS, 2007. Integrated Global Observing Strategy Cryosphere Theme Report - For the
Monitoring of our Environment from Space and from Earth. Geneva: World Meteorological
Organization. WMO/TD-No. 1405. 100 pp.
ISMASS Committee, 2004: Recommendations for the collection and synthesis of Antarctic Ice
Sheet mass balance data. Glob. Planet. Change, 42, 1-15.
Kapitsa, A.P., J. Ridley, G de Q Robin, M. Siegert, and I. Zotikov, 1996. A large freshwater lake
beneath the ice of central East Antarctica. Nature, 381, 684-686
Neal CS; “The dynamics of the Ross Ice Shelf revealed by radio echo sounding,” Journal of
Glaciology, 24(90), 295-307, 1979.
NRC Committee on Earth Science and Applications from Space, 2007. Earth Science and
Applications from Space: National Imperatives for the Next Decade and Beyond. National
Research Council, ISBN 0-309-66714-3, 456 p.
Oswald, G., P. GOgineni, 2008, Recovery of subglacial water extent from Greenland radar
survey data. J. Glac, vol 54, no 184, p. 94-106
Skolnik,
Van der Veen and ISMASS, 2010. Ice sheet mass balance and sea level rise: A science Plan.
SCAR Report 38, ISSN 1755-9030, 37 p.
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