Enhancing NASA's Contributions to Polar Science A Review of Polar Geophysical Data Sets (2001) / Chapter Skim
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4. Assessment
4. Assessment
Pages 72-94
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From page 72... ...
After the variable-by-variable review, the committee summarizes NASA contributions and identifies key gaps in the current generation of polar geophysical data sets. This summary provides the basis for recommendations presented in Chapter 5.
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From page 73... ...
) generally do not provide adequate spatial and temporal coverage, especially over ocean and sea ice portions of the polar regions.
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From page 74... ...
Given the challenges associated with surface-based sensors in the extreme and isolated polar regions, flight of a wind-profiling mission could be very useful, but the persistent cloudiness of polar regions could be a constraint. Cloud Properties Existing satellite data sets (two global, two Arctic only)
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From page 75... ...
cannot provide surface temperatures when cloud fractions exceed 90%, which leads to a bias and to an incomplete description of energy exchanges at the surface. Although there are four relevant data sets (the two Arctic-only data sets are held at the NSIDC, one global data set is held at the Langley DAAC, and the remaining global data set is partly archived at the Goddard DAAC)
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From page 76... ...
Measurement uncertainties in several components of the surface energy balance make the combined errors large, thereby limiting the accuracy of estimated changes of surface elevation due to melt, as well as estimation of runoff from snow packs on land. Integrated analysis of available and future satellite measures of albedo and surface temperature, in combination with sensible and latent fluxes, may provide a solution; however, representative in situ studies and instrumentation are required.
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From page 77... ...
The one extensive snow cover data set does not contain any information about snow cover on ice. Global precipitation data sets combining satellite and ground-based data are available from other agencies such as the WMO's Global Precipitation Climatology Center and the Global Precipitation Climatology Project.
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From page 78... ...
Despite the potential opportunity, NASA makes only a limited contribution to the direct measurement of precipitation, primarily in support of microwave analyses of snow cover (water content) on land.
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From page 79... ...
In particular, data sets that describe the timing of freeze and thaw with adequate spatial coverage and spatial resolution may overcome some constraints. The NSIDC provides a land-based freezing/thawing/degreeday product based on climatology station records, but it does not provide adequate resolution or coverage of the polar regions, particularly in mountainous terrain.
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From page 80... ...
Products to measure land surface characteristics are not adequate for applications in high latitudes. For example, satellite-based technology is not able to monitor temporal changes in land surface characteristics of high-latitude regions, such as gradual changes in canopy characteristics associated with gradual vegetation changes and abrupt changes in land surface characteristics associated with disturbances, particularly fire.
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From page 81... ...
NASA supports the collection of such information either directly through its sensors or indirectly through data support. NASA efforts result in the provision of clear-sky surface temperature, surface albedo, radiation and cloud properties, and clear-sky surface air temperature.
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From page 82... ...
NASA makes a valuable contribution in terms of ground-based flask and tower monitoring for carbon fluxes through FLUXNET, as part of a NASA EOS calibration and validation activity. Canopy chemistry, indicating vegetative stress and potential rates of nutrient recycling in ecosystems, can be derived from hyperspectral sensor data.
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From page 83... ...
Note that extended time series of these measurements are necessary to understand the background variability of the processes that determine surface elevation and provide the opportunity to assess the causes of change; this is required to develop a predictive understanding of ice sheet variations.
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From page 84... ...
The development of an operational version of this radar will greatly enhance our knowledge of spatial variations of accumulation over ice sheets and allow point measurements from ice cores to be placed in a regional context. An ice-penetrating radar that measures both accumulation variations and ice thickness would provide a unique opportunity for completing the ice thickness map of Antarctica.
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From page 85... ...
Of primary interest in the Arctic is the capability of passive microwaves to characterize snow depth and water equivalent; while this is being applied for snow water equivalent on land, snow data on ice and on ice sheets is not available. Again, NSIDC activities to collect and distribute land-based data sets for precipitation and, in this context, snow cover provide important auxiliary information for satellite snow retrievals.
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From page 86... ...
Passive microwave sensors have provided a time series starting in the 1970s that has been used to document climate change (e.g., Cavalieri et al., 1997) and validate numerical models.
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From page 87... ...
Sea Ice Velocity Until recently sea ice motion fields were produced solely by tracking a relatively sparse network of drifting buoys. This has changed dramatically, to the point that there now exists a variety of ice motion data sets derived from satellites (SAR, AVHRR, passive microwave sensors, scatterometers)
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From page 88... ...
This Pathfinder Project has also created a preliminary integrated surface temperature data set that includes ocean, ice, and land, but only for the polar regions. This type of activity should be strongly encouraged, and on a global basis.
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From page 89... ...
In addition to ocean color, estimating ocean productivity and CO2 flux of open oceans requires other measurements, including photosynthetically active radiation, clouds, sea ice, sea surface temperature, surface albedo, and surface winds. Because sea ice is a bright target that saturates highly sensitive marine sensors, it is difficult to accurately determine ocean color in the vicinity of sea ice.
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From page 90... ...
For instance, NASA has done well in supporting demonstration projects and field campaigns that have led to identifying the potential of applying remote-sensing technologies for the development of data sets depicting the spatial and temporal variability of key land-surface quantities: terrestrial CO2 and CH4 flux, freeze and thaw dynamics, snow cover, evapotranspiration, soil moisture, and other land surface characteristics. NASA also makes a valuable contribution in terms of groundbased flask and tower monitoring for carbon fluxes through FLUXNET, as part of a NASA EOS calibration and validation activity.
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From page 91... ...
BETTER MANAGEMENT OF NASA'S CURRENT GENERATION OF POLAR GEOPHYSICAL DATA SETS Many of the variables identified in this chapter are already estimated in re-analyses, including atmospheric profiles, cloud properties, surface temperature, surface heat fluxes, surface albedo, precipitation, evapotranspiration and soil moisture. While the reliability of many of these estimates remains to be established, re-analyses and other data assimilation techniques are powerful vehicles for assessing the impacts of data, especially because re-analyses can accept observations whenever and wherever they are made.
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From page 92... ...
The needed quantities include land surface temperature, precipitation, evapotranspiration, photosynthetically active radiation, soil moisture, biomass, leaf area index, and land surface characteristics. Although some of the variables are already being measured by remote-sensing techniques, these data sets need to be evaluated to determine whether they are adequate for high-latitude applications.
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From page 93... ...
Surface temperatures over terrestrial and ocean regions are needed for a variety of applications, including turbulent flux determinations, freeze and thaw mapping, and controls on trace gas fluxes. NASA can take steps to enhance surface temperature data sets by supporting programs that measure in situ SST (particularly surface temperature)
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From page 94... ...
Tying measurements from an accumulation radar to shallow core sites where background rates and variability have been determined in both Greenland and Antarctica would greatly enhance efforts to determine appropriate forcing for ice sheet models. Altimetry offers the most promising approach to sea ice thickness mapping for change detection; this approach is the subject of an upcoming European Space Agency mission.
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