About the Mascon Visualization Tool
The Gravity Recovery and Climate Experiment (GRACE) is a dual-satellite mission continuously monitoring and mapping Earth's changing gravity field. A joint mission between NASA and DLR, GRACE has been in operation since 2002, providing a roughly 14 year time history of Earth gravity. This time history has been an invaluable resource for tracking large-scale flooding, droughts, and glacial melting. Led by the University of Texas's CSR, centers at NASA JPL and GSFC, as well as Germany's GFZ direct the release of periodic time-variable gravity solutions.
A variety of GRACE solutions are available to the community for study and use. Two such solutions are the JPL RL05M Mascon solution and the GSFC Global Mascons v01 solution. This visualization tool uses these mascons to recover localized data from GRACE. Both sets of mascons provide a depiction of the temporal and spatial variations of Earth's gravity field. It is important to note that both of these mascon solutions are determined directly from the GRACE Level-1B data, as opposed to several proposed post-processing strategies that seek to estimate “mascons” from the Level-2 spherical harmonic solutions (e.g. Jacob et al., 2012).
Represented by 4551 equal-area, 3 arc-degree mascon cells located across the surface of the earth, this solution captures variations in the gravity field at each localized cell. Changes in any given mascon, therefore, represent the total changes to mass over the area of the cell - in other words, each cell represents the resolution of the GRACE data, and localized signals in areas smaller than that mascon cell are averaged over the whole mascon.
JPL uses a Coastline Resolution Improvement (CRI) filter to separate land signals from ocean signals in those mascons that are along the coast. As a result, land areas and ocean areas within the same mascon will return different signals when this filter is applied. This tool optionally allows the user to turn off this filter.
If the coastline filter is selected, land areas will return a different signal from ocean areas within the same mascon, as shown:
For more information regarding the JPL mascons, methodologies used in determining the JPL RL05M Mascon solution, or information regarding the CRI filter, please visit JPL's Mascon page.
This mascon solution is comprised of 41,168 equal-area, 1 arc-degree mascon cells. These mascons are solved for using spatial constraints that tie nearby mascons to each other. Six separate constraint regions are defined where mascons in different regions are uncorrelated, significantly reducing signal leakage across region boundaries. The constraint regions are: Greenland Ice Sheet above 2000 m, Greenland Ice Sheet below 2000 m, Antarctic Ice Sheet, Gulf of Alaska, land, and ocean.
It is important to note that these mascons are defined to 1 arc-degree in order to accurately define the constraint regions, but the fundamental spatial resolution of the solutions within a particular region is essentially the same as other GRACE solutions (~300 km). In other words, the time series of mass change for a particular mascon is highly correlated to the nearest mascons within the same constraint region.
This tool displays the standard global mascon product over land areas, and the Ocean Bottom Pressure product over oceans. For both, Glacial Isostatic Adjustment has been removed. Details about this solution's constraint techniques and data versions can be found at GSFC's Mascon page.
For the GSFC mascon solution, users can view timeseries for more than just individual mascons. By default, the tool initially displays continental regions and large seas. Clicking on any of these regions will bring up a timeseries for that entire region along with information about that region, as seen in the image below.
Choosing the "Basins" toggle breaks down the continental regions to distinct drainage basins. Selecting any mascon within a given basin will bring up the timeseries for that basin as well as information about the basin. The image below shows an example, selecting the Gulf of Alaska basin.
If you are interested in the timeseries of an individual mascon, these can be viewed by first setting the map view to "Trend", "Annual", or "RMS", and then selecting the mascon you wish to view. Then timeseries will plot below, just as with the Regions or Basins.
Using the Tool
To use the Mascon Visualization Tool, simply select a location by clicking on the map on the main page or by inputing latitude and longitude coordinates into the form next to the map. On the map, land and water are shaded tan and blue, respectively. These shadings represent locations determined as land versus as ocean by the CRI filter. If you would like to disable the CRI filter, select the radio button next to the map before clicking on the map. When you select a location on the map, the map will zoom to an appropriate level and show the outline of the mascons, as shown in the image below.
After selecting a mascon on the map, the tool will retrieve a timeseries for that mascon and plot it below. A slider at the bottom of the plot lets you select the timeframe over which data is plotted, as shown in the image below. Radio buttons at the right allow you to enable or disable plotting the best fit trend overtop the data, and also allow you to de-season the data. De-seasoning removes annual and semi-annual signals from the timeseries, as well as a 161-day aliasing signal that results from GRACE's orbit design. Best fit parameters are estimated for the selected time interval - changing the interval will change the fitted parameters.
Acknowledgements and References
GRACE land are available at http://grace.jpl.nasa.gov, supported by the NASA MEaSUREs Program.
D.N. Wiese. 2015. GRACE monthly global water mass grids NETCDF RELEASE 5.0. Ver. 5.0. PO.DAAC, CA, USA. Dataset accessed 2017-07-01 at http://dx.doi.org/10.5067/TEMSC-OCL05.
Watkins, M. M., D. N. Wiese, D.-N. Yuan, C. Boening, and F. W. Landerer (2015), Improved methods for observing Earth’s time variable mass distribution with GRACE using spherical cap mascons, J. Geophys. Res. Solid Earth, 120, doi:10.1002/2014JB011547.
Wiese, D. N., F. W. Landerer, and M. M. Watkins (2016), Quantifying and reducing leakage errors in the JPL RL05M GRACE mascon solution, Water Resour. Res., 52, 7490–7502, doi:10.1002/2016WR019344.
Data used by this tool is available at Tellus with version name "RL05M_1.MSCNv02CRIv02".
Luthcke, S.B., T.J. Sabaka, B.D. Loomis, et al., 2013, Antarctica, Greenland and Gulf of Alaska land ice evolution from an iterated GRACE global mascon solution, J. Glac. 59(216), 613-631, doi:10.3189/2013JoG12J147.
Data used by this tool is available at http://ssed.gsfc.nasa.gov/grace/, supported by the NASA MEaSUREs Program. Version v02.3b. Data accessed 2017-07-20.
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