Principal Investigator (PI): Felix Landerer, NASA's Jet Propulsion Laboratory

We will continue our previous Making Earth System Data Records for Use in Research Environments (MEaSUREs) activities (“An Earth System Data Record of Changes in Earth Masses from GRACE, CHAMP and Other Satellites”, PI V. Zlotnicki, 2006) and provide a high-quality, consistent Earth System Data Record (ESDR) of Earth s surface mass changes, derived from temporal gravity field variations as observed with the twin Gravity Recovery and Climate Experiment (GRACE) satellites and with geodetic satellites (e.g., LAser GEOdynamics Satellite-1 [LAGEOS-1], Starlette, Ajisai, Stella) tracked by satellite laser ranging (SLR).

The GRACE satellites, launched in 2002, resolve monthly time changes of gravity from spherical harmonic degrees 2 up to approximately 60 (resolving length scales from roughly 9000 to 300 km, respectively). The GRACE mission has proven to be the single-best technology available for comprehensive, global, near real-time monitoring of surface mass changes. GRACE has observed climate-relevant phenomena such as ice mass loss (from Greenland, Antarctica, glaciers and ice caps), groundwater depletion (e.g., over India or California), and ocean mass changes (e.g., global sea level rise from ocean mass changes as well as regional variability associated with ocean currents).

While offering less spatial resolution (only up to approximately 5000 km), surface mass changes from satellite laser ranging techniques provide a much longer data record, since 1976. In addition, SLR methods are highly complementary to GRACE in that SLR is particularly sensitive to low-degree harmonics, and can help to constrain these components accurately during the GRACE period. Thus, we propose to combine GRACE and SLR data, augmented by geocenter estimates. To derive a consistent record, SLR data needs to be processed with the same geophysical background models as GRACE. Upon improving and homogenizing the SLR and GRACE records, we propose to use established EOF-reconstruction methods to derive a long-term data record with more information content than SLR alone. This activity will support bridging the likely data gap between the current GRACE and the GRACE Follow-On mission, scheduled for launch in 2017.

The gravity field solutions delivered by the GRACE-Science Data System (SDS) are still improving through better background models as well as from improved knowledge of internal satellite alignments and orbit determination. While the GRACE-SDS does not provide gridded surface mass fields, these types of products are what most geoscientists require for their research & applications projects. Going from spherical harmonics to a gridded surface mass product requires several GRACE-specific complex filter and correction steps, which need to be carefully implemented and adjusted for a variety of processing parameter choices.

A central goal of our project is to broaden the user community of data products derived from GRACE and geodetic satellites beyond the traditional users, dominated by geodesists. We will provide a long-term, climate-quality ESDR of global surface mass variations that is consistent over time, optimized for relevant applications, with uncertainty estimates, and well documented for user-ready applications. Based on our past successful ESDR projects, we are confident that the outlined strategies to improve and add value to the existing time-variable gravity observations can achieve this goal.

The proposed ESDR products fulfill a critical need for the precise understanding of Earth’s climate system dynamics, providing data records for land, ocean and cryosphere applications. We will distribute all data via our GRACE tellus website in CF-compliant formats, and partner with NASA's Physical Oceanography Distributed Active Archive Center (PO.DAAC) to use several of their data services. The demonstrated utility and broad uses of GRACE data are behind NASA’s decision to approve GRACE-FO rather than wait until the 2020s for a GRACE-II mission as recommended in the Decadal Survey in 2007. GRACE and GRACE-FO also figure prominently in the 2010 NASA Science Mission Directorate Science Plan.