Principal Investigator (PI): Victor Zlotnicki, NASA's Jet Propulsion Laboratory

Researchers have utilized data from the Gravity Recovery and Climate Experiment (GRACE) mission for many new research applications in hydrologic, cryospheric, and ocean studies, made uniquely possible by GRACE, for example, determining the mass losses of Greenland and Antarctica. GRACE measures time changes in the gravity field, from which we derive time changes in surface mass distributions. Because this is a totally new data type, we continue to find ways to make major improvements on the accuracy of the derived mass distributions. The GRACE time series started in mid 2002, and resolves monthly time changes in gravity between spherical harmonic (SH) degrees 2 and about 60 (roughly, wavelengths 180º to 6º, in degrees of latitude or longitude); longer time averages can resolve shorter wavelengths).

About 2 years earlier, the German mission Challenging Minisatellite Payload (CHAMP) started to measure the Earth's gravity field, and it continues to do so today, albeit with lower accuracy for comparable time-space scales since it uses a different technology to measure gravity. Researchers have retrieved CHAMP gravity coefficients for SH degrees 2-10 (roughly, wavelengths longer than 360 in degrees of latitude or longitude) averaged over 3 months with accuracy comparable to GRACE's one month averages, at the same wavelengths.

A long time series (3 decades at monthly spacing) of the Earth's gravity field's SH degree 1, 2, and 3 coefficients (roughly, wavelengths longer than 120º) exists from satellite laser ranging (SLR) to Starlette, Ajisai, Stella, LAGEOS 1 and 2, Etalon 1 and 2, Beacon-C. The degree 1 coefficients pose a special problem. GRACE and CHAMP tracking alone provide weak constraints on them. Combinations of GPS data from ground network stations and low Earth orbiters, with ocean bottom pressure estimates, allow retrieval of their seasonal and some interannual variability but not yet trends, while SLR provides the strongest constraint on degree 1. GRACE intersatellite ranging data do not improve the degree 1 coefficient beyond the value provided by the GPS tracking. Thus an adjustment to all the known data is necessary better to constrain these degree 1 terms, a task we also propose here.

This proposal focuses on reprocessing as needed by the various missions in order to obtain a consistent time series of global mass distributions, for use in estimates of mass change in the hydrologic, cryospheric and oceanic components of the Earth System, for the time period 2000-present, extended into earlier decades at much lower resolution. We understand that different length scale and frequency bands, and different accuracies, can be expected from the different missions.

We aim to eliminate systematic differences in processing as a source of discrepancy, to generate different sets of time series of mapped mass variability, with the same wavenumber and frequency content. We will generate science-enabling, consistent gridded mass distributions, (expressed as cm of equivalent water thickness), with improved spatiotemporal coverage, resolution and accuracy for Earth mass studies, and with error estimates. We will provide the data through a public Web and ftp interface, using standard formats for data and metadata, together with such other web services as may become standard.

Distributed by NASA's Physical Oceanography Distributed Active Archive Center (PO.DAAC)