IGCP 565 Project Workshop

IGCP 565 Workshop 1: Science of geodetic monitoring of the hydrological cycle

Abstract:

Surface Mass Loads from GRACE, GPS, and Earth Rotation
Richard Gross(1), Geoffrey Blewitt(2), Hans-Peter Plag(2)
(1) Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA
(2) Nevada Bureau of Mines and Geology and Seismological Laboratory, University of Nevada, Reno, NV, USA

The rearrangement of mass within the surficial fluid layers of the Earth, including the atmosphere, oceans, and water, snow and ice stored on land, causes the Earth’s gravitational field to change, causes the Earth’s rotation to change by changing the Earth’s inertia tensor, and causes the Earth’s shape to change by changing the load acting on the solid, but not rigid, Earth. On time scales of months to a decade, loading of the solid Earth by surface fluids dominates non-secular variation in each of these three fundamental areas of geodesy (gravity, rotation, and shape). From a very fundamental point of view, geodesy naturally provides constraints on the global water cycle at multiple spatial and temporal scales. Space geodetic observations of surface mass changes are inherently strong at the regional to global scale, and so these observation provide for an opportunity to complement traditional in-situ measurements.

For more than 6 years, GRACE has been producing the best ever estimates of continental scale variation in terrestrial hydrology. This presents an opportunity to use GRACE to validate new inversion methods that use continuous GPS (CGPS) data on Earth’s shape, which can now be monitored continuously with millimeter precision. These new methods are based on the theoretical correspondence between Earth’s geometrical shape and its gravity field. Unlike GRACE, high quality CGPS data now span ~14 years, and so these new CGPS methods have the potential to produce the first ever estimates of decadal scale variation in continental-scale water storage. Since the rotation, gravitational field, and shape of the Earth all change in response to changes in the surface mass load, measurements of these quantities must be consistent with each other. We therefore use independent GPS and Earth rotation measurements to assess GRACE measurements of the gravitational field. This will be done both globally, thereby assessing the low-degree harmonics of the GRACE-measured gravitational field, and regionally, thereby assessing the GRACE-measured gravitational field for those regions that GPS-measured mass loads can be determined.