Cooperative Institute for Research in Environmental Sciences

Abstracts: 19

CIRES and Gravity

John Wahr, Sean Swenson, Isabella Velicogna, Douglas Robertson and Roger Bilham

CIRES scientists are working in space, on the continents and in the laboratory in the application of gravity to monitoring processes on and within the Earth. Laboratory studies include improvements to the FG5 Absolute Gravimeter, designed here in Boulder, with an accuracy of 1 microgal. The device operates by dropping a mirror in a vacuum and timing its fall with an Iodine stabilised laser and an atomic clock. The device is a field instrument that has been operated on the equator (in the Algerian Sahara) and in Arctic conditions (New Zealand the Antarctic. We have recently identified a number of noise factors in the instrument and measurement process whose correction now permits both faster and more accurate measurements. We have applied gravity measurements to determine uplift rates of the mountains in the Himalaya and New Zealand, and our instrument has been used in the Antarctic and in Greenland to measure post-glacial rebound. But by far the most exciting new development is the GRACE mission (Gravity Recovery and Climate Experiment) onboard a pair of satellite launched 6 months ago by NASA. The separation of the two satellites as they follow each other around the earth is a sensitive measure of the earth's gravity field. When fully operational the satellite will produce (for the first time ever) monthly "maps" of the earth's gravity field - the geoid - sufficiently accurate to detect a sheet of water 1 cm thick in most large river catchments, and global sea level changes of the order of 1 mm. Work to date consists of a series of simulations to determine the sensitivity of future data in distinguishing between various ice-melt/mantle-flow models, mantle rheology, and climate forcing factors. For example, it is essential to remove the mass of the atmosphere from the observed signal. The most societally important signals will be those associated with the prediction of potential spring floods following the gravitational observation of winter snow accumulation. Long-term goals are to resolve issues associated with accelerating sea level rise and the mass transfer of ice to the world's oceans. Interesting geodynamic quests include searches for the stability of the flexural bulge associated with plate subduction, and erosion and uplift processes in the western North America and the Tibetan Plateau.