Jean Dickey (PI)
Jet Propulsion Lavoratory
jean.dickey@jpl.nasa.gov

Geodetic Validation of Geophysical Models: Applications to Glacial, Hydrological and Oceanic Variability on Seasonal-Interannual Timescales

Monitoring of the Earth’s sub-systems to understand their evolution in the context of global change poses unique challenges to NASA. In particular, better understanding of water mass redistribution within and between its glacial, hydrological and oceanic reservoirs is needed to advance our understanding of the overall state and evolution of the climate system. In this proposal, we use independent constraints provided by space geodetic techniques to improve our knowledge and modeling of mass fluxes within the Earth system, and to evaluate and intercompare results from geophysical models of these processes. Included in the suite of measurements we will use are length-of-day (LOD), polar motion, geocenter motions, and low-order spectral components of the Earth’s gravity field (geopotential). Individually and in combination, these measurements can provide effective global-scale constraints on redistribution of water substance between its available reservoirs. They can thus be used as independent checks on the quality of simulations by the related process models, and can also be used in combination with more comprehensive, global models to increase our ability to track and understand ongoing changes in the cryosphere-hydrology-ocean system. We will investigate models from several sources with heavy focus on those produced by the Global Modeling and Data Assimilation Office at Goddard Space Flight Center.  Two examples from our ongoing work clearly demonstrate the strengths of geodetic methods in model validation and analysis. Dickey et al. (2002) combined results from glacial monitoring by the National Snow and Ice Data Center (NSIDC) with output from the altimeter- and XBT-assimilated ECCO model of ocean circulation to show that a pronounced anomaly in the Earth’s dynamic oblateness (J2), beginning in 1997-98, can be largely explained by a concomitant surge in sub-polar glacial melting and an equatorward mass shift originating in the Southern Ocean. In this proposal, we employ a detailed regional data-base of glacier mass balance under development by our collaborator M. Dyurgerov, along with high-resolution, global simulations of ocean circulation (including the Arctic basin) performed with the upgraded “cubed-sphere” version of the ECCO model, to more completely source and understand the recent and ongoing anomalous behavior in the Earth’s geopotential; data on interannual polar ice sheet mass balances will be included as it becomes available. We have also used geodetic series in combination with the output from hydrologic models to study closure of the J2 seasonal cycle, with the models being tentatively ranked according to the degree of closure achieved. In the course of our proposed work, we will expand this approach to include other geodetic observables (in particular center-of-mass and polar motion) in order to provide more robust consistency checks on models of water/ice mass balance and transport processes.  In summary, our overall strategy will be to investigate the effect of variations in separate reservoirs of water substance – sub-polar glaciers, polar ice sheets, ground water and snow (hydrology), and the ocean – on the Earth’s rotation, geopotential and center-of-mass. Comparisons with space-geodetic data will be used to evaluate different models of these sub-systems, and to gain insights into the physical processes which drive large-scale mass fluxes and their impact on geodetic observables. The results of our research will provide new approaches for process-model evaluation, and new tools for monitoring and understanding the origins of large-scale mass shifts in the Earth system.

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