Recent Accomplishments of the GRACE Mission I
Presiding: B Tapley, University of Texas; C Reigber, GeoForschungsZentrum Potsdam
G13A-01 13:30h
The GRACE Mission Status
The Gravity Recovery And Climate Experiment (GRACE) is in the 3rd year of its nominal 5-year operating lifetime. GRACE data have been providing unprecedented insights into the mass distribution and variability evident through measurements of the Earth's gravity field variations. The mean geoid errors have been reduced to less than 1-cm to harmonic degree 70; and monthly variations at 500 km and longer wavelengths are being tracked to 2-mm geoid accuracy. The seasonal and inter-annual variability is being used for applications in hydrology, oceanography, glaciology and geodesy. This presentation presents an overview of the Science applications from GRACE mission. We also review the mission status, including plans for improving the data quality and future data product releases.
http://www.csr.utexas.edu/grace/
G13A-02 13:50h
On Pertinent Use of GRACE K-Band Data
Thanks to its novel K-band measurement between its twin satellites orbiting at 470 km altitude, the GRACE mission is bringing enormous progress in the knowledge of the Earth gravity field. These measurements are made available as three different types of information: biased range, range-rate and range-acceleration, which are provided normally at 5 s interval (level-1B data files). Each one of these data types presents advantages and disadvantages for deriving gravity field coefficients. Biased range, specified at best between 10-4 and 10-1 Hz, should bring the most complete information, although it is exposed to some mismodelled device behavior, which requires estimating many empirical parameters in semi-dynamical orbit processing mode. Range-acceleration information coupled with accelerometer data can be processed faster to generate directly linear combinations of gravity field coefficients, avoiding the costly process of integrating partial derivatives together with the accelerations; however the gravity signal is weakened in the low to middle frequency band by the double derivation and its recovery depends on the quality of the applied derivation process. Alternatively, range-rate data, which are mostly used by present day investigators, could be seen as a good compromise. In order to quantify the interest of these three data types for gravity field determination, we have investigated their impact in terms of information contents, performance in gravity field model recovery, computational convenience... A summary and the conclusions of this study based on test cases will be presented.
G13A-03 14:05h
Observing Steric Sea Level Variations with GRACE and Satellite Altimetry
Satellite altimeters observe the combination of steric and non-steric sea level variations. Theoretically, GRACE should observe only non-steric sea level variations. Thus, one should be able to combine monthly maps of SL derived from GRACE with those from altimetry in order to estimate steric sea level. This is useful for combining with steric sea level computed from direct temperature/salinity profiles, or as a measure of steric sea level where there are no direct observations. We will describe how to properly combine the two data types (altimetry and GRACE), commenting on important corrections that need to be applied to each data type. We demonstrate the accuracy of the recovered steric SL variability by comparing the results with those determined from direct temperature/salinity profiles during 2002 to 2004, where there are sufficient numbers available.
G13A-04 14:20h
An Integrated Study of Sea Level Change Using Altimetry, Gravity, and In Situ Measurements
The TOPEX/Poseidon (T/P) and Jason satellite altimeter missions have provided a 12- year record of sea level change, which show an increase of global mean sea level of 2.8 +/- 0.4 mm/year, with considerable geographic variation. An important question for climate studies is to determine the cause of this change - specifically how much of the change is due to steric (heating) versus eustatic (runoff, melting ice, etc.) contributions? One method for answering this question is to analyze ocean temperature measurements to estimate the steric contribution, which when differenced with the altimetry leaves the eustatic component. These studies suggest that the steric and eustatic contributions are roughly equal over the last decade. The launch of the GRACE satellite gravity mission in 2002 now provides a method to directly measure the eustatic contribution through the changes it causes in the Earth's gravity field. GRACE measurements of monthly eustatic variations over the ocean are balanced by changes over the land that GRACE also measures, thus the GRACE dataset provides a unique opportunity to interpret global sea level variations in the context of the global water cycle. Current efforts have focused on interpreting the annual signals, but the analysis of interannual and secular variations is also being attempted. In addition, we compare monthly upper ocean heat storage estimates from in situ temperature measurements with thermal steric height variations derived from GRACE and altimetry. We will review the latest results from T/P, Jason, GRACE, and the in situ measurements, and place them in context with an overall evaluation of our current knowledge of sea level change in the altimetric era, and how this relates to changes in water storage over the continents.
G13A-05 14:35h
Excitation of Earth Rotation Variations "Observed" by Time-Variable Gravity
Time variable gravity measurements have been made over the past two decades using the space geodetic technique of satellite laser ranging, and more recently by the GRACE satellite mission with improved spatial resolutions. The degree-2 harmonic components of the time-variable gravity contain important information about the Earth's length-of-day and polar motion excitation functions, in a way independent to the traditional "direct" Earth rotation measurements made by, for example, the very-long-baseline interferometry and GPS. In particular, the (degree=2, order=1) components give the mass term of the polar motion excitation; the (2,0) component, under certain mass conservation conditions, gives the mass term of the length-of-day excitation. Combining these with yet another independent source of angular momentum estimation calculated from global geophysical fluid models (for example the atmospheric angular momentum, in both mass and motion terms), in principle can lead to new insights into the dynamics, particularly the role or the lack thereof of the cores, in the excitation processes of the Earth rotation variations.
G13A-06 14:50h
Long Wavelength Dynamics of an Ice Age Earth: Implications for GRACE
The remnant gravitational signature of the Earth's response to the Late Pleistocene glacial cycles, or glacial isostatic adjustment (GIA), is a primary target of the GRACE satellite gravity mission. At the longest wavelengths, numerical simulations of the secular variation in the geopotential due to GIA exhibit a pronounced (degree two, order one) anomaly associated with so-called `rotational feedback'. That is, a perturbation in the geopotential arising from GIA-induced deflections in the Earth's rotation vector. The amplitude of the feedback signal is a function of the adopted Earth model and it has been the source of ongoing debate within the GIA literature. We revisit this issue using a new treatment of the equations governing load-induced rotation perturbations on spherically symmetric, viscoelastic Earth models. We demonstrate that previous estimates of the rotational feedback signal in present day rates of change of the geopotential (and also crustal velocities) have been significantly overestimated by the traditional rotation theory. This result has important implications for the analysis of secular trends constrained by the GRACE satellite mission.