INSIGHTS INTO EARTH SYSTEM SCIENCE: VARIATIONS IN THE
EARTH’S ROTATION AND ITS GRAVITATIONAL FIELD (IAG,
IAPSO, IAMAS, IASPEI, IAGA, IAHS)
Location: Law Building 303 LT
Location of Posters: Old Gym
Wednesday 21 July AM
Presiding Chairs: J.O. Dickey, JPL/Caltech, Pasadena
C.R. Wilson, NASA/HQ, USA
JSG14/L/07-A3 0830
EARTH ROTATION MONITORING: STATUS AND PROSPECTS
Christoph REIGBER (GeoForschungsZentrum Potsdam, Div. Kinematics andDynamics of the Earth, Telegrafenberg A 17, D-14473 Potsdam, Germany, e-mail: reigber@gfz-potsdam.de)
Rotation is the major feature of our planet’s motion and its precise knowledge is required for various navigational, global geodetic and astronomical tasks. Irregularities in both the rotation rate and the position of the rotation axis in space are manifestations of changing mass redistributions within, on and outside of the planet and of torques acting on it. Their precise knowledge provide a unique and global measure of changes taking place in the various spheres of the Earth system.
International cooperative efforts for observing the Earth rotation date back almost exactly a century and have culminated in the space geodetic observation programs of the International Earth Rotation Service (IERS) which include satellite (SLR, LLR, GPS, DORIS) and radioastronomy (VLBI) based techniques. Analyses of the observations provide nowadays a monitoring of the Earth orientation and realizations of the terrestrial reference frame at the "0.3 mas accuracy level, corresponding to 1 cm on the Earth surface. This level of accuracy is continuously improving thanks to advances in technology, more robust tracking networks and improved data reduction and analysis techniques. The precise terrestrial reference frame sets the framework for regional networks being established for e.g. deformation monitoring, water vapor mapping and surveying tasks. The improved accuracy and temporal resolution of the Earth’s rotation parameters is triggering investigations of their subdiurnal to multi-year variability caused by geophysical processes taking place in the Earth system components. In order to support such practical applications and frontier research activities best, the IERS is presently adjusting its structure and functions to the user needs. One of the major challenges for the future will be to realize within an integrated global geodetic monitoring system highly accurate, consistent, long-term Earth rotation series with very high temporal resolution. The presentation will desribe the present status in monitoring the Earth rotation and is trying an outlook into near future developments and needs.
JSG14/E/12-A3 0900
ASSESSING THE PLANETARY ANGULAR MOMENTUM BUDGET WITH ATMOSPHERIC UPPER AIR AND OCEAN MODEL DATA
DAVID A. SALSTEIN, Rui M. Ponte, Richard D. Rosen (Atmospheric and Environmental Research, Inc., 840 Memorial Dr., Cambridge, MA 02139, USA, email: salstein@aer.com) Detlef Stammer (Massachusetts Institute of Technology, Department ofEarth, Atmospheric and Planetary Sciences, Cambridge, MA 02139, USA)
Atmospheric and oceanic information are used in combination with Earth rotation parameters to assess the angular momentum budget of the planet about the axis of rotation, on a variety of timescales. For the atmospheric data the 50-year long NCEP/NCAR reanalysis products capture variability at levels as high as 10 hPa. In addition, separate data sets of the stratosphere include comprehensive calculations on levels as high as 0.3 - 1 hPa. Because stratospheric winds vary strongly at semiannual, annual, and quasi-biennial timescales, incorporating these data is particularly important. We assess the regions within the atmosphere that have the strongest variations on intraseasonal, seasonal, and interannual timescales. In the last case, El Niño events are clearly related to the strength of the zonal circulation in the atmosphere. Available estimates of oceanic axial angular momentum, calculated from model-derived zonal current and mass fields, are also used to assess the role of the ocean in explaining some of the discrepancies still present in the solid Earth and atmosphere's combined angular momentum budget. In considering excitations for polar motion in the equatorial plane, atmospheric and oceanic effects can approach similar magnitudes on a number of timescales. Within the atmosphere, the mass field, related to surface pressure, is the agent that most excites polar motion on subseasonal scales. We discuss the extent to which a closed budget is achieved within the estimated uncertainties in all momentum quantities, as a function of timescale.
JSG14/L/08-A3 0930
INFLUENCE OF CORE DYNAMICS ON THE ROTATION OF THE EARTH
Gauthier HULOT (Institut de Physique du Globe de Paris, 4 Place Jussieu, 75252 Paris Cedex 05, France, email: gh@ipgp.jussieu.fr)
The visco-elastic body rotation of the Earth's mantle, usually referred to as the "Earth's rotation", is well known to display variations. These variations occur as a result of some mechanical forcing acting on the mantle and produced by both external interactions (Luni-Solar tidal forces) and internal interactions (with the fluid enveloppes and the core). This forcing produces changes within the angular momentum of the mantle, which results in the observed changes within both the rate of rotation (Length of Day, LOD) and the position of the rotation axis. The present review will focus on the specific influence of core dynamics, thought to play a dominant role in modifying the Earth's rotation on decade time scales. We will briefly recall the two approaches currently used for studying this influence. One consists in inferring the core angular momentum (AM) from magnetic observations, assuming conservation of core+mantle AM and checking that variations within the Earth's rotation are correctly predicted. The other consists in searching for core-mantle coupling mechanisms that can produce such variations. We will then first focus on the LOD variations and present a set of results obtained by various teams. It will be shown that the AM approach clearly leads to the conclusion that core dynamics is responsible for the decade variations within the LOD. Results obtained through the second approach are not as clear cut, and arguments in favor or against each type of coupling (electromagnetic, topographic, or gravitational) will be discussed, together with the possible role of the inner core. We will next turn to the possible influence of core dynamics on the position of the rotation axis. This subject has received much less attention up to now. This is likely due to the combined facts that the dynamics of the core (under strong influence of the Coriolis forces) does not permit an easy approch of the problem in terms of AM conservation, and that the variation of the position of the rotation axis (usually determined in terms of the position of the pole of rotation within the mantle frame of reference) is not very well constrained on decade time scales. It will however be argued that core dynamics can also produce variations that are nearly compatible with the available observational constraints. Finally, the possible influence of core dynamics occuring on time scales less than one year (as testified by the occurrence of so-called geomagnetic jerks) will be briefly discussed.
JSG14/L/09-A3 1000
NUTATIONS: OBSERVATIONS, THEORETICAL MODELING, AND IMPLICATIONS
P. M. Mathews, (Department of Theoretical Physics, University of Madras, Chennai 600025, India) email: mathews@imsc.ernet.in T. A. Herring, (Department of Earth, Planetary and Atmospheric Sciences, M.I.T., Cambridge, MA, U.S.A), B. A. Buffett (Department of Geophysics and Astronomy, University of British Columbia, Vancouver, B.C., Canada)
The VLBI data available now permit estimation of the amplitudes of a large set of nutation terms with a formal precision much better than 10 except for a few terms. Recent advances in theory enable these estimates to be closely matched by model predictions. Apart from an excess (nonhydrostatic) flattening of the core mantle boundary (CMB), inferred earlier from nutation data, and the effects of mantle anelasticity and ocean tides on nutations, the couplings of the fluid outer core to the mantle and to the solid inner core through magnetic fields at the CMB and the inner core boundary (ICB) are found to have a significant role in accounting for the observations. The inner-core-induced normal mode (PFCN or FICN) plays an important part in this context. We shall present the estimates obtained for various parameters by a least squares fit of analytical theory to data, and their interpretation in geophysical terms. A precise estimate for the Earth's ellipticity, magnetic coupling parameters at the CMB and ICB and their implications, revision of the estimate for the CMB flattening, questions regarding the inner core flattening, and possibilities for constraining an elasticity and ocean tide models, will be discussed
JSG14/W/05-A3 Poster 1100-01
CORE: 19+ YEARS OF VLBI UT1/LOD, AAM, EL NIÑO AND LA NIÑA
THOMAS A CLARK (Email: clark@tomcat.gsfc.nasa.gov), Chopo Ma (Email: cma@leo.gsfc.nasa.gov) and Ben Chao (Email: chao@denali.gsfc.nasa.gov), all at NASA/GSFC, Laboratory for Terrestrial Physics, Greenbelt MD 20771 USA.
Since 1980, the global VLBI network has produced more than 2000 one-day determinations of UT1 with a frequency of at least one measurement per week. The accuracy of the early measurements was hundreds of msec, while more recently ~5 msec is common. We have used this 19+ year record of UT1 to derive Length of Day (LOD), which mirrors changes in angular momentum in the fluids in earth’s interior, atmosphere and hydrosphere. Observed decadal-scale variations arise largely from changes in the earth’s core, and from global-scale climatic and ocean circulation variability which affect LOD with a peak-to-peak range ~1 msec over the 19+ year data set.
On shorter time scales, LOD is highly correlated with total atmospheric angular momentum (AAM). The annual average LOD signature due to AAM has a peak-to-peak amplitude ~900 msec. LOD is highly correlated with AAM derived from various global climate models (GCMs) and we present detailed comparisons with the NCEP GCM. We note that VLBI data has proven to be a useful, independent proxy for validation of various GCMs.
After removing the decadal and average annual components, LOD shows the interannual to intra-seasonal variability of the atmosphere. Of particular interest is the LOD signature of the 1997/8 El Niño/La Niña events when LOD increased ~ +400 msec in ~1997.2 , followed by a ~-600 msec transition into La Niña in 1998.4, and with an integrated "sundial error" of ~0.2 seconds. The observed LOD variations are highly correlated with conventional ENSO indices; it is rather remarkable that radio astronomical observations of quasars can be so closely correlated with barometric pressure differences between Darwin and Tahiti!
The VLBI community has begun a program that will improve the temporal coverage and accuracy of earth orientation measurements. The new Mark-4 VLBI data system will give a twofold increase in the sensitivity and a significant improvement in correlation capability. This program has been named CORE: Continuous Observation of the Rotation of the Earth.
JSG14/E/03-A3 Poster 1103-02
BENEFITS FROM A COMBINED GPS/GLONASS ANALYSIS FOR EARTH ROTATION STUDIES
Markus ROTHACHER (Astronomical Institute, University of Berne, Sidlerstr. 5, M CH-3012 Berne, Switzerland, email: rothacher@aiub.unibe.ch) Robert Weber (Department of Advanced Geodesy, Technical University of Vienna, Gusshausstr. 27-29, A-1040 Vienna, Austria)
Since the start of the International GLONASS experiment (IGEX) in October 1998, the Center for Orbit Determination in Europe (CODE) is processing a global network of GPS/GLONASS receivers on a routine basis. This development opens up the possibility to study the benefits that may result from a combination of the IGEX solutions with the global GPS-only solutions (IGS network) for the estimation of earth rotation parameters (ERPs). An improvement in the ERP estimates is to be expected mainly because of the following circumstances: (1) by adding GLONASS data, the number of satellites involved increases from 27 (present GPS constellation) to about 41;(2) the different orbit inclinations of the two systems (55 and 65 degrees in the case GPS and GLONASS, respectively) should help to separate LOD and nutation rate estimates from variations in the orbital elements common to all satellites; (3) because the revolution periods of the GLONASS satellites, in contrast to those of GPS, are not in a 2:1 commensurability with the Earth's rotation, the impact of orbital biases on the determination of subdaily ERPs are less critical; (4) the improved satellite geometry should result in a better decorrelation of station heights and troposphere zenith delay parameters and thus, indirectly, to more accurate ERP estimates.RPs from combined GPS/GLONASS solutions will be presented and they will be compared to values from GPS-only solutions to assess the impact of the factors listed above.
JSG14/E/22-A3 Poster 1106-03
COMBINATION OF VLBI, GPS AND SLR DATA AT THE OBSERVATION LEVEL-NEW RESULTS
Per Helge Andersen (Forsvarets forskningsinstitutt, P. O. Box 25, N-2007 Kjeller, Norway, and
Institute of Theoretical Astrophysics, P. O. Box 1029, Blindern 0315, University of Oslo, Norway,
email: per-helge.andersen@ffi.no)
A significant number of VLBI and SLR stations are equipped with GPS receivers. A few true fundamental stations with all three techniques even exist. Each technique has its strength and weakness with respect to the determination of geodetic parameters and together they complement each other in a way that should be fully taken advantage of in the data analysis. The simultaneous analysis of different data types at the observation level, due consideration of the physical interrelations, and presentation of results in a common reference system, are the main ideas behind the development of the GEOSAT software. A new and improved version of the software has been implemented with an automatic generation of 1) observation residuals and observation partial derivatives for VLBI, GPS and SLR, consistent at the 0.1 ppb level, and 2) a simultaneous arc-by-arc UD-filtering at the observations level. A very advanced multi-level (presently four parameter levels including stochastic parameter representations at each level) SRIF arc combination software for long-term solutions has been developed and validated. The main elements of the processing scheme will be presented. The first results with a simultaneous analysis of different space geodetic data types (VLBI, GPS, and SLR) at the observation level were presented at the IERS-98 symposium in Potsdam. That analysis was based on data from Jan 12 - Jan 23 in 1994. The analysis has recently been extended with significantly more observations. Results from the new analysis will be presented.
JSG 14/E/04-A3 Poster 1109-04
MONITORING GLOBAL GEOPHYSICAL FLUIDS BY SPACE GEODESY
B. F. CHAO (Space Geodesy Branch, NASA's Goddard Space Flight Center, Greenbelt, Maryland 20771, USA; email: chao@denali.gsfc.nasa.gov), V. Dehant, R. S. Gross, R. D. Ray, D. A. Salstein, M. Watkins
Since its establishment on 1/1/1998 by the International Earth Rotation Service, the Coordinating Center for Monitoring Global Geophysical Fluids (MGGF) and its seven Special Bureaus have engaged in an effort to support and facilitate the understanding of the geophysical fluids in global geodynamics research. Mass transports in the atmosphere-hydrosphere-solid Earth-core system (the "global geophysical fluids") will cause the following geodynamic effects on a broad time scale: (1) variations in the solid Earth's rotation (in length-of-day and polar motion/nutation) via the conservation of angular momentum and effected by torques at the fluid-solid Earth interface; (2) changes in the global gravitational field according to Newton's gravitational law; and (3) motion in the center of mass of the solid Earth relative to that of the whole Earth ("geocenter") via the conservation of linear momentum. These minute signals have become observable by space geodetic techniques, primarily VLBI, SLR, GPS, and DORIS, with ever increasing precision/accuracy and temporal/spatial resolution. Each of the seven Special Bureaus within MGGF is responsible for calculations related to a specific Earth component or aspect -- Atmosphere, Ocean, Hydrology, Ocean Tides, Mantle, Core, and Gravity/Geocenter. Angular momenta and torques, gravitational coefficients, and geocenter shift will be computed for geophysical fluids based on global observational data, and from state-of-the-art models, some of which assimilate such data. The computed quantities, algorithm and data formats are standardized. The results are archived and made available to the scientific research community. This paper reports the status of the MGGF activities and current results.
JSG14/W/18-A3 Poster 1112-05
THE ROTATIONAL AND GRAVITATIONAL EFFECT OF EARTHQUAKES
Richard GROSS (Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA, email: Richard.Gross@jpl.nasa.gov)
The static displacement field generated by an earthquake has the effect of rearranging the Earth's mass distribution and will consequently cause the Earth's rotation and gravitational field to change. Although the coseismic effect of earthquakes on the Earth's rotation and gravitational field have been modeled in the past, no unambiguous observations of this effect have yet been made. However, the Gravity Recovery And Climate Experiment (GRACE) satellite, which is scheduled to be launched in 2001, will measure time variations of the Earth's gravitational field to high degree and order with unprecedented accuracy. In this presentation, the modeled coseismic effect of earthquakes upon the Earth's gravitational field to degree and order 100 will be computed and compared to the expected accuracy of the GRACE measurements. In addition, the modeled second degree changes, corresponding to changes in the Earth's rotation, will be compared to length-of-day and polar motion excitation observations.
JSG14/E/15-A3 Poster 1115-06
LONG-PERIOD TIDAL VARIATIONS OF THE EARTH'S ROTATION RATE
SHAILEN D. DESAI and Richard S. Gross (both at Jet Propulsion Laboratory, California
Institute of Technology, Pasadena, CA 91109, U.S.A, email: Shailen.D.Desai@jpl.nasa.gov, Richard.Gross@jpl.nasa.gov), John M. Wahr (Department of Physics, University of Colorado, Boulder, CO 80309-0390, U.S.A., email: wahr@lemond.colorado.edu)
Long-period tidal variations of the Earth's rotation rate are caused by the redistribution of mass associated with the respective elastic solid Earth tides, the ocean tide heights, and the anelasticity of the Earth's mantle, and by the relative angular momentum associated with the long-period ocean tide currents. The dominant contribution, from the elastic solid Earth tides, is known to high accuracy from theoretical and numerical analyses. However, the lack of global observations of the long-period ocean tides has limited the accuracy of predicted contributions from these ocean tides. The anelastic response of the mantle at frequencies smaller than the seismic frequencies are somewhat uncertain but could be inferred from observations of the long-period tidal variations of the Earth's rotation rate with improved predictions of the contribution from the ocean tides.
Almost global empirical models of the monthly and fortnightly ocean tides estimated from TOPEX/POSEIDON altimetry are used to predict their respective contributions to variations of the Earth's rotation rate. Observed long-period tidal variations of the Earth's rotation rate are estimated from the SPACE98 time series and the residual between these observations and the sum total of the predicted contributions from the elastic solid Earth tides and the ocean tides is used to infer anelastic properties of the Earth's mantle at the monthly and fortnightly periods.
JSG14/L/01-A3 Poster 1118-07
TEMPORAL VARIATIONS OF OCEAN TIDAL TERMS IN UT1–UTC
Oleg TITOV (Astronomical Institute of Saint-Petersburg University, Bibliotechnaya sq., 2, Petrodvorets, Saint-Petersburg, 198904, Russia, email: oleg_titov@usa.net)
Global redistribution of mass water causes a variations in Universal Time. VLBI technique allows to detect and estimate the corresponding oceanic tides with periods near 12 and 24 hours. Usually, scientists obtain the estimates using global set of VLBI data. Possibility of the tides variations from year to year is considered this paper. About 500 VLBI experiments (NEOS-A, CONT’96, etc.) from 1991 till 1998 year were analyzed. All reductional calculations have been made using OCCAM software. Least squares collocation method (LSCM) provides us the time series of UT1–UTC variations with high temporal resolution (one point for 3-5 minutes). The full number of points exceeds 100.000. Therefore the standard deviation of yearly estimates is reasonable. Variations of UT1–UTC obtained using the LSCM are in a good agreement with ones recommended by IERS Conventions. Estimates of main tidal terms in UT1–UTC independently for every year as well as for full set of data are presented. It is shown that the yearly estimates are slightly differ from each other. The various reasons of the effect are discussed.
JSG14/W/09-A3 Poster 1121-08
ATMOSPHERIC ANGULAR MOMENTUM FUNCTIONS SIMULATED WITH THE JMA GLOBAL MODEL FOR THE PERIOD OF 1955-1994
I. Naito (National Astronomical Observatory, Mizusawa, Iwate, Japan), Y-H Zhou (Shang Hai Observatory, Shang Hai, China), M. Sugi (Meteorological Research Institute, Tsukuba, Japan),
R. Kawamura (Toyama University, Toyama, Japan), and N. Sato (Japan Meteorological Agency, Tokyo)
Axial and equatorial atmospheric angular momentum (AAM) functions calculated from an ensemble mean data of the three independent 40-year integrated values for the period of 1955-1994 under the observed near-global sea surface temperature (SST) condition by Japan Meteorological Agency (JMA) global model are discussed in comparison with those calculated from the reanalysis data during 1968-1994 of National Center for Environmental Prediction (NCEP) and from JMA's operational objective analysis data during 1984-1994 and the inferred functions during 1968-1994 from the observed length of day (LOD) and polar motion data, respectively. For comparisons in seasonal variation during 1984-1994, in general, the wind term for the axial AAM function agrees well with those from JMA's operational and NCEP's reanalysis data and roughly with the inferred function from the observed LOD. However, its semi-annual term is over-estimated, suggesting that the model unreliably simulates the subtropical zonal winds. On the other hand, the pressure term for the equatorial AAM function is considerably over-estimated with respect to those from JMA's operational and NCEP's reanalysis data, indicating that the SST driven global atmospheric motion induces too large seasonal variations in redistribution of atmospheric mass occurring mainly between the Eurasian Continent and the North Pacific Ocean.
JSG14/W/14-A3 Poster 1124-09
SEASONAL-SCALE THREE DIMENSIONAL ANGULAR MOMENTUM
BUDGETS FOR ATMOSPHERE-MANTLE SYSTEM
Yuichi Aoyama (School of Mathematical and Physical Science, The Graduate University for Advanced Studies, Mizusawa, 023-0861, Japan, email: aoyama@miz.nao.ac.jp) Isao Naito (National Astronomical Observatory, Mizusawa, 023-0861, Japan, email: naito@miz.nao.ac.jp)
Atmospheric contributions to seasonal variations of length of day (LOD) and wobble are evaluated during 1988 - 1997. The data sets used here are SPACE97 for LOD, EOP97C04 for wobble, and two atmospheric angular momentum (AAM) functions calculated from objective analysis data of Japan Meteorological Agency (JMA) and reanalysis data of National Centers for Environmental Prediction (NCEP). The axial AAM functions agree well in annual variation and roughly in semi-annual variation with inferred function from the observed LOD. However, there is a discrepancy between the equatorial AAM functions and the inferred function from the wobble in annual variation, where the wind terms show a considerable disagreement between JMA and NCEP, which arises mainly from tropospheric wind. Spatial distributions of regional wind contributions in annual variation to the LOD and the wobble are further discussed. In semi-annual variation for equatorial component, on the other hand, the AAM functions explain only a small part of the observed wobble, indicating that there remain a major other source.
JSG14/W/23-A3 Poster 1127-10
ON THE CHANDLER WOBBLE AND ITS EXCITATION
Dr Aleksander Brzezinski
For the estimation of the Chandler wobble parameters it is important that the input time series expressing polar motion and its excitation are possibly long and homogeneous in time. The two series which seem to meet such requirements, hence are potential sources of new information about the Chandler wobble, are: 1) polar motion (PM) series spanning 1899.7-1992.0, derived from a new global adjustment of the optical astrometry observations by Vondrak et al. (1998), and 2) 40-years time series of the atmospheric angular momentum (AAM) estimates, computed on the basis of results of the NCEP/NCAR reanalysis project by Salstein and Rosen (1997). In this presentation we report an attempt to obtain from the new PM and AAM series improved estimates of the Chandler wobble parameters, the frequency of resonance F and the quality factor Q. Different stochastic models are used to express the wobble excitation. Finally, based on the AAM reanalysis data we address the problem, to which extent the observed Chandler motion can be explained by the atmospheric excitation.
JSG14/L/03-A3 Poster 1130-11
VARIATIONS IN EARTH'S GRAVITATIONAL FIELD CAUSED BY THE POLE TIDE
S.R. DICKMAN (Department of Geological Sciences, State University of New York, Binghamton, NY 13902-6000, USA, email: dickman@bingvmb.cc. binghamton.edu)
The pole tide is the oceanic response to the Chandler wobble. The re-distribution of mass accompanying the pole tide causes periodic changes in Earth's gravity field. If the Chandlerian polar motion m is decomposed into prograde and retrograde components according to m = MP exp(ist) + MR exp(-ist), where s is the Chandler frequency, then the pole tide's effects on gravity can be theoretically predicted in terms of MP, MR, and oceanic response factors.
I have employed a previously developed spherical harmonic dynamic ocean tide model to calculate the open-ocean response factors for the pole tide. Using these factors and the Chandler wobble amplitude observed over the past decade, I have computed the 14-month periodic contributions to the spherical harmonic coefficients of the gravitational potential. For example, including tidal loading of the underlying solid earth, the pole tide's contribution to J2 is typically ~1.3x10^-12, whereas its contributions to C21 and S21 are typically ~3.6x10^-11 and ~2.2x10^-11, respectively.
In addition to these results, I will also discuss how observationally based oceanic response factors -- determined, e.g., from satellite analyses or tide gauge data -- could be used to produce alternate estimates of the pole tide contributions to gravity. An illustration including the North and Baltic Seas, where the pole tide has long exhibited dramatic enhancements, will be presented.
JSG14/L/04-A3 Poster 1133-12
BOUNDS ON CORE-MANTLE COUPLING AT SEASONAL PERIODS FROM
COMBINED ROTATIONAL AND GRAVITATIONAL DATA
S.R. DICKMAN (Department of Geological Sciences, State University of New York, Binghamton, NY 13902-6000, USA, email: dickman@bingvmb.cc. binghamton.edu)
Excitation of variations in the Earth's rotation is often separated into 'matter' and 'motion' terms, which act to change Earth's inertia tensor and provide relative angular momentum, respectively. The outcome of any such excitation also depends on how well the core and mantle are rotationally coupled. Geophysicists have generally assumed zero axial coupling on time scales shorter than decadal. Recent work [Dickman & Nam 1995], in which observed and predicted tidal changes in the length of day were compared, found that such coupling is indeed weak on a nine-day time scale. However, the situation is unclear at longer periods. Knowing the frequency dependence of rotational coupling would yield more accurate estimates of excitation, and could provide clues to the state of Earth's deep interior.
A wide variety of processes -- some of which are not very well quantified -- contribute to seasonal changes in the length of day. Comparisons between observations and predictions, with the latter based on differing amounts of core-mantle coupling, may be too uncertain to allow the true extent of coupling to be resolved. Alternatively, recognizing that second-degree gravity coefficients reflect the total mass re-distribution contributing to rotational excitation [e.g. Eanes et al. 1997], we can incorporate the seasonal components of time- variable gravity data into our predictions, leaving only the 'motion' terms to be modeled (e.g. ocean currents) or measured (e.g. winds).
Preliminary bounds on the extent of axial coupling at seasonal periods will be discussed.
JSG14/E/08-A3 Poster 1136-13
ATMOSPHERIC FORCING OF POLAR MOTION AND CLIMATE PATTERNS
Jolanta NASTULA (Space Research Centre of the PAS, 00-716 Warsaw, Bartycka 18a, Poland,
Email: nastula@cbk.waw.pl) and David A. Salstein (Atmospheric and Environmental Research, Inc. 840 Memorial Drive Cambridge, MA 02139, USA, Email: salstein@aer.com)
Atmospheric excitation of polar motion occurs by means of angular momentum exchange with the solid Earth in the equatorial plane. We assess such atmospheric forcing by comparing geodetic polar motion and atmospheric excitation functions. Atmospheric excitation of polar motion has different strengths on various time scales; we contrast the "subseasonal" and the "seasonal-interannual" regimes, with 150 days, the boundary between the two. Atmospheric excitation functions were computed from the new 50 year - long reanalysis system of the U.S. National Centers for Environmental Prediction and National Center for Atmospheric Research, which include effects of winds up to 10 hPa and surface pressure. In exploring how different regions were responsible for high frequency polar motion, we had already isolated the Eurasia, North American, and other regions as important for exciting high frequency polar motion, with the northern and central Eurasian region (EUR) especially prominent in this regard. Here we search for connections between EUR and certain well-known atmospheric behavior: the El Nino, the North Atlantic Oscillation, and the North Atlantic/Eurasian patterns. Additionally, empirical orthogonal function were computed for both temporal bands. The first such modes contain patterns with strong variations over southern oceans.
However, use of the inverted barometer correction (IB) reconfirm the overriding importance of Eurasia. Using the results of an ocean model run, we assess the regional variability of the excitation function within the oceans also.
JSG14/W/15-A3 Poster 1139-14
INTERANNUAL VARIATIONS IN LENGTH-OF-DAY AND ATMOSPHERIC ANGULAR MOMENTUM WITH RESPECT TO ENSO CYCLES
Joachim Hoepfner (GeoForschungsZentrum Potsdam, Division 1: Kinematics and Dynamics of the Earth, Telegrafenberg, D-14473 Potsdam, Germany, email: ho@gfz-potsdam.de)
At interannual time scales, the excitation of variations in Length-Of-Day (LOD) is caused by two prominent signals in the atmosphere: The El Nino-Southern Oscillation (ENSO) and the Quasi-Biennial Oscillation (QBO). In this study, the axial Atmospheric-Angular-Momentum (AAM) component CHI3 is related to changes in LOD. Focussing on the interannual variations in the solid Earth-atmosphere axial angular momentum budget, we consider the Low- Frequency Component and the Quasi-Biennial Oscillation in LOD and AAM in their temporal variability. In particular, we use the time series of LOD data of the series EOP (IERS) 97C04 from 1962 to 1998 and of CHI3 data of the series AAM (NCEP) Reanalysis from 1958 to 1998, AAM (JMA) from 1983 to 1998, AAM (ECMWF) from 1988 to 1996, and AAM (UKMO) from 1986 to 1998. To separate the interannual signals, we have applied low-pass and band- pass filters. For comparison, the monthly data of the Southern Oscillation Index (SOI), as given by the NOAA by the difference in the surface pressure between Tahiti and Darwin, Australia, have been processed and are exhibited in the same manner. Concentrating on interannual time scales, the main results are quantitative estimates of the variability of two components, in particular of the Low-Frequency Component and the Quasi-Biennial Oscillation, in the LOD and AAM variations and also in the SOI variations and quantitative estimates of the total interannual AAM and SOI signals. Besides, the decadal LOD component is available as a function of time. The results show the character and the time evolution of the various portions. They should contribute to the problem of the interpretation concerning the interannal LOD changes associated with the global-scale ENSO cycles.
JSG14/L/05-A3 Poster 1142-15
ATMOSPHERIC ANGULAR MOMENTUM VARIABILITY SIMULATED WITH THE ECHAM3-T21 GLOBAL CIRCULATION MODEL AND ITS INFLUENCE ON THE EARTH'S ROTATION PARAMETERS
JOCHEN ELBERSKIRCH, Andreas Hense (both at Meteorologisches Institut, Universit at Bonn,
Auf dem Hugel 20, 53121 Bonn, Germany, e-mail: jelbers@uni-bonn.de
The simulated Atmospheric Angular Momentum (AAM) budget of the ECHAM3-T21 global circulation model (GCM) is analysed. Five simulations with different initial conditions are forced by observed monthly means of sea surface temperature and global ice coverage (GISST-dataset from Hadley center, Bracknell) for the corresponding period from 1949 to 1994. The three components of AAM are compared to the Earth's rotation parameters (change of length of day and polar motion), respectively. A frequency analysis is performed with Fourier- and Wavelettransformation. The third component of simulated relative AAM for the mean of the five simulations shows significant variability on different timescales, particular on the interannual timescale and it correlates very well with the timeseries of the change of LOD. The correlation of the other components are somewhat lower, which is not surprising. For the understanding of the physical process of the AAM transfer between atmosphere, ocean and solid earth, a canonical correlation analysis is carried out. This technique yields typical spatial anomaly patterns of maximum correlation between AAM and torques for all components. The application of lagged correlation analysis shows the spatial development of anomalous mountain torques if anomalies of relative AAM are existent. The mountain torque offers a very surprising pattern before and when the maximum anomaly of relative AAM occurs. To compare the results of the ECHAM3 model the NCEP Reanalysis data from 1958 - 1997 are also used.
JSG14/L/08-A3 Poster 1145-16
INTERANNUAL VARIABILITY IN EARTH ROTATION AND ATMOSPHERIC ANGULAR MOMENTUM: EL NIÑO CONNECTIONS
J. O. DICKEY (Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109-8099; tel 818–354–3235; e-mail: jean.o.dickey@jpl.nasa.gov); P. Gegout (Ecole et Observatoire des Sciences de la Terre Laboratoire de Dynamique Globale, rue Rene Descartes 67084 Strasbourg Cedex, France; Tel: [33] 3 88 41 66; email: Pascal.Gegout@eost.u-strasbg.fr); S. L. Marcus (Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109-8099; tel: 818–354–3477; e-mail: steven.l.marcus@jpl.nasa.gov)
Comparisons between length of day (LOD) and the strength of the ENSO cycle, represented by the Southern Oscillation Index (SOI, the difference in sea level pressure between Darwin and Tahiti) series, have indicated striking agreement, with high interannual values of LOD generally coinciding with ENSO events. During an ENSO event, the SOI reaches a minimum, leading to an increase in atmospheric angular momentum (AAM) associated with the collapse of the tropical easterlies. Further increases in AAM may result from a strengthening of westerly flow in the subtropical jet streams. Conservation of total angular momentum then requires the Earth’s rate of rotation to slow down, thus increasing LOD.
The impact of the 1997-98 ENSO event will be presented in context of angular momentum exchange utilizing LOD, SOI and AAM (both global and latitudinally belted) data. We have utilized the NCEP reanalysis from 1959 to 1998 from the surface to 100mb to examine the effect of the tropospheric zonal winds. Special emphasis will be placed on the globally coherent polarward propagation observed on interannual time scales; decadal variability will also be addressed. The techniques utilized include traditional recursive filter and multi-channel singular spectrum analysis (M-SSA); comparisons will be made with previous events, especially the 1982–83 event.
JSG14/E/06-A3 Poster 1148-17
OCEAN INDUCED VARIATIONS OF EARTH ROTATION ON TIME SCALES FROM HOURS TO YEARS - RESULTS FROM A NUMERICAL WORLD OCEAN MODEL
Juergen SUENDERMANN, Maik Thomas (both at Institut fuer Meereskunde der Universitaet Hamburg, Troplowitzstr. 7, D-22529 Hamburg, Germany, email: suendermann@ifm.uni-hamburg.de)
The world oceans motion and mass fields cause significant variations of Earth orientation parameters on various time scales. Generally, these ocean induced changes are attributed either to tidal forces or variations of the general circulation. Analogously, numerical world ocean models can still be divided into Ocean General Circulation and tidal models. But it is questionable if the neglect of non linear interactions by a linear superimposition of both components of motion is accurate, especially with respect to frequencies that are typical for both circulation and tides. To include non linear interactions an OGCM is additionally forced with the complete astronomical tidal potential without decomposition into fourier components.
Long-time integration and a time step of one hour offer a wide spectrum of analysis of ocean induced oscillations of Earth rotation. Time series of relative and rotational angular momentum resulting from three model runs from 1949 to 1994 with different forcing conditions are used to examine the role of the oceans with respect to excitation of polar motion and changes in length-of-day on time scales from hours to years. Since the model allows to treat the particular contributions of the three oceans separately, characteristic frequencies of global ocean basins can be identified.
JSG14/W/10-A3 Poster 1151-18
THE ROLE OF ATMOSPHERE AND OCEAN IN EXCITING VARIABILITY IN THE EARTH'S ROTATION AND GRAVITY FIELD
Thomas J. JOHNSON (U.S. Naval Observatory, 3450 Massachusetts Ave. NW, Washington, DC 20392-5420, USA, email: tj@casa.usno.navy.mil) Clark R. Wilson (NASA Headquarters, Code YS, 300 E St. SW, Wash. D.C. 20546; and The University of Texas at Austin, Austin, TX. 78712,
email: cwilson@hq.nasa.gov) Benjamin F. Chao (NASA Goddard Space Flight Center, Code 926, Space Geodesy Branch, Greenbelt, MD 20771 email: chao@denali.gsfc.nasa.gov)
This research examines the role of the atmosphere and ocean in exciting the temporal variability in the Earth's rotation and gravity field. The main focus of this paper will be on the role of the ocean as predicted by 10 years of data products from the Parallel Ocean Climate Model. Unlike earlier rigid-lid models, this free-surface ocean model allows the estimation of the variability in the oceanic angular momentum and Stokes coefficients resulting from mass redistribution and relative motion that is more representative of the real ocean. First, we compare the atmosphere and ocean model's angular momentum time series to the polar motion and length of day variations. Then, we use the Stokes coefficients computed from the atmosphere and ocean model's mass redistribution to examine the Earth's temporally varying gravity field. These temporal variations are observable as variations in the Earth's geocenter location and as perturbations of an orbiting satellite's orbital elements. We also investigate the seasonal and interannual variability predicted by the atmosphere and ocean models. These comparisons can be useful in estimating the possible role of other geophysical processes, such as continental water storage in exciting the variations in the Earth's rotationand gravity field.
JSG14/W/07-A3 Poster 1154-19
HYDROLOGICAL AND OCEANIC EXCITATIONS OF EARTH ROTATION
C. R. Wilson [1,2,3] and J. L. Chen [1] Center for Space Research, University of Texas at Austin
[1] Department of Geological Sciences, University of Texas at Austin [2] NASA Headquarters,
Washington D.C. [3]
Motion and redistribution of water on the continents and in the oceans are likely to be the major contributors to the residual polar motion excitation not accounted for by the atmosphere. We investigate continental water storage change and its excitation of polar motion and length-of-day (LOD) variation using both a data assimilating hydrological model and climatological average datasets. We estimate oceanic mass contributions to earth rotation changes from TOPEX/Poseidon sea level observations adjusted by a simplified steric sea level change model. The results are compared with the residual excitations after atmospheric contributions are removed using the NASA GSFC GEOS-1 atmospheric model. Our findings include the following: Continental water storage change plays an important role in exciting polar motion and LOD at seasonal time scales; hydrological models exhibit significant differences with regard to the predicted contribution to earth rotation changes, indicating the immaturity of understanding of the global hydrological cycle; The assimilated-data hydrological model provides generally better agreement with earth rotation observations; and non-steric sea level changes indicate a significant polar motion and LOD excitation source from ocean mass redistribution over a wide range of time scales.
JSG14/W/08-A3 Poster 1157-20
EARTH ROTATION AND OCEAN CIRCULATION INTERACTION
Nils-Axel Mörner Paleogeophysics & Geodynamics, S-10691 Stockholm, Sweden,
email: morner@pog.su.se
The ocean circulation is not only driven by thermo-haline forces. The interchange of angular momentum between the hydrosphere and the solid Earth is another fundamental factor. This can be demonstrated for the ENSO-events, for the last centuries' instrumental data, for decadal-to-century changes during the Holocene, for the high-amplitude climatic-eustatic changes some 13-10 ka ago, and for the main glacial/interglacial cycles. The oceans have a remarkable heat-storing capacity. Any change in the ditribution of the water masses will have strong effects on the global climatic temperature distribution. The oceans contain huges masses of water of a reasonably high density that are constantly circulating both horizontally and vertically over the globe. Any irregilarity in this circulation leads to a redistribution in total mass which affects the sea level and has to be compensated by an interchange of angular momentum between the hydrosphere and the solid Earth. This means that we in the recorded past changes in climate and in sea level can obtain information about possible past changes in ocean circulation, too. And this is exactly what our paleo-records seem to indicate. During sunspot minima, the solar wind intensity decreased (read by increased in-fall of cosmic ray increrasing the 14C production and 10Be content) which led to a speeding-up of the Earth’s rate of rotation affecting the ocean circulation so that the a major current like the Gulf Stream sent less warm equatorial water along its northern branch towards NW Europe at the same time as Arctic water penetrated further south giving rise to severe cold periods in NW Europe and warm periond in S. Europe and N. Africa. The sudden changes in rotation at around 13,000 BP and 1000 AD, were both linked to trans-polar geomagnetic VGP-shifts and deformations of the gravitational potential surface besides their strong signals in the ocean circulation system.
JSG14/W/04-A3 Poster 1200-21
TEN-YEARLY POLAR MOTION CONNECTED WITH PRECIPITATION CHANGES OVER NORTH AMERICAN AND EURASIAN CONTINENTS
Tetsuya Iwabuchi (The Graduate University for Advanced Studies, Mizusawa, 023-0861, Japan,
email: iwabuchi@miz.nao.ac.jp) Isao Naito (National Astronomical Observatory, Mizusawa, 023-0861, Japan, email: naito@miz.nao.ac.jp)
A hydrologic excitation model of ten-yearly polar motion appearing in the combined polar motion data of five-daily IPMS and SPACE94 during 1962-1995 is proposed. The model is based on the exponential decay of the land water storage supplied by precipitation. The NOAA monthly gridded precipitation anomaly data in 5 degrees-square are used for the model estimation. By assuming that the land water decreases exponentially with time, about 37% of the ten-yearly polar motion can be explained when decay parameter of 3.7 month. The seesaw-like changes of precipitation that efficiently excite the ten-yearly polar motion are also confirmed between the North American Continent and western region of the Eurasian Continent. From these results, a new look at the ten-yearly hydrological cycle system is advocated, which consists of atmospheric variations connected with the North Atlantic Oscillation, see-saw like sea level changes along both coastal regions in the North Atlantic Ocean, and precipitation changes in the North American Continent and the Eurasian Continent. The essential factor to determine the ten-year scale in the system is considered to be hydrological cycle in the two continent.
JSG14/W/16-A3 Poster 1203-22
INFLUENCES OF HYDROLOGICAL MASS REDISTRIBUTIONS ON EARTH ROTATION
R. DILL, H. Drewes, B. Richter, H. Schuh DGFI, Marstallplatz 8, D-80539 München, Germany
email: dill@dgfi.badw.de
Hydrological processes cause local, regional and global redristributions of water masses which have an impact on the rotation of the Earth. We distinguish between direct and indirect influences of these mass motions on the rotation vector. Direct effects result from the displacements of water (matter term) which change the tensor of inertia of the whole Earth and from the velocity of water masses relative to a terrestrial reference frame (motion term). Redistributions of water masses influence also indirectly the Earth rotation by changing the tensor of inertia of the solid Earth due to load deformations. The indirect effects of atmospheric and oceanic loading and also the influence of small surface deformations caused by snow and ice loads or groundwater level variations are studied. Different realistic models of global snow cover are used to estimate and compare the magnitude of the resulting perturbations in Earth rotation. By a combined approach of atmospheric water balance, river runoff and sea level variation,the monthly continental ground water storage is determined. The influence of global and regional ground water variations will be discussed. Small regional effects are also looked at, especially the drying up of the Aral Sea and the filling of big new dams, like the proposed Three Gorges Dam in China.
JSG14/P/01-A3 Poster 1206-23
CLIMATE CYCLES IN VARIATIONS OF THE GRAVITY FIELD AND POLAR MOTION
Horst JOCHMANN (GeoForschungsZentrum Potsdam, Division of Kinematics and Dynamics of the Earth, Telegrafenberg, D-14473 Potsdam, Germany, email: vroni@gfz-potsdam.de)
Global change is accompanied by mass redistributions (e.g. mass exchanges between the hydrosphere and the cryosphere, and variations of the global atmospheric and oceanic circulations). These changes of the mass distribution at the earth produce temporal variations of the earth's rotation and the gravity field. The discussions with respect to earth rotation are confined to polar motion, because the variation of this parameter is mainly caused by redistributions of masses, while length of day variations depend nearly completely on motion effects. So, polar motion is more closely related to gravity field variations than changes of the length of day.The amplitude spectrum of the temporal variation of polar motion contains well known climate cycles (e.g. 11, 13, 22, 35 and 80 years). Similar cycles are found in the atmospheric excitation function of polar motion. But, this excitation function can not be identified as the only source of these cycles. It was found that inner core motion contributes to a certain extend to the climate cycles in polar motion.
Modem satellite missions (CHAMP and GRACE) will determine the gravity field with increased accuracy in near future. This allows a more accurate calculation of its temporal variation. Estimating the influence of long term atmospheric mass redistributions on the gravity field it could be shown that besides the dominating annual period a number of low frequent periods are expected to be contained in the gravity field. The amplitudes of these terms are in the same order of magnitude as the annual one and can be partly identified as climate cycles. The expected temporal behaviour of the Stokes coefficients infers the global property of the climate cycles.
JSG14/W/12-A3 Poster 1209-24
EARTH ROTATION, ICE AGES AND SEA-LEVEL CHANGES: INFLUENCE OF MANTLE VISCOSITY AND SOLID-EARTH STRATIFICATION ON THEIR INTERDEPENDENCE
L.L.A. Vermeersen, R. SABADINI (Dipartimento di Scienze della Terra, Sezione Geofisica, Universita di Milano, Via L. Cicognara 7, I-20129 Milano, Italy)
It has been realized long before the plate tectonics revolution of the sixties that there might exist close connections between changes in the position of the earth's rotation axis with respect to the earth's surface (polar wander), variations in sea level and the rise and decline of Ice Ages on geological time scales. Quantitatively, the modeling of these interrelationships has received renewed attention in the last few years, after initial attempts to relate these phenomena in the eighties by means of viscoelastic solid earth relaxation models had shown its feasibility. We show that the radial mantle viscosity profile and the layering of the solid earth have important influences on these interactions.
JSG14/E/11-A3 Poster 1212-25
CORE MANTLE ANGULAR MOMENTUM EXCHANGE IN GEODYNAMIC MODELLING
Weijia Kuang and Benjamin F. Chao (Space Geodesy Branch, Code 926, NASA, Goddard Space Flight Center, Greenbelt, MD 20731, USA, email: Kuang@santafe.gsfc.nasa.gov)
Kinematic studies of geomagnetic data and Earth rotation observations have demonstrated that the decadal fluctuation of the length of day (LOD) results from the exchange of the angular momentum between Earth’s core and the solid mantle. However, our knowledge about the dynamics responsible for the angular momentum exchange, i.e. the coupling torques across the core-mantle boundary (CMB), is limited in the past based on very simple theoretical models.
To understand better the dynamics of the angular momentum exchange across the CMB, we introduce a heterogeneous mantle into our geodynamo model that simulates 3-D convective flow in the Earth’s outer core. In the model, the mantle rotates subject to the coupling torques exerted on the CMB. In particular, we assume that the CMB is spherical but superimposed with a small amplitude (< 3 km), large scale (~ 1000km) and non-axisymmetric topography. We also assume that the electrical conductivity of the lower mantle (D"-layer) is finite and spatially variant. The heterogeneity introduced in the model follows the lower-mantle seismic tomography. With this model, we are able to study the topographic coupling and the magnetic coupling along with the convective flow in Earth’s outer core. Furthermore, this model can be used to examine the effect of the couplings on the polar motion and nutations of the Earth.
JSG14/W/06-A3 Poster 1215-26
THE EARTH'S POLAR ZONALITY MOTION DURING THE LATE WEICHSELIAN BASED ON PALAEOCRYOSPHERE STUDIES
Robert MOKRIK (Institute of Geology, Sevcenkos 13, 2600 Vilnius, Lithuania,
email: mokrik@geologin.lt)
Periglacial periods during the Pleistocene in the Estonian Homocline have formed a permafrost up to 500m thick. Final phase of permafrost according to radiocarbon data had an extensive period of periglacial activity occured 30-26 ka BP during the Huneborg-Denekamp Stadial time before Late Weichselian ice maximum. It is 6 ka BP earlier than in the West Siberian Craton (24-15 ka BP). That was related with polar zonality movement of the Earth. Permafrost processes caused metamorphism in the Cambrian-Vendian aquifer. As a result, the mineralization of groundwater decreased in comparison with the initial pre-Pleistocene period from 4-10 to 0,5-1 g/l. Hydrocarbonate-sodium chlorides prevail in the groundwater owing to degrees of calcium and magnesium carbonates and sulphates. The last cryogenese (30-26 ka BP) had lightened oxygen -18 isotope composition of groundwater at 2 per mil. The last Scandinavinian Ice Sheet 11 ka BP had retreated northwards from the Estonian Homocline. It formed ice-dammed Baltic Ice Lake (BIL), having to 80m higher level in comparison with the Ocean. BIL took part in recharge of the Cambrian-Vendian aquifer for a short time. That is why an additional oxygen-18 lightening (1,5-2 per mil) of groundwater has been happened. Total isotope oxygen-18 lightening of groundwater during the Pleistocene is 13-14 per mil. Existence of the permafrost in Estonia is confirmed also by data of ratio uranium isotopes 234 U/238U activity in the Cambrian-Vendian aquifer. This ratio varies up to 40. These waters are unique in the European hydrogeological practice and reflect the cyclic cryosphere change during Quaternary. The permafrost in the Estonian domain caused great thermokarst activity which originated the valley shapes of lakes Peipus, Vortsjarv, Gulf of Parnu and Gotland Proper.
JSG14/L/02-A3 Poster 1218-27
ON COSMOLOGICAL ORIGIN OF CERTAIN GEODETIC FEATURES OF THE EARTH
A.O. Adekugbe-Joseph (Center for Fundamental Study, P.O.Box 22415 University of Ibadan P.O., Ibadan, Oyo State, Nigeria. Email: nigeria@netbox.com)
The angle 66.5 degrees of inclination to the ecliptic of earth's rotational axis (rotational slanting of the earth), has been predicted accurately, and the ratio of polar diameter to equatorial diameter,(polar flattening of the earth), of 0.997 has been recalculated exactly, within a new cosmological model.
An instantaneous rate of increase of earth's radius of 0.108cm/yr, as well as instantaneous relative drift rate of 0.108y cm/yr of two points with longitudinal separation of y degrees along the equator, have been calculated as consequences of the expansion of the earth with the universe in the new model. A remarkable agreement of the predicted rate of increase of earth's radius of 0.108cm/yr, earth's rotational slanting of 66.5 degrees and earth's polar flattening of 0.997 is shown.
The slight westward deflection in the north and eastweard deflection in the south of continental plates have also been explained as local consequence of the new cosmological model.
These numbers as well as the westward deflection in the north and eastward deflection in the south of continental plates are increasing, but discernible changes occur in time scales of between 10 million and billion years only.
Wednesday 21 July PM
Presiding Chairs: C.R. Wilson , NASA/HQ, Washington D.C
J.O. Dickey JPL/CALTECH, USA.
JSG14/L/10-A3 1400
THE EARTH GRAVITY MODEL: STATUS AND PROSPECTS
Byron D. Tapley The University of Texas at Austin Center for Space Research R1000 Austin, TX 78712
The requirements for an Earth Gravity Model with both high spatial resolution and high accuracy have been articulated for over three decades. During the past decade, there has been considerable improvement in the global properties of the current models. A number of new, high accuracy tracking systems has provided data sets with enhanced accuracy and better global coverage. The use of satellite altimeter measurements has substantially improved the coverage over the oceans, and the release of several important gravity sets collected over the continents has allowed These improvements. Along with the improved models, the demands for gravity to support a number of important scientific investigations has increased. The utilization of the data from the highly successful TOPEX/Poseidon and ERS altimeter missions is still limited by geoid error. Further, the interest in mass variations within, and themass exchange between, the atmosphere, oceans and solid surface have lead to increased interest in the temporally varying characteristics of the gravity field. To satisfy the increased demands, there have been a number of new gravity mappingprograms that span the spectrum from improved surface measurements tonew an exciting satellite missions. In this presentation, we summarize the status ofcurrent knowledge in the modeling of the earths gravity field, note some of the limitations in the current data, discuss the currents requirements and describe future opportunities to improve the gravity model. In particular, the promising suite of space missions, CHAMP, GRACE and GOCE, which are scheduled to be implemented during the first decade of the next century will be described and their contributions to the gravity model development will be discussed.
Finally, the conceptual and computational challenges associated with these new data sets will be described.
JSG14/L/11-A3 1430
OCEANOGRAPHY FROM GRAVITY MEASUREMENTS.
Chris W Hughes and Trevor F Baker (CCMS Proudman Oceanographic Laboratory, Bidston Observatory, Birkenhead, Merseyside L43 7RA, UK, Email:cwh@ccms.ac.uk, tfb@ccms.ac.uk)
In principle, measurements of the global gravity field should provide information about tides and ocean circulation. In practice, to date, this has been greatly complicated by the fact that terrestrial observations of time variations in these quantities have been at a limited number of sites, for which the movement of the observation point due to deformation of the earth must be taken into account. Satellite observations have the advantage of being global and decoupled from the solid earth, and have provided useful information about changes in gravity at tidal periods and in relation to post-glacial rebound, but are generally limited to very coarse spatial resolution. The launch in 2001 of a pair of satellites dedicated to monitoring the earth's gravity field and its variations on length scales down to 300 km will dramatically change the way such information can be used. Over the oceans it will be possible to invert this gravity information to give a measure of changes in ocean plus atmospheric mass distribution, and therefore in ocean bottom pressure. This is a fundamental quantity which relates not only to the moment of inertia of the ocean, but also to its coupling with the solid earth. New theoretical results suggest, and model diagnostics confirm, the important role of ocean bottom pressure in western boundary currents and at high latitudes, via a coupling of the oceanic angular momentum and vorticity balances. Results will be presented showing that it should be possible to use variations in global gravity to infer changes in abyssal ocean currents and important aspects of the thermohaline circulation - a unique ability for a remote sensing technology.
JSG14/E/23-A3 1500
HYDROLOGICAL AND GLACIOLOGICAL APPLICATIONS OF GRACE; ESPECIALLY GRACE/GLAS SYNERGY AND ANTARCTIC MASS BALANCE
John Wahr (Dept of Physics and CIRES, U of Colorado, Boulder, CO 80309, US,
email: wahr@lemond.colorado.edu) CHARLES BENTLEY (Dept of Geology and Geophysics, U of Wisconsin, 1215 West Dayton Street, Madison, Wisconsin 53706, US,
email: bentley@geology.wisc.gov) Duncan Wingham (Dept of Space and Climate Physics, University College London, 17-19 Gordon Street, London, England WC1H OAH, email: djw@mssl.ucl.ac.uk)
GRACE is a dedicated satellite gravity mission, approved and scheduled for a 2001 launch as part of NASA's Pathfinder Program and with assistance from DLR. GRACE will map the earth's gravity field to unprecedented accuracy and spatial resolution every few weeks. Time-variable gravity inferred from these measurements has applications for virtually all of the earth sciences, including hydrology, oceanography, glaciology, climatology, and solid earth physics. We will discuss the sort of hydrological and glaciological information that can be derived from the GRACE data. We will then concentrate specifically on using the data to learn about the mass balance of the Antarctic ice sheet. We will show that by combining GRACE data with laser altimeter measurements from NASA's GLAS mission, it should be possible to determine Antarctica's contribution to the present-day rise in global sea level to an accuracy approaching 0.2 mm/yr over a five year period.
JSG14/W/19-A3 1600
GLOBAL AND REGIONAL GEOPHYSICAL PROCESSES AFFECTING THE GRAVITY FIELD AND ITS TIME VARIATIONS
R. SABADINI, L.L.A. Vermeersen, G. Di Donato (Dipartimento di Scienze della Terra, Sezione Geofisica, Universita di Milano, Via L. Cicognara 7, I-20129 Milano, Italy,
Email: bob@sabadini.geofisica.unimi.it); R. Devoti, V. Luceri, P. Rutigliano, C. Sciarretta (Telespazio, Matera, Italy); G. Bianco (ASI, Matera, Italy).
Integrated studies based on physical models of global and regional processes are of paramount importance to understand the complexities of the evolving Earth and, at the same time, to face the need of society to mitigate the consequences of environmental changes. Gravity data acquisition and modelling of global and regional geophysical processes at short and long time scales are keys to these issues. Joint inversions of secular changes in low degree harmonics of the geopotential based on new SLR analyses constrain the viscosity profile of the mantle and ongoing mass redistribution over the surface of the Earth, which is likely to be due to present-day ice mass instabilities in alpine glaciers and/or to changes in the mass balance in Greenland and Antarctica. The rheological parameters inferred from the long wavelength, time dependent gravity field, and the density anomalies at crustal and lithospheric level, obtained from high resolution gravity space missions (GOCE), can be used as input lithospheric and mantle parameters in regional tectonic models. These allow to make predictions on important issues affecting our environment, such as sea level rise and stress accumulation in active seismic areas. Results are shown for the Mediterranean, severely affected by these natural risks.
JSG14/E/14-A3 1630
ABSOLUTE SEA LEVEL AND GLOBAL GLACIAL ISOSTATIC ADJUSTMENT
W.R. Peltier(Department of Physics, University of Toronto, Toronto, Ontario, Canada M5S-1A7,
email: peltier@atmosp.physics.utoronto.ca)
The global viscoelastic and self-gravitating theory of glacial isostatic adjustment(GIA), as recently reviewed in Peltier(1998, Rev. Geophys., 36, 603-689), has been extensively tested against a wide range of geological and geophysical observations. Since the future GRACE mission of NASA will deliver a rather high resolution model of geoid hight time dependence and since a major, and perhaps dominant, contribution to this time dependence is expected to be due to the GIA process, we clearly require the best possible theoretical model of this field. Geoid height is of course synonymous with absolute sea level. I will describe a complete analysis of the expected signal that includes the full influence of rotational feedback due to the changes of earth rotation that accompany this dynamical process.
JSG14/E/09-A3 Poster 1700-01
CHANGES IN THE GEOPOTENTIAL OBSERVED WITH SLR
C. COX, S. Klosko (Raytheon ITSS Corp., Greenbelt, MD 20770; 301-441-4010;
email: ccox@magus.stx.com) B. Chao (Laboratory for Terrestrial Physics, NSAS/GSFC,
Greenbelt, MD 20771)
Existing analyses of Satellite Laser Ranging (SLR) tracking data has yielded solution rates for low-degree zonals, in some cases up through J5. The secular zonal rate solution is improved using additional data and a new analysis technique applied in the processing of the SLR data. The new method effectively results in a continuous ephemeris in selected Keplerian orbit components, providing better estimates of the orbit plane accelerations, while retaining some of the accuracy of the shorter orbit solution arcs. The preliminary results of this technique, as applied to multiple satellite (LAGEOS, LAGEOS-2, Starllete, Stella, TOPEX/POSEIDON, and Ajisai) solutions for the secular rates will be presented. Additionally, the results of other new approaches for improving the resolution of the time varying gravity model by extending the temporal solutions to the low degree tesseral terms will be discussed. The secular zonal results, and those obtained earlier by Nerem and Klosko (1996) and Cheng et al. (1997) are combined with observed global sea level rise, and the secular polar motion rates in an inverse solution to provide constraints on geophysical models describing post-glacial rebound and ice sheet mass balances. Although the number of well resolved geopotential terms is limited, the inverse solutions are promising and provide results consistent with IPCC estimates of ice sheet mass balance and realistic contrasts between upper and lower mantle viscosity.
JSG14/W/11-A3 Poster 1703-02
OBSERVING THE TIME-VARYING EARTH'S GRAVITY FIELD USING SATELLITE TRACKING DATA
M. K. Cheng and B. D. Tapley, Center for Space Research The University of Texas at Austin Austin, Texas 78759-5321, U.S.A., e-mail: cheng@csr.utexas.edu
Mass redistribution within the Earth causes the temporal variations in the Earth's gravity field. Analysis of the long-term satellite laser ranging (SLR) tracking data has provided the capability for measuring the time-varying Earth's gravitational field. In this paper, we present recent results for the secular rates of low degree and order geopotential coefficients and the seasonal variations of the zonal harmonics from analysis of over 23 years SLR data collected from 8 geodetic satellites, including Starlette, Ajisai, Stella, LAGEOS I and II, Etalon I and II, BE-C. In addition, DORIS data provide an important complement to the SLR data to monitor the temporal variations in the non-zonal terms. Comparisons of the results from analysis of the satellite tracking data and predicted from the geophysical models demonstrate that analysis of long-term SLR data can provide a global constraint on modeling of the long-term changes of the Earth system, such as the post glacial rebound and the mass balance of polar ice, and be able to observe the meteorological mass transport between the atmosphere, solid earth and ocean. The continuous long-term SLR data are invaluable for enhancing our understanding of the Earth system dynamics and its role of the meteorological excitation on the Earth's gravity field.
JSG14/E/05-A3 Poster 1706-03
GEODETIC CONSTRAINTS ON MASS REDISTRIBUTION IN THE EARTH SYSTEM
Erricos C PAVLIS (JCET/UMBC, NASA Goddard, Code 926, Greenbelt, MD 20771, U.S.A.,
E-mail: epavlis@Helmert.gsfc.nasa.gov)
The continuous redistribution of mass within the Earth system causes concomitant changes in the Stokes' coefficients describing the terrestrial gravity field. Secular changes in J2 due to post-glacial relaxation have been observed since many years. Similar changes in J3 have been attributed to changes in the ice sheets of Greenland and Antarctica. Seasonal changes in these coefficients have also been closely correlated with mass transfer in the atmosphere and oceans. The system is also affected by the hydrological cycle, which however is the most difficult to measure accurately so far. It is thus important to find means to measure the total effect so that we can infer the least well-known component after subtraction of the other ones. Satellite laser ranging data to LAGEOS 1 and 2 have already contributed in this effort the most accurate results yet. The sparseness of the SLR data (due to the lack of several Lageos-like targets), places limits on the maximum resolution of these series of results. This new analysis doubles the resolution of our previous results and extends our series up to the recent past. This data set of the past six years has been analyzed in a consistent manner using NASA Goddard's GEODYN/SOLVE II software and following the IERS Conventions 1996. We present and discuss our low-degree Stokes' coefficients time-series with emphasis on the geocenter variations and compare them to those inferred geophysically. The spectral contents of the series are also compared and discussed.
JSG14/W/24-A3 Poster 1709-04
Seasonal variations of the Earthís gravity field determined from satellite laser ranging
R. S. NEREM, R. J. Eanes, P. F. Thompson, and J. L. Chen (all at the Center for Space Research, The University of Texas at Austin, Austin, TX, 78712, email: nerem@csr.utexas.edu)
Mass redistributes itself in the Earth system on a variety of temporal and spatial scales reflecting complex interrelated processes in the oceans, atmosphere, groundwater, glacial/polar ice, among others. The measurement of these variations is thus important for a variety of studies attempting to understand the interrelations of the different components of the Earth system, and how they may change with time due to anthropogenic influences. We have used Lageos-1 and Lageos-2 satellite laser ranging (SLR) data to determine long wavelength seasonal variations of the Earthís gravity field from 1993 to the present. Due to the altitude of these satellites, and the non-continuous nature of the measurements, these data can detect seasonal gravitational variations only for wavelengths of roughly 10,000 km and longer (a degree 4 spherical harmonic expansion). We have compared the observed annual variations for a complete 4 x 4 spherical harmonic expansion as observed by Lageos 1/2 SLR data to those predicted from a variety of atmospheric, oceanic, and hydrologic models. We have used the observed variations to optimally select the best set of model predictions. The correlation of the maps of the observed and modeled annual geoid variation is as high as 0.8, with an rms difference of close to 1 mm. Given the sparse temporal and spatial distribution of the SLR data, and the limitations of the geophysical models, we consider this agreement to be as good as can be expected before the launch of a dedicated satellite gravity mission. Similar results will also be presented for variations in the Earthís geocenter location. The implications of these results for planned future dedicated satellite gravity missions will be discussed.
JSG14/E/02-A3 Poster 1712-05
GEOCENTER MOTION DETERMINED WITH GEODETIC DATA. COMPARISON WITH SURFACE LOADING DATA
J-F CRETAUX, A. Cazenave, L. Soudarin (LEGOS-GRGS/CNES, 18, Avenue Edouard Belin, 31401 Toulouse Cedex 4, France, e-mail: Jean-Francois.Cretaux@cnes.fr)
DORIS and SLR data were processed and combined in order to determine the Earth center of mass over the 6-year period from January 1993 to december 1998. The seasonal variations of the DORIS-SLR reference frame origin were computed by determining 3 translation parameters between monthly coordinates of the DORIS and SLR networks. These translation parameters are interpreted as the geocenter motions. These variations that are due to the mass redistribution inside the surface fluid envelops of the Earth were compared to the variations of the 1 degree geopotential coefficients also computed with the same data. During the 1993-1998 period the geocenter coordinates obtained by space geodetic measurements were then compared to geocenter variation obtained from various geophysical sources, i.e. atmospheric pressure, ocean mass, and surface ground water load.
JSG14/W/13-A3 Poster 1715-06
THE GRAVITY CHANGE INDUSED BY THE ANNUAL POLAR MOTION AND THE EFFECT OF SEA SURFACE HEIGHT VARIATIONS ON ITS CHANGE
Tadahiro SATO (National Astronomical Observatory, 2-12 Hoshigaoka-cho, Mizusawa-shi, 023-0861 Japan, e-mail: tsato@miz.nao.ac.jp) Yuichi AOYAMA (The Graduate University for Advanced Studies, 2-12 Hoshigaoka-cho, Mizusawa-shi, 023-0861 Japan, e-mail: aoyama@miz.nao.ac.jp) Masatusgu OOE (National Astronomical Observatory, 2-12 Hoshigaoka-cho, Mizusawa-shi, 023-0861 Japan,
e-mail: ooe@miz.nao.ac.jp) Yoich FUKUDA (Graduate School of Science, Kyoto University, Kitashirakawa, Oiwake-cho, Sakyo-ku, Kyoto, 606-01 Japan, e-mail: fukuda@kugi.kyoto-u.ac.jp)Kazuo SHIBUYA (National Institute of Polar Research, 1-9-10 Kaga, Itabashi-ku, Tokyo, 173-8515 Japan, e-mail: shibuya@nipr.ac.jp)
Recent relative and absolute gravimeters have enough potential to detect such weak and long-period gravity change due to the effect of earth's polar motion for instance. In addition to the observations of polar motion itself at high precision, the observation of the polar motion effect on gravity gives us an important data to understand the characteristics of earth's behavior at a low-frequency band. The polar motion effect on gravity is mainly consisted of the three components of semi-annual, annual and Chandler (of 14 months). In this study, the annual component is mainly discussed based on the comparison of the 5-year observation from a superconducting gravimeter at Syowa Station, Antarctica. One great advantageous thing in the gravity observation at Syowa is that its observation is almost free from such irregular and long-term effects due to the variations in under ground water, which are one of the major noise sources in the measurement of polar motion effect. The observed gravity changes were compared with the theoretical estimated ones, i.e. the sum of the effects of solid tide, ocean tide, polar motion and SSH (Sea Surface Height) variations. The effect of SSH variations were estimated using the data obtained from POCM (Parallel Ocean Climate Model). Based on the SST (Sea Surface Temperature) data which were used to derive the POCM, the effect of SST variations on SSH was corrected. The comparison results suggest; (1) both the observed annual amplitude (1.2 micro Gals) and phase (25.5 degrees) measured by referring on an epoch of 2000, are well explained by the above four effects, and (2) the gravity effect of SSH variations (about 0.2 micro Gals in amplitude) plays an important role to explain the observed annual gravity change especially to its phase.
JSG14/E/13-A3 Poster 1718-07
INTERANNUAL VARIABILITY OF THE EARTH'S GRAVITATIONAL FIELD: EL NINO CONNECTIONS
P. GEGOUT (Ecole et Observatoire des Sciences de la Terre, Laboratoire de Dynamique Globale, 5 rue Rene Descartes, 67084 Strasbourg Cedex, France; tel: [33] 3 88 41 66 94;
email: Pascal.Gegout@eost.u-strasbg.fr); J. O. Dickey (Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109-8099; tel: 818-354-3235;
email: Jean.O.Dickey@jpl.nasa.gov); D. Dong (Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA91109-8099; tel: 818-393-1827;
email: dong@cobra.jpl.nasa.gov)
Temporal variations of the even zonal combination of the geopotential are investigated using the geodetic satellite Lageos I from 1980 to 1998. Our purpose is to investigate the interannual variability of the Earth's gravitational field and its connections with El Nino and La Nina, events.
Gravity variations relative to redistributions of groundwater, atmospheric and oceanic masses are estimated and compared to Lageos I determination. After the removal of a composite seasonal cycle, the use of recursive filtering and multi-channel singular spectral analysis reveals strongsimilarities between observed interannual variability and El Nino Southern Oscillation proxies.
JSG14/E/20-A3 Poster 1721-08
Gravity Variations Computed From a Coupled Atmosphere-Ocean Model
Eric W. LEULIETTE and R. Steve Nerem (both at Center for Space Research, University of Texas at Austin, 3925 W Braker Lane Suite 200, Austin TX 78759, USA, email: leuliette@csr.utexas.edu) Gary L. Russell (NASA/Goddard Institute for Space Sciences, New York, NY 10025, USA,
email: russell@giss.nasa.gov)
The Gravity Recovery and Climate Experiment (GRACE) will return high resolution measurements of the Earthís gravity field for five years beginning in 2001. The projected accuracy of this mission will allow for the observation of mass flow in various components of the coupled solid Earth/ocean/atmosphere system. General circulation models offer a way to assess the impact of these mass changes on the gravity field and the prospects for detecting climate signals in the gravity field. Using monthly-averaged fluid mass diagnostics from a coupled atmosphere-ocean model developed at GISS, we estimate the seasonal and secular geoid signals from sea level, snow, sea ice, soil moisture, and surface pressure can be compared to estimated errors for GRACE. We have used different climate scenarios to access the ability of gravity missions to identify climate change. All annual mass flows are well above the preliminary GRACE measurement errors at half wavelengths of 1000 to 10000 km. In addition, differences between the annual ocean column mass and snow mass annual as predicted by each model scenario (control versus %1 annual CO2 increase), differ at amplitudes that are significant when compared to the GRACE errors. Mass flows with significant secular trends have been used to estimate the magnitude of climate change over five years. These redistributions should be detectable by GRACE in principle, although they can not be separated using GRACE data alone.
JSG14/W/03-A3 Poster 1724-09
VARIATIONS OF GEOCENTER AND EARTH ORIENTATION DEDUCED FROM ESTIMATES OF OCEAN MASS REDISTRIBUTION
Wolfgang BOSCH (Deutsches Geod‰tisches Forschungsinstitut, Marstallplatz 8, D-80539 München, Germany, email: bosch@dgfi.badw.de )
A consistent and homogeneous time series over more than six years is now available for global sea level variations monitored by Topex/Poseidon altimetry. The variations, dominated by a well known annual oscillation, are known to be caused by heating and cooling of the thermocline. The sea level variations therefore represent basically a volume change rather than a mass redistribution. We estimate the residual mass redistribution by using available climatologies to remove the steric effect and compute simultaneously the impact on both, variations of the Earth's center of origin and the Earth's orientation. The results are illustrated and analyzed on spatial and temporal scales.
JSG14/W/01-A3 Poster 1727-10
FUTURE CONSTRAINTS FOR POST-GLACIAL REBOUND FROM GRACE
Mark TAMISIEA and John Wahr (Department of Physics and CIRES, C.B. 390, University of Colorado, Boulder, CO 80309, USA, Email: tamisiea@colorado.edu)
GRACE, a dedicate gravity mission to be launched in 2001, will provide much more information on the secular change of the geoid coefficients than is currently available. Secular variations of the geoid could provide useful constraints for determining the radial viscosity profile of the earth from post-glacial rebound. However, other mechanisms such as hydrology, current ice mass changes, and sea level rise also cause such variations. With the new GRACE data, the spatial pattern of the variations may allow for separation of these mechanisms.
In this talk, we create a synthetic geoid that includes various secular effects and determine the contamination of the post-glacial rebound signal from the other mechanisms' signals. We also begin to determine the extent to which the original input viscosity profile can be recovered.
JSG14/W/20-A3 Poster 1730-11
ESTIMATION OF THE GRAVITY CHANGES INDUCED BY THE MASS VARIATIONS IN THE FLUID EARTH
Yoichi Fukuda (Department of Geophysics, Graduate School of Science, Kyoto University, Japan, KitashirakawaOiwakecho, Sakyo-ku, Kyoto 606-8502, Japan, email: fukuda@kugi.kyoto-u.ac.jp) Tadahiro Sato (National Astronomical Observatory, Mizusawa, 2-12 Hoshigaoka-chyo, Mizusawa-shi, 023- 0861 Japan, email: tsato@miz.nao.ac.jp) Yuichi Aoyama (The Graduate University for Advanced Studies, 2-12 Hoshigaoka-chyo, Mizusawa- shi, 023-0861 Japan, email: tsato@miz.nao.ac.jp) Lorant Foldvary and Kazuo Tsutsui (both at Department of Geophysics, Graduate School of Science, Kyoto University, Japan, Kitashirakawa Oiwakecho, Sakyo-ku, Kyoto 606-8502, Japan, email: fl@kugi.kyoto-u.ac.jp )
The recent progresses of the precise gravity measurements using superconducting gravimeters and absolute gravimeters enable us to observe very weak gravity signals induced by the various kinds of dynamic processes in and on the Earth. Because gravity changes reflect actual mass movements, the data of gravity measurements give us useful information to understand the dynamic processes. As an experimental study, we estimated the gravity changes induced by the sea level variations. We first employed SSH (Sea Surface Height) variation data from POCM (Parallel Ocean Climate Model) to evaluate their gravity effects, and then applied the EOF analysis to investigate the characteristic of the gravity changes. The results show that one of the EOF components is strongly correlated with ENSO like SSH variations. The amplitude of expected gravity changes in the pacific equatorial regions possibly reaches 2 to 3 micro Gals. This amount of amplitude could be enough detectable by a careful gravity observation. On the other hand, several satellite gravity missions are now under planning and some of them will soon be realized. Thus we also estimated the gravity effects of the sea level variations at the satellite orbit height level. Although the spatial coverage of the satellite gravity measurements are much superior than the ground measurements, the ground measurements are still important because of their high precision and temporal sampling rate. Using the simulated data set, we investigate the effective distribution of gravity observation points on ground. The same investigation has been carried out for gravity effects of the atmospheric pressure and soil moisture variations.
JSG14/E/16-A3 Poster 1733-12
VARIATION OF THE GEOCENTER AT SECULAR AND GEOLOGICAL TIME-SCALE
GREFF-LEFFTZ Marianne, Institut de Physique du Globe de Paris, 4 place Jussieu, 75252 Paris
Cedex 05, France.
The degree one deformations of the Earth, in a reference frame related to the centre of mass of the planet, are computed using a theoretical approach (Love numbers formalism) at secular time-scale, where the Earth has a viscoelastic behavior and at geological time-scale, where the mantle is a vis cous fluid. For a Maxwell model of rheology, the degree one relaxation modes associated with the viscoelastic Love numbers have been investigated : the Mo mode does not exist and there is only one transition mode (instead of two) generated by a viscosity discontinuity.The translations at each interface of the incompressible layers of the Earth's model ( especially at the surface, at the Core-Mantle boundary (CMB) and at the Inner Core boundary (ICB)) are computed. They are viscoelastic when the Earth is submitted to Pleistocenic deglaciation and of about the meter. In a quasi-fluid approximation (Newtonian fluid) because of the mantle density heterogeneity, their order of magnitude are of about 100 m at the surface, 1 km at the CMB and 10 m at the ICB (which is in a quasi-hydrostatic equilibrium at this time-scale).
JSG 14/E/01-A3 Poster 1736-13
NON-TIDAL PLUMB LINE VARIATION: A NEW GEODYNAMICAL QUANTITY IN STUDYING THE VARIATIONS IN THE EARTH'S GRAVITATIONAL FIELD AND ITS ROTATION
Zheng-xin LI (Shanghai Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai, 200030, China, email: lzx@center.shao.ac.cn)
Non-tidal plumb line variation (PLV) on ground is related to the variations of the Earth gravitational field but it remains unsolved whether or not one can actually measure them in practice. The paper describes the efforts in using the traditional techniques to detect the non-tidal PLV. Based on the results obtained recently at two different locations, Jozefoslaw of Poland and Yunnan of China, where parallel gravimetry and astrometry observations have been carried out more than 20 and 10 years, interannual PLV, in the order of 0.01"-0.02", have been discovered. It is clear now the existence of the non-tidal PLV at these two locations and the possibility in measuring them in practice by either of the two techniques at interannual time scales. Although only results of the two locations have been obtained, and only interannaul part ot the non-tidal PLV has been studied at the moment, it is already very interesting to see the correlationship between the interannual PLV derived from some astrometric observations and a certain number of geodynamical quantities. It is quite possible that the non-tidal PLV discovered here will be used as a new geodynamical quantity one day with which the studying related to the variations in the gravitational field and its rotation might be carried out in a better way.
JSG14/E/18-A3 Poster 1739-14
GLOBAL GRAVITY CAMPAIGNS - FROM THE GROUND (GGP) TO THE SKY (GRACE)
DAVID CROSSLEY (Saint Louis University, Department of Earth and Atmospheric Sciences, 3507 Laclede Ave., St. Louis, MO 63103, USA, email: crossley@eas.slu.edu) Jacques Hinderer (Ecole et Observatoire des Science de la Terre, 5 rue Ren Descartes, 67084 Strasbourg, FRANCE,
tel +33.88.60.50.63 fax +33.88.61.67.47 email: jhinderer@eost.u-strsbg.fr)
The GGP global geodynamics campaign is now returning high precision data from 17 superconducting gravimeter stations. These sites are clustered in Europe and Japan and more widespresd in other parts of the globe. When the 6 years of the campaign are over we will for the first time be able to separate various types of global gravity signals, associated for example
with rotation of the Earth, to local gravity changes caused by environmental factors.
At the same time, the planned mission GRACE will look at the global gravity field in a very different way. Connecting the two campaigns is the need for ground truth for the GRACE mission. This talk will discuss ways in which the GGP sites, supplemented with absolute gravimeter measurements, may be useful to the GRACE mission.
JSG14/W/02-A3 Poster 1742-15
A 750 DAY RECORD OF GRAVITY VARIATIONS AS SEEN BY A SUPERCONDUCTING GRAVIMETER (GWR C026) AND AN ABSOLUTE GRAVIMETER (FG5-206) IN STRASBOURG (FRANCE)
Jacques Hinderer, Martine Amalvict, Jean-Paul Boy and Pascal Gegout (all at Ecole et Observatoire des Sciences de la Terre, 5 rue Descartes 67084 Strasbourg Cedex, France, e-mail: jhinderer@eost.u-strasbg.fr)
This paper is devoted to a comparative analysis of a 750 day long record of gravity changes as observed both by an absolute gravimeter (FG5 model 206) and a superconducting gravimeter (GWR model C026) which are operating in parallel in Strasbourg. This study extends previous related studies by adding one more year of relative and absolute gravity data at the same station. The relative gravity (as well as the atmospheric pressure) is sampled every 2 sec and numerically decimated to 1 min in agreement with the GGP (Global Geodynamics Project) recording standard. The absolute gravity values (with a drop repetition of 15 sec) consist in several days (about a week) of continuous measurements performed generally once every month. Two main objectives will be sought: on one hand, the alibration capability of AG/SG parallel registrations will be further tested, especially with respect to stability in time, duration requirement and precision; in particular, we will show the mean value of the scale factor resulting from a least squares adjustment using all the AG/SG values available during the 750 day time span. On the other hand, the absolute gravity values will be superimposed onto the continuous superconducting gravimeter observations in order to estimate long term gravity changes at our station. We will attempt to separate true (geo)physical effects from instrumental causes (e.g. drift of the cryogenic meter). A special attention will be paid to the gravity signature of the polar motion and to the respective capability of both types of instruments to exhibit it.
JSG14/E/19-A3 Poster 1745-16
RELATIVE AND ABSOLUTE GRAVIMETERS IN LARGE AREAS
A.Sas-Uhrynowski, Y.ZANIMONSKIY ( Institute of Geodesy and Cartography, Warsaw, Poland,
e-mail: zgf@igik.edu.pl )
The advantages of the satellite missions to map the Earth's gravity field are increased with presence of the additional terrestrial data. Fast accumulation of the gravimetrical data with high accuracy in special and least known areas especially is necessary. In this connection the traditional methods of gravimetric serveys should be improved. The modernization of the Polish Network shows that usage of relative gravimeters on a long (up to 350 km) distances between stations is quite effective. It is a great desire to use the Portable Ballistic Gravimeter as an additional device This device was under R&D at the Institute of Geodesy and Cartography for a few years . It consists of an original parts and operates under special algorithm. The base of this algorithm is randomization of the systematic errors. PBG have been extensive field tested as autonomous as alongside other absolute gravimeters . The repeatability was estimated 0,01mGal for weekly and monthly intervals. Now we have the autonomous small-size and resistant to field conditions unit. The payment for this advantages is change of the instrument's metrological status. PBG is a working measuring instrument and it have to be calibrated at the standard such as Polish Gravity
Reference Network. The relative connection is obtained by looping between two stations three days in a row. During this time PBG works automatically at the second consecutive station. The purpose of new algorithms for data processing is to bring together results obtained by means of so different instruments. Results of the recent measurements are going to discuss too.
JSG14/E/10-A3 Poster 1748-17
EVALUATION OF SIX YEAR SUPERCONDUCTING GRAVIMETER OBSERVATION AT GFZ POTSDAM SITE
JUERGEN NEUMEYER, Franz Barthelmes, Hans-Juergen Dittfeld (GeoForschungsZentrum Telegrafenberg A17, 14473 Potsdam, Germany, e-mail: neum@gfz-potsdam.de, bar@gfz-potsdam.de, ditti@gfz-potsdam.de)
The high accurate Superconducting Gravimeter TT 18 has been continuously recorded gravitational variations at GFZ Potsdam site for six years. This data set and environmental parameters recorded at the same time have been used for analysing the data and separation of different geophysical effects.
In a first step the short and long period Earth tides are analysed and the tidal parameters are compared with the Wahr Dehant model. Selected tidal parameters are used for determination of the complex eigenfrequency of the Earth¥s Nearly Diurnal Free Wobble and the result is compared with theory.
Gravity variations caused by atmospheric pressure, groundwater table and rainfall are estimated by using different approaches. The admittance coefficients are determined and the quality of the reduction is discussed.
After reduction of the environmental effects the polar motion is extracted from the residual data set and compared with smoothed records from space techniques. The free oscillation of the Earth caused by big Earthquakes is used for separation and splitting of the different modes.
The decay of the modes is illustrated by means of the Wavelet Transform.
Finally an attempt is made for determination of the three translational modes of Earth solid inner core (Slichter Triplet) using the Cepstrum method.
JSG14/E/21-A3 Poster 1751-18
STUDY TO PROPOSE A GRAVITY NETWORK AS GROUND TRUTH FOR SATELLITE MISSIONS
Bernd RICHTER, W. Schwahn, D. Simon, R. Falk, H. Wilmes (all Bundesamt fuer Kartographie und Geodaesie, Richard Strauss Allee 11, D-60598 Frankfurt a.M. / Germany (email: richter@ifag.de)
Special satellite missions (CHAMP, GRACE) are initiated to study the fine structure of the Earth gravity field and its temporal variations. For the first time space borne derived gravity variations can be compared with series observed at the Earth surface. Various gravity variations and their contributions to the Earth borne and space borne sensors will be discussed. Due to the temporal resolution of the satellite data only gravity variations can be analysed which have a long-term periodical or non-periodical variation nature or a secular trend. As a test case periodical signals are preferable because of the higher sensitivity of models to periodical pattern. In addition the spatial disturbance for long-period variations is at least regional, usually of global character. The annual gravity variation is a potential candidate which should be detectable by satellite and ground borne.