G43B-0806 1340h
Oceanic Excitations On Polar Motion: A Cross Comparison Among Models
Recent studies based on various oceanic general circulation models (OGCMs) demonstrated that the oceans are a major contributor to polar motion excitations. In this paper, we analyze and compare observed non-atmospheric polar motion excitations with oceanic angular momentum (OAM) variations determined from four OGCMs, which include the parallel ocean climate model (POCM), a barotropic ocean model (BOM), the Estimating the Circulation and Climate of the Ocean (ECCO) non-data-assimilating model (ECCO-NDA), and the ECCO data-assimilating model (ECCO-DA). The data to be analyzed span a 5-year overlapped period from 1993 to 1997. At annual time scale, these four OAM estimates do not agree well with each other, while POCM shows relatively larger discrepancies than other three models. At intraseasonal time scales, ECCO-DA yields the best agreement with observations, and reduces the variance of non-atmospheric excitations by about 60%, 10-20% more than those explained by other three models. However, at the very short periods of 4-20 days, the BOM estimates could explain about half of the observed variance, twice as much as that by ECCO-NDA, and also shows considerably better correlation with observations. Due to different modeling schemes and methods, significant discrepancies could arise with respect to the quantity of modeling large-scale oceanic mass redistribution and current variation. A clear understanding of global oceanic contributions to polar motion excitation still remains a challenge.
G43B-0807 1340h
Diurnal and semi-diurnal effect of the atmosphere on the Earth rotationand geocenter motion
The diurnal cycle in the atmosphere, and some of its geodetic consequences, as Earth rotation variation and geocenter motion, are studied, from a one year simulation. We show that the effect on the polar motion is at the level of 0.2 milli arcsecond, and that a geocenter motion of the order of the millimeter is to be expected at diurnal timescale. Nevertheless, the large differences between the results from different models indicate that those results should be taken with caution. In particular, we show that the differences in the pressure cycle between the NCEP, the ECMWF and LMD models are very large, which can have important consequences for the de-aliasing of GRACE, for instance.
G43B-0808 1340h
Near Real-time UT1 Measurement by Using e-VLBI Technique
Recent progress of e-VLBI technique and the increase of network speed made rapid UT1 measurements possible. On June 30, 2004 we performed a one-hour e-VLBI session with the baseline between Kashima and Westford stations to estimate UT1 as rapid as possible. Observation data were recorded with the K5/VSSP system at Kashima and the Mark-5 system at Westford. Immediately after the session was finished, Mark-5 data were transferred to Kashima through the Internet. The Mark-5 data were then converted to K5 format data. In the next step, the converted data were correlated with those recorded at Kashima by using the K5 software correlator combined with the network-distributed processing system named VLBI@home. Finally we succeeded to obtain estimated UT1 value in as short as 4.5 hours after the session was over. To shorten the turn-around time of UT1 estimation further, we are improving the K5 software correlator so as to correlate K5 data with Mark-5 data directly. We are also developing software to send K5 data over network according to the standard data format (VSI-E). In addition to the rapid UT1 measurement results, we will report about current status of these software developments.
G43B-0809 1340h
PREDICTION OF UNIVERSAL TIME (UT1) FROM ATMOSPHERIC ANGULAR MOMENTUM
Accurate positioning by satellite (GPS, GLONASS, DORIS) requires prediction of Universal Time UT1, as do systems of interplanetary spacecraft navigation. Below 10 days, the UT1 variations are mostly due to the exchange of angular momentum between the atmosphere and the crust. Therefore UT1 predictions at these time scales can be based on the predicted values of the axial Atmospheric Angular Momentum (AAM). Such values are derived daily by miscellaneous meterorological centers over the 5-10 forthcoming days. In the present study we characterize the quality of three current AAM forecasts, those of the U.S. National Centers for Environmental Prediction, the European Centre for Medium-Range Weather Forecasts, and the United Kingdown Meteorological Office. We analyze the associated prediction of UT1, especially compared to one determined without the use of any geophysical information, namely based classically on the EOP series history, and we check the skill of the current forecast procedure.
G43B-0810 1340h
TRF Kinematics From Satellite Laser Ranging.
The continuous redistribution of mass within the Earth system causes concomitant changes in the Stokes coefficients describing the terrestrial gravity field. Seasonal changes in these coefficients have been closely correlated with mass transfer in the atmosphere, hydrosphere and oceans. The origin of the Terrestrial Reference System (TRS) is realized through the adopted coordinates of its defining set of positions and velocities at epoch, constituting the conventional Terrestrial Reference Frame (TRF). Its stability, integrity and applicability are directly related to the accuracy and fidelity with which such motions can be observed or modeled during the position determination of these sites. Since over two decades now, these site coordinates are determined through space geodetic techniques, in terms of absolute or relative positions of the sites and their linear motions. The new gravity-mapping missions, CHAMP and GRACE, and to a lesser extent the future mission GOCE, address directly the temporal gravitational changes. For the very low degree and order terms, there is also a geometric effect that manifests itself in ways that affect the origin and orientation relationship between the instantaneous and the mean reference frame, as well as the axes of figure orientation. Satellite laser ranging (SLR) data to LAGEOS 1 and 2 contributed in this effort the most accurate results yet, demonstrating millimeter level accuracy for weekly averages. Other techniques, like GPS and DORIS, have also contributed and continue to improve their results with better modeling and more uniformly distributed (spatially and temporally) tracking data. Improvements in the analysis methodology and the underlying models, resulted in a new series of weekly results, consistent with the recently adopted IERS Conventions 2003, and using the latest improvements in modeling SLR observations. We will present our results from several years of LAGEOS 1/2 and ETALON 1/2 SLR data, assess their accuracy and compare them to results from the various other techniques and geophysical monthly series from missions such as GRACE. A comparison of the SLR-derived trajectory of the "geocenter" with respect to the TRF, reveals a strong correlation with the geophysical events.
G43B-0811 1340h
Rotation and impact of the Earth's core on Earth's rotation
This paper investigates the dynamical behavior of a liquid core inside the Earth related to the mantle by inertial coupling. The core-mantle interaction was included in a realistic model of the Earth's rotation, the SONYR model (acronym of Spin-Orbit N-bodY Relativistic model). This model integrates simultaneously the spin-orbit N-body problem and permits to identify the different families of librations/nutations of the terrestrial planets, with special emphasis on the Earth's rotational motion. By using the SONYR model, we study the impact of the core on the rotational motion of the Earth. By comparing the rotation of the Earth considered as a homogeneous body with the rotation of an Earth model with two layers, we establish the signature of the core motion on the librations. We estimate the well-known proper frequencies of the Free Core Nutation and the Chandler Wobble for the two-layer Earth model. Moreover, we investigate for the first time in detail the motion of the core inside the mantle and find, for periods of the order of a year, that the core motions have a larger amplitudes than those of the dynamical motions of the mantle. We present results obtained for various initial conditions and for various oblatenesses.