Statistics – Computation
Scientific paper
May 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agusm...a42a01s&link_type=abstract
American Geophysical Union, Spring Meeting 2001, abstract #A42A-01
Statistics
Computation
1223 Ocean/Earth/Atmosphere Interactions (3339), 1227 Planetary Geodesy And Gravity (5420, 5714, 6019), 1239 Rotational Variations, 3339 Ocean/Atmosphere Interactions (0312, 4504)
Scientific paper
Advancements in the study of the Earth's variable rate of rotation and the motion of its rotation axis have given impetus to the analysis of the torques between the atmosphere, oceans and solid Earth. The output from global general circulation models of the atmosphere (pressure, surface stress) is being used as input to the torque computations. Gravitational torque between the atmosphere, oceans and solid Earth is an important component of the torque budget. Computation of the gravitational torque involves the adoption of a gravitational model from a wide variety available. The purpose of this investigation is to ascertain to what extent this choice might influence the results of gravitational torque computations. The gravitational torque is the second largest contributor to the equatorial torque budget. It is due to the non-radial part of the Earth's gravitational field. The computation of the gravitational torque requires both the local value of the atmospheric surface pressure and the unit vector normal to the local equipotential surface. Given a value for the surface pressure from an atmospheric model, different gravitational models yield variations in the computed gravitational torque to the extent that their equipotential surfaces differ. Variations in the computed gravitational torque are also a function of the degree and order used to compute the equipotential surfaces of a given gravitational model. Various current gravity field models have been used to compute the atmospheric gravitational torque on the solid Earth and oceans : EGM96, TEG-3, GRIM4-C4, JGM-2, and JGM-3. Time variations with respect to the nominal (EGM96 to degree and order 70, i.e., EGM96.70) are negligible for the equatorial (x, y) components of torque for all the 70x70 gravitational models, as well as for EGM96 to degree and order 240 (EGM96.240). Time variations in the computed axial component of torque (z) are not negligible compared to the nominal, being of the order of 13% for the 70x70 fields, and 17% for EGM96.240. Mean value differences range between 9%-22% of the nominal for the 70x70 fields and 100% for EGM96.240. The equatorial torque budget is determined to a large extent by the balance between the pressure torque due to polar flattening and the gravitational torque. The axial torque budget, however, is dominated by the orographic or mountain torque and the wind stress torque over land and oceans, the gravitational torque being a distant third. The time variations of the gravitational axial component of torque found here most likely fall within the uncertainties due to the other larger components. This might change in the future as the torque approach becomes more accurate in representing the wind stress and orographic contributions. At present, the variations in the mean might be more important from the standpoint of attaining zero mean for the sum of all components: wind friction, orographic, and gravitation.
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