A global, monthly snow depth data set has been generated from the Nimbus 7 satellite observations using passive microwave remote‐sensing techniques. In this paper we analyze 7 years of data, 1979–1985, to compute the snow load effects on the earth's rotation and low–degree zonal gravitational field. A uniform sea level decrease has been assumed in order to conserve water mass. The resultant time series show dominant seasonal cycles. The annual peak‐to‐peak variation in J2 is found to be 2.3 × 10 −10, that in J3 to be 1.1 × 10−10, and believed to decrease rapidly for higher degrees. The corresponding change in the length of day is 41 μs. The annual wobble excitation is (4.9 marc sec, −109°) for the prograde motion component and (4.8 marc sec, −28°) for the retrograde motion component. The excitation power of the Chandler wobble due to the snow load is estimated to be about 25 dB less than the power needed to maintain the observed Chandler wobble. The superior quality of the satellite data over conventional data acquired by ground observations and modeling is demonstrated. We also discuss the role of atmospheric water and the problems arising from the lack of snow load observations over the Antarctic and Greenland ice sheets.
Redistribution of mass near Earth’s surface alters its rotation, gravity field, and geocenter location. Advanced techniques for measuring these geodetic variations now exist, but the ability to attribute the observed modes to individual Earth system processes has been hampered by a shortage of reliable global data on such processes, especially hydrospheric processes. To address one aspect of this deficiency, 17 yr of monthly, global maps of vegetation biomass were produced by applying field-based relationships to satellite-derived vegetation type and leaf area index. The seasonal variability of biomass was estimated to be as large as 5 kg m−2. Of this amount, approximately 4 kg m−2 is due to vegetation water storage variations. The time series of maps was used to compute geodetic anomalies, which were then compared with existing geodetic observations as well as the estimated measurement sensitivity of the Gravity Recovery and Climate Experiment (GRACE). For gravity, the seasonal amplitude of biomass variations may be just within GRACE’s limits of detectability, but it is still an order of magnitude smaller than current observation uncertainty using the satellite-laser-ranging technique. The contribution of total biomass variations to seasonal polar motion amplitude is detectable in today’s measurement, but it is obscured by contributions from various other sources, some of which are two orders of magnitude larger. The influence on the length of day is below current limits of detectability. Although the nonseasonal geodynamic signals show clear interannual variability, they are too small to be detected.
Global meteorological analyses from the European Centre for Medium Range Weather Forecasts are employed to compute the atmospheric excitation Ψ of the polar motion for the 9‐year period of 1980–1988. Both the matter component Ψ(matter) and the motion component Ψ(motion) are computed, the former with and without the oceanic inverted barometer (IB) effect. It is found that Ψ(motion) contributes significantly to the total excitation Ψ overall and nonnegligibly to the annual signal in Ψ, or the annual wobble excitation, in particular. Our results for the annual wobble excitation, in terms of the prograde component Ψ+ and the retrograde component Ψ− for January 1, are Ψ+ = (16.8 milliarc seconds (mas), −93°) and Ψ− = (15.6 mas, −98°) with IB, and Ψ+ = (17.3 mas, −101°) and Ψ− = (28.1 mas, −112°) without IB. These results are within the (rather large) range of previous estimates. The IB effect has a small impact on Ψ+, whereas its impact on Ψ− is considerable. The (better determined) prograde components Ψ+ are then compared with that observed from the Lageos satellite laser ranging data: (17.3 mas, −61°). Although the amplitudes are nearly equal, large phase discrepancies exist between the atmospheric and this observed value. The resolution of this discrepancy awaits a better knowledge of the seasonal angular momentum budget of the Earth's surface fluid elements.
After 3 years of intense work by some two dozen collaborating scientists at three institutions and after scores of evaluation tests, the Earth Gravitational Model 1996 (EGM96) was completed and released to the scientific community in September 1996. This model was developed jointly by the NASA Goddard Space Flight Center (GSFC), the National Imagery and Mapping Agency (NIMA, formerly the Defense Mapping Agency), and The Ohio State University. EGM96 provides a more accurate reference surface for the topography, improves models of the ocean circulation, improves orbit determination for low‐orbiting satellites, and contributes to global and regional studies in tectonics and geodynamics. The new spherical harmonic model, is complete to degree 360, corresponding to a global resolution of about 55 km. EGM96 incorporates newly released surface gravity data from around the globe, over three decades of precise satellite tracking data and altimeter measurements of the ocean surface from the TOPEX/POSEIDON, ERS‐1 and GEOSAT missions. Figure l a shows a global map of the geoid undulations implied by EGM96, while Figure l b shows the corresponding gravity anomaly field.
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