This report does not cover all mass transport processes and mass distributions in the Earth system.
The fields proposed as core elements of this joint research framework (Figure 4.1) are those, for which the gravity field missions in combination with satellite altimetry enable completely new approaches.
On purpose, the framework will not include projects on magnetic field effects (because these are subject to a DFG priority program on its own), and Earth rotation (which is subject of a DFG research group). The interrelations with these fields will be taken into account, however, to the necessary extent.
The combination of gravity and magnetic field data for a more complete Earth system understand-ing could be very interestunderstand-ing for future research, not only for the study of the Earth’s core, but also for the atmosphere and possibly for ocean circulation and tides. On CHAMP, this combination is realized from the observational side. However, due to the currently running DFG priority program, for the proposed framework it is recommended to restrict to gravity and altimetry as core data.
Variations of Earth rotation are subdivided into nutation, variation in spin rate and polar motion.
Any mass change in Earth system and any relative forcing between Earth system components re-sult in variations of Earth rotation. As gravitation, the observed Earth rotation parameters (length-of-day and polar motion) reflect the integral effect of all exchanges of angular momentum in Earth system. The separation of effects is possible by means of typical time periods of the
indi-1. Complementarity of gravity and altimetry missions: For many of the proposed applications, the exploitation of satellite gravity or altimetry data alone is not sufficient, but the combination of both is needed, as well as the combination with other complementary missions. This is particularly challenging, because the mass signals to be determined are so small, and because the sensor systems and data types to be combined are profoundly different.
2. Mass exchange: For many of the described processes, the exchange of masses between system components, i.e. between oceans, ice and land masses is of central importance.
Output from one process represents intput for other processes.
3. Separation of integral signals: The satellite observations result from the sum of all mass changes in the Earth system. A strategy has to be established to identify the contribution of each individual mass change. The better we understand all other signal components, the better we can extract and use one single component of interest.
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vidual effects or based on external models. Earth rotation analysis ideally complements the grav-ity field and altimetry approach to mass transports. At the time of writing, a special research group on Earth rotation and global geodynamic processes is being established by the DFG, based on a concept document by Schuh et al. (2004). A close cooperation with this group is planned.
. Synopsis of signal components and amplitudes
In Chapters 2 and 3, numbers of required accuracies and estimated signal amplitudes are dis-cussed in various places. For the gravity field, a summary of this information is compiled in Ta-bles 4.1 and 4.2, augmented by some further estimates from other sources (NRC 1997, Rummel et al., 2003). Table 4.1 shows the requirements on the static gravity field in terms of geoid heights and gravity anomalies, and the required spatial resolution, from the main areas of modelling.
These requirements will essentially be met by the GRACE and GOCE gravity field models.
Table 4.2 lists the gravity field time variations, showing the typical signal amplitudes, typical spa-tial scales and time periods, as estimated today. The signal amplitudes are still subject of discus-sions, as simulation results vary considerably, some giving regional extreme values, others global standard deviations. The consolidation of these numbers will be among the tasks of the proposed
Figure 4.1: The new satellite data serve as common basis for the modelling of mass transport and mass distribution in the Earth system. The right hand side of the figure gives an overview of the main research fields. The coloured rings symbol
ize the interconnections which are due to physical reasons, but also resulting from observational data characteristics.
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research program. Therefore, in Table 4.2 (as in Table 4.1) only orders of magnitude are given, which are rather certain in most cases.
The comparison of these signal amplitudes with the expected mission performance of CHAMP, GRACE and GOCE as discussed in Chapter 2 (see Table 2.1, Figures 2.7, 2.8 and 2.9) shows that the amplitudes of most signal components exceed the gravity field errors and are thus in the meas-urable range, at least for large spatial scales. For the small numbers for secular changes in Table 4.2, it has to be taken into account that the accuracy of linear trend determination improves fast with increasing mission duration: From 5 years of monthly gravity field solutions, each of them with an accuracy of 0.01 mm in terms of geoid for the best resolved spherical harmonic degrees (see Figure 2.9), a linear trend in geoid variation can be estimated with an extremely high accu-racy of about 0.8 µm/y for these degrees. Thus, also very small secular changes such as the signals expected from mantle plumes will possibly be detectable.
When looking at the numbers for the time variations in Table 4.2, one should keep in mind that any observation even being only slightly above the error level is already very valuable and repre-sents a completely new piece of information on mass transport processes.
An impression of the type of interconnections between the processes and disciplines is given by Figure 4.2, which is showing the spatial and temporal scales of geoid signals caused by the vari-ous Earth system processes in synoptic way (adapted from Rummel et al., 2003). For both, space and time, one has to deal with a wide variety of scales, ranging from static down to very high frequent signals in the time domain, and from local to global scales in the space domain. The bubbles of the various signals superimpose each other in many places. Some processes have sig-nals ranging over nearly the entire spectrum, such as ocean circulation, or hydrology. The Figure also shows the expected limits of spatial and temporal resolution for the gravity field missions CHAMP, GRACE and GOCE. It becomes clear that the signal of some processes is partly or en-tirely beyond the time or space resolution of these satellite missions. Signals with spatial scales smaller than 70 km will not be resolved in the next few years, although we hope to extend this limit with future missions. In the time domain, the current resolution limit is given by the monthly GRACE gravity field solutions. Higher frequency signal components, with periods from hours to days, e.g. tides or high frequency atmosphere variations, tend to alias into the observed time se-ries. To keep this under control, for these components reductions using the best available models have to be applied. Altogether, with the current missions we can achieve considerable progress in
Application Accuracy Resolution
Table 4.1: Static gravity field requirements for mass transport and mass distribution modelling
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many areas, but we will not be able to cover all processes relevant for mass transport and mass distribution. The present missions are a first important step for mass transport and mass distribu-tion monitoring in Earth system research.
. Common challenges for satellite data analysis
This section addresses the topics in the yellow rectangle of Figure 4.1, which are prerequisite for the derivation of mass signals from the satellite data products, and concern all geophysical appli-cations. The box represents the interface between the satellite geodetic sensor data and the various geophysical disciplines. It provides the processing chain from sensor data to mass signals, and supports geophysical modelling in transforming the mission data into a format and representation adequate to data assimilation requirements.