Temporal gravity field variations from oceanic, atmospheric and inner core mass redistributions and their sensitivity to new gravity missions CHAMP and GRACE

Temporal gravity field variations are caused by mass redistributions in
the atmosphere, on the Earth’s surface and in the Earth’s interior. With the
upcoming new gravity missions CHAMP and GRACE, for the first time such
variations can be measured from space on a global scale. To estimate the time
variable gravity signals and their sensitivity to the new missions, simulation
studies for specific gravity variation sources in all three areas are performed.
Starting from a long series of monthly mean atmospheric air pressure data from
1900 to 1988, monthly atmospheric density variations with respect to the long-
term mean are computed and transformed into monthly gravity coefficients. A
similar approach was used to estimate monthly gravity field coefficients from
oceanic mass redistributions. For an 8-year period, monthly spherical harmonic
series up to degree and order 6 from ocean bottom pressure fields, derived from
the POCM ocean circulation model, are estimated. Another method for estimating
the impact of ocean mass redistributions on the gravity field is based on three-year
monthly residual sea-surface models from altimetry, which are corrected for the
thermal water expansion. Attraction of these residual water masses is transformed
into monthly gravity field coefficients up to degree and order 100. Finally, gravity
changes caused by the precession of the inner with respect to the outer core and
their density differences are predicted for a long time series from 1900 to 1991.
Half-yearly gravity coefficients are estimated up to degree 2. Time series for all
gravity field coefficients from these different sources are then analyzed to detect
their amplitudes and phase lags. All calculated gravity signals are compared to the
expected sensitivity of the CHAMP and GRACE missions.