Seite - 16 - in Contributions to GRACE Gravity Field Recovery - Improvements in Dynamic Orbit Integration, Stochastic Modelling of the Antenna Offset Correction, and Co-Estimation of Satellite Orientations

Atmospheric mass flux The atmospheric mass distribution is changed through winds at a very high frequency in a complex pattern. Oceanic mass flux Oceanic mass is redistributed by changing currents at a high fre- quency.Further, thenetmassof theoceancanbechangedbyrun-offofcontinental water and evaporation. Continental water cycle Net mass is added to the continental mass balance through precipitation and can be lost through run-off, evaporation, or transpiration. Mass is stored and transported in surface water bodies, groundwater systems, and aquifers. Cryosphere The cryosphere acts mainly as a mass store, with mass flux occurring at longer time scales than in the remaining continental water cycle. Mass is added to the cryosphere through precipitation and freezing, and lost through evaporation and melting with subsequent run-off. The major constituents of the cryosphere are the Antarctic ice caps, continental glaciers, and the Arctic permafrost. Solid Earth Motion The detectable fractions of solid Earth mass flux are mainly due to seismic and post-seismic activity. Other fluxes in the solid Earth like tidal deformation and glacial isostatic adjustment are described separately. Glacial Isostatic Adjustment Due to isostatic adjustment, mantle material was dis- placed from below glaciated land surfaces in the past. After ice masses melt, the crust elastically rebounds towards its initial configuration. Further, the viscous mantle material slowly flows back to the areas it was previously displaced from. Glacial isostatic adjustment (GIA) describes the combination of these two effects. 3.2 Tides and Tidal Effects In addition to gravitational signals due to processes internal to the Earth system, other solar system bodies induce forces acting on the GRACE satellites through direct and indirect tidal effects. The individual effects are described in more detail in the IERS Conventions (2010) (Petit and Luzum, 2010). Direct tides The gravitational field of other bodies, primarily the Sun and the Moon, induce a tidal potential which directly influences the GRACE satellites. This potential varies with time due to the change in the relative positions of the involved bodies. Solid Earth tides The direct tidal potential leads to a deformation of the solid Earth body, the resulting mass redistribution leads to a secondary tidal effect, the solid Earth tides. Ocean tides Similarly, the changing direct tidal potential induces a mass flux in the oceans, with a corresponding gravitational effect from the redistributed masses. Atmospheric tides Equivalently to the ocean tides, atmospheric masses also move in response to direct tides, with a corresponding gravitational effect. Pole tides Polar motion, the motion of Earth’s rotational axis with respect to its solid body, induces centrifugal forces acting on the solid Earth. These forces in turn Chapter3 Gravity and Other Signals16