The GRACE monthly gravity fields are not perfect. Sensor and modeling noise are present in the solutions, the largest of which manifests itself as North-South striping. Filtering the solution eliminates a large part of the stripes and smoothing attenuates the noise, but of course not all the noise is removed. One way to evaluate the noise level in the monthly gravity field solutions is by analyzing the variations in equivalent water height (EWH) seen over regions with very little rainfall and hydrological mass redistribution. The largest of such zones is the Sahara desert. When calculating the standard deviation of the water signal over the Sahara, with the EGSIEM plotter for example (lowest figure below), the noise level as a function of spatial resolution can estimated. The figure below shows the largest surface used in the estimation, which is about 500 x 3000 km. For such a large surface the noise level (i.e. the standard deviation of the EWH signal) is about 17 mm. The time series plot, made with the EGSIEM plotter, displays the variations in EWH for 5 different solutions. When we reduce the blocks to 500 x 1000 km, the noise increases to 20-35 mm. For 500 x 500 km blocks, the noise level is estimated at 30-60 mm (depending on the selected solution).
