The influence of pressure waves in tidal gravity records

For the reduction of atmospheric effects, observed gravity has initially been corrected by using the computed barometric admittance k of the in situ measured pressure, expressed in nms⁻²/hPa units and estimated by least squares method. However, the local pressure changes alone cannot account for the atmospheric mass attraction and loading when the coherent pressure field exceeds a specific size, i.e., with increasing periodicities. To overcome this difficulty, it is necessary to compute the total atmospheric effect at each station using the global pressure field. However, the direct subtraction of the total gravity effect, provided by the models of pressure correction, is not yet satisfactory for S2 and other tidal components, such as K2 and P1, which include solar heating pressure tides. This paper identifies the origin of the problem and presents strategies to obtain a satisfactory solution.

First, we set up a difference vector between the tidal factors of M2 and S2 after correction of the pressure and ocean tides effects. This vector, hereafter denoted as RES, presents the advantage of being practically insensitive to calibration errors. The minimum discrepancy between the tidal parameters of M2 and S2 corresponds to the minimum of the RES vector norm d.

Secondly we adopt the hybrid pressure correction method, separating the local and the global pressure contribution of the models and replacing the local contribution by the pressure measured at the station multiplied by an admittance k_ATM.

We tested this procedure on 8 stations from the IGETS superconducting gravimeters network (former GGP network). For stations at an altitude lower than 1000 m, the value of d_opt is always smaller than 0.0005. The discrepancy between the tidal parameters of the M2 and S2 waves is always lower than 0.05% on the amplitude factors and 0.025° on the phases. For these stations, a correlation exists between the altitude and the value k_opt. The results at the three Central European stations Conrad, Pecny and Vienna are in excellent agreement (0.05%) with the DDW99NH model for all the main tidal waves.