Spurious annual vertical deformations over Japan due to mismodelling of tropospheric delays

SUMMARY We quantitatively estimate the spurious annual vertical deformations due to the mismodelling of tropospheric delays over Japan, through a numerical simulation. For this purpose, we simulate the GPS data for present‐day satellite distributions, using realistic cumulative tropospheric delays that are calculated using ray tracing and numerical weather models. To obtain the spurious deformations, the simulated GPS data are then analysed by using the same GPS processing software and the same analysis strategies as those used in the analysis of the observed GPS data. In the estimated spurious annual vertical deformations, it is found that the amplitudes of the deformations increase toward the north, reaching up to 3 mm at around 45°N, whereas the phase lags are uniform throughout Japan and vary by around 220°, indicating spurious subsidence in the middle of February. In the western part of Japan, the observed annual vertical deformations are less affected by spurious annual vertical deformations and more likely caused by the atmospheric and oceanic loading. In the northern part of Japan, we observe large (up to 8 mm) annual vertical deformations with phase lags corresponding to subsidence in winter; subsidence in winter is interpreted to be the effect of snow loading. However, some part of the observed subsidence is found to be spurious due to the mismodelling of the tropospheric delays. These spurious signals may be mitigated by using mapping functions that are constructed by using direct ray tracing and numerical weather models in the GPS analysis.