科研之友
微信
新浪微博
Facebook
分享链接
摘要
Both GPS displacements and Gravity Recovery and Climate Experiment (GRACE) harmonics are important measurements for studying surface loading. The discrepancies of the annual variations of horizontal displacements observed by GPS and GRACE are much larger than in the verticals. So some previous studies only use GPS verticals to study surface loading. However, the precision of GPS horizontals is much higher than the verticals and GPS horizontals can be used to extend GPS studies of surface loading. Here we analyze and explain the discrepancies of the annual variations of horizontal displacements observed by GPS and GRACE. To investigate the possible origins of the discrepancies between GPS and GRACE (especially in the horizontals), we analyze the effects of GPS residuals uncertainty, thermoelastic deformation and region-scale loading variations. We compare ITRF2008-GPS residuals (232 IGS global stations) with surface displacements derived from GRACE gravity fields up to degree 60 (CSR RL05 version). All the ITRF2008-GPS residuals in the east, north and up components include only the non-linear component of station motion with respect to ITRF2008. Therefore, we do not need to remove linear terms. Outliers and offsets have also been detected and removed. For GRACE data, we firstly restore atmospheric and oceanic contributions, using a separate file (GAC) with monthly average numerical model values. Additionally an estimate of degree-1 gravity coefficients is added. Finally GRACE gravity coefficients are transformed to surface displacements according to. The difference of the annual phase is smaller than 60 days for about 50% stations and the annual amplitude of GPS is about 2 similar to 3 times larger than GRACE. Comparison between different GPS residuals (ITRF2008-GPS vs JPL) shows that some of the discrepancy between the GPS and GRACE is due to technique uncertainties in the GPS data processing. However further analyses show GPS-related uncertainties are not the major reason for the discrepancy. Firstly a large part of the discrepancies in the N-S direction between GPS and GRACE can be explained by GPS displacements induced by temperature variation, using the global thermoelastic solution from (Fang et al., 2014). Thermoelasitc displacements observed by GPS (not captured by GRACE) should be removed for loading study using GPS data. Secondly we find that the annual horizontal variations observed by GPS are not as coherent as GRACE, especially over Europe, although the uncertainty of the annual horizontal variations over Europe is better than other regions. Analyses demonstrate that the variability from station to station within Europe may be induced by small-scale loading variations. Agreements between GPS and GRACE in the horizontals are not as good as in the verticals. Firstly, it is difficult to exactly separate seasonal variations from GPS horizontals with large trends and smaller seasonal signals. Secondly, for the verticals, the annual phases caused by temperature variations and mass variations are almost in phase and the annual amplitude (similar to 0. 4 mm) caused by temperature variations is much smaller than that caused by mass variations. So even if the thermoelastic displacements are not removed from the verticals, GPS verticals still agree well with GRACE. The situation is more complex for the horizontals. Finally, the horizontals are more sensitive to the local loadings, as it is not only related to the mass variations (loss or gain), but also the location of the mass.
- 出版日期2015-9
- 单位武汉大学
