Study on regional scale mass variation using GRACE satellite gravity data

The dedicated gravity mission GRACE (Gravity Recovery and Climate Experiment) launched in 2002 is the first satellite which provides temporal gravity field variation of the Earth with an unprecedented accuracy. GRACE is providing us monthly solutions of global gravity fields as a set of spherical harmonic coefficients, and several studies have already showed that GRACE has successfully revealed the seasonal varying signals in the spatial scale about 1000 km. On the other hand, although it is known that GRACE can detect further small scale mass variation at least about several hundred km, such smaller spatial scale mass recovery has not been sufficiently studied. Thus, regional scale mass recovery from GRACE monthly gravity field solutions is discussed in this study. The seasonal landwater mass variation in the Indochina Peninsula and the interannual ice sheet mass change in Antarctica are picked up and studied. In the Indochina Peninsula, regional scale mass variations of 4 major river basins, i.e., Salween, Chao Phraya, Irrawaddy and Mekong river basins, were estimated. The results show that GRACE detected at least the Mekong and Irrawaddy basin scale variations, while the detection of the mass variations associated with Salween and Chao Phraya river basins is still difficult. The comparison over the combined area of the 4 river basins shows fairly good agreement, although there remain small discrepancies quantitatively. Amplitudes of the annual signals from the GRACE solutions are 0.9-1.4 times larger than the one from the hydrological model, and the phases are delayed about 1 month compared with the model signal. The phase differences are probably due to improper treatments of the groundwater storage process in the hydrological model and this suggests that the GRACE data possibly give constraints to the model parameters. Further, it was proved that the obtained regional mass variation can be used for the separation of the local and the regional signal containing in-situ gravimetry data. Thus, regional mass variation estimated by GRACE data also contributes to discuss the local signal precisely. For the ice sheet mass variation in Antarctica, the sources of the 3 areas with striking interannual mass trend was investigated. In Antarctica, it is well known that post glacial rebound (PGR) also causes large mass trend. Thus to discuss the mass sources by each regional mass trend area contributes to discuss the large (continental or sub-continental) scale ice sheet mass change more precisely. Besides of the GRACE data, other various models and observations were used for discussion. Among these 3 areas, the source of the positive mass trend in Enderby Land, East Antarctica, has not been well explained due to the uncertainties in the estimation of the surface ice sheet mass balance, the post glacial rebound (PGR) and the errors in the GRACE data processing. In this study, the GRACE mass trend was compared with the ones estimated from in-situ snow-stake measurements and the Ice Cloud and land Elevation Satellite (ICESat) data. The result shows that the main part of the mass trend can be explained by the snow accumulation. Further, interannual mass trends in East, West, and the whole area of Antarctica were also estimated. The result shows that the Antarctic ice sheet mass is decreasing and the rate is large in West Antarctica. However, because of the large uncertainties of now available PGR models, the quantities of the decrease cannot be discussed precisely, at present.