Basin Mass Changes in Finland From GRACE: Validation and Explanation

The signal leakage effect in the use of the Gravity Recovery and Climate Experiment (GRACE) level-2 products limits the accuracy of the mass inversion, especially in small spatial scales (less than similar to 100,000 km(2)). In this study, we try to use GRACE observation to evaluate the basin mass changes in Finland, where the basins are quite small (less than 93,000 km(2)) and the accurate mass inversion is challenging. To this end, we carry out a series of numerical simulation experiments and optimize the parameters of the GRACE inversion methods. We propose a hybrid inversion strategy, which takes the differences of both inversion methods and basins into consideration. This strategy is expected to achieve the statistically optimal estimation of the basin mass changes in Finland with Nash-Sutcliffe Efficiency values ranging from 0.82 to 0.92, corresponding to Index Of Agreement values from 0.95 to 0.98. Further, based on similar to 19.7 years of real GRACE data, we analyze the seasonal and interannual terrestrial water storage (TWS) changes in Finland. We find that snow plays a key role in the annual cycle of the TWS change. A remarkable phase difference (similar to 5 months) between TWS and precipitation is mainly attributed to the effects of snow accumulation and melting. We also find that there is an obvious interannual oscillation with a period of similar to 3.8 years in the TWS change of Finland, which seems to be a superposition effect of the real hydrological signal and the tide aliasing error.