Description: The variation in the sea level of the semi-closed Baltic Sea has been monitored in several complimentary ways. Now GRACE provides a method to directly measure the total mass variability in the Baltic. Using in situ and modelled Baltic data, we show that GRACE is able to recover the variation in the total water mass. We derive sea level surfaces from tide gauge data and estimate steric effects using hydrodynamic models as well as in situ salinity and temperature measurements for their verification. With its areal extent (~400 km x 1000 km) as well as fast temporal variations (hourly to monthly), the Baltic Sea provides a challenging test field for the temporal and spatial resolution of GRACE. We use both the standard monthly GRACE gravity field solutions and regional solutions and compare their capability to recover Baltic water mass variations. Due to spatial averaging, the GRACE mass estimates over the elongated area are contaminated by signals outside the region. The contribution of continental hydrology can be removed using water storage models to estimate mass variations on surrounding land areas. We discuss the processing steps required for the different GRACE solutions to improve the GRACE mass estimates for the Baltic, including mitigation of signal leakage as a result of spatial filtering. The capability of GRACE to recover internal mass redistributions in the Baltic is also investigated. Finally, we discuss the reduction of the Baltic contribution for studying land-uplift signal due to post-glacial rebound.

Description: The monthly variation in the water mass of the semi-enclosed Baltic Sea is about 60 Gt RMS over an area of 390000km2 . The Baltic has a dense network of tide gauges (TGs), and several high-resolution regional hydrodynamic models, making it one of the best-monitored seas for mass variations of this size in the world. We investigate the performance of different GRACE gravity field solutions to recover this oceanic mass variation using in situ measurements of sea-level heights. For GRACE, we use both the standard monthly solutions as well as regional solutions to estimate the total water storage in the Baltic Sea. For the “ground truth”, we use sea-level measurements in the network of tide gauges around the Baltic Sea. For comparison, we have access to data from two hydrodynamic models. The water mass estimated from the constructed sea surfaces is then compared with different GRACE estimates. At present, we ignore the steric anomalies, as they are small in the Baltic Sea. We also discuss the “leakage” between land hydrology and the Baltic Sea in the GRACE estimates of water storage.

Description: We compare the GRACE water storage estimates in Finland with the total water storage from a high-accuracy local hydrological model. Being a significant source of mass variations, we also discuss the leakage of the Baltic mass into estimates of continental water storage in GRACE solutions. For the total water storage, we use the model of the Watershed Simulation and Forecasting System (WSFS) of the Finnish Environment Institute. WSFS covers the whole hydrological cycle, including surface and subsurface water and snow. For GRACE water storage, we use three estimates obtained through different processing methods. First, we use the standard monthly GRACE gravity field solutions and apply appropriate filtering. To improve the spatial and temporal resolution, two regional solutions are studied. NASA has recently made available GRACE estimates of 10-days mass change in 4x4 degree blocks (mascons) over the worlds' continents covering the period April 2003 to April 2006. In addition to the mascon solutions, we will also use the regional solutions employing GRACE KBR data derived from in situ disturbance potential measurements via the energy conservation method. The monthly variation in the water mass of the semi-enclosed Baltic sea is about 60 Gt RMS. It is governed by the water exchange with the North sea through the Danish straits, and is difficult to catch in the global ocean circulation models. We study the contribution of Baltic mass variation to the GRACE estimates of water storage. On the other hand, the Baltic has both a dense network of tide gauges and several specific high-resolution hydrodynamical models, making it possibly the best-controlled mass variation of this size in the world. We discuss the possibilities of using Baltic mass to validate and compare GRACE solution methods.