U21C-0613
Further assessments of CSR RL04 GRACE gravity field solutions
The GRACE Release-04 gravity field data products from UTCSR are considerably improved compared to the previously available Release-01 products. This improvement has been realized through improvements in background gravity models as well as algorithmic changes in the GRACE data processing. These improved products have been available to the user community since late Feb 2007. The interpretation of RL-04 products is slightly different when compared to RL-01. Besides the improvement in quality, a different set of background gravity field models have been applied to the data before the RL-04 products were created. This leads to a potential change in the way these products must be interpreted. The paper will summarize the feedback received from the user community on the applications of these new data products. The error estimates of the RL-04 fields and their effect on interpretation will be presented. A brief review of future directions for product improvement will be provided, as well.
U21C-0614
Exploration of the Spatio-temporal Spectrum of the Mass-Transport Information Resolvable by GRACE
The spatio-temporal resolution of GRACE is altered by a variety of windowing filters acting on the satellite data. Shorter time scales are resolved at limited cost in terms of spatial resolution and data quality, thus allowing GRACE to recover sub-monthly variations in gravity. The error characteristics of these filters are presented, and the spectra of their effects on the mass-transport signal are analyzed both regionally and globally.
U21C-0615
High-Resolution Analysis and Modeling of GRACE Accelerometer Observations
A better understanding and modeling of high-resolution GRACE accelerometer data serves three purposes: (1) to ensure that the best possible data are used in the GRACE gravity field processing, (2) to obtain precise and clean non-gravitational accelerations for aeronomy research, and (3) to understand and quantify disturbances which may play a role for future space-borne accelerometry. The external non-gravitational forces acting on the twin GRACE satellites are superimposed by a complex signal pattern of satellite-induced effects, originating from switching events in electrical circuits of on-board heaters and magnetic torquers, from vibrations and thruster accelerations. For each of these processes, we compared and averaged 10 Hz acceleration signals from a large number of events from long accelerometer time series. The analysis results provide constraints, e.g., on thrust accuracy, misalignments, and vibration frequencies. These constraints may help to understand the underlying physics. We modeled and reduced acceleration signals due to thrusters and heater switching and obtained considerably smoother and cleaner signals of external non-gravitational accelerations which may be useful for applications in aeronomy research.
U21C-0616
Monthly land waters solutions by least-squares inversion of GRACE geoid data (2002- 2006)
A least-squares inversion procedure is used to derive the Land Waters (LW) variations at the resolution of ~400 km by combining: (1) monthly GRACE solutions from August 2002 to February 2007 recently made available by GFZ; and (2) a priori information from hydrology models. This approach proves to be an efficient strategy to separate different hydrological components (i.e., continental water storage included snow) from other geophysical phenomena, residual errors and high-frequency noise. Time-series of continental water storage for large drainage basin can be derived from these LW solutions, and proved to be consistent in comparison with sparse in situ measurements of water levels and global hydrology models. A posteriori uncertainties on the estimated LW coefficients and errors from spectrum truncation and leakage can also be computed.
U21C-0617
Determination of dominant periodic components of water storage changes from GRACE and global hydrology models
Variations of mass anomalies at the Earth's surface derived from monthly time series of global models of the Earth's gravity field of the US-German twin satellite mission GRACE (Gravity Recovery and Climate Experiment) clearly trace mass redistributions induced by continental hydrology. Previous studies indicate that such mass redistributions are dominated by annual and - in some regions - semiannual variations. In this contribution, we explicitly estimate for the first time the spectra of such dominant periodic patterns (in terms of periods, phases and amplitudes) that are not restricted to the fundamental annual frequency and its overtones. To this end we use a novel method that combines conventional empirical orthogonal functions (EOF) analysis with a determination of sine waves of arbitrary periods from the principal components based on time series of spatial maps of surface mass anomalies from GRACE and global hydrology models. The significance of the GRACE-derived spectra in view of the correlated GRACE data errors is assessed by means of a Monte-Carlo technique using available GRACE error covariance information. Considering only the significant, but dominating terms we can construct filtered GRACE data series which will serve for an improved validation and calibration of global hydrology models. This is demonstrated using the results from about 5 years of GRACE gravity fields of the GFZ-RL04 series and from independent state-of-the-art hydrology models. As one result our study reveals a systematic advance of the phases of the dominating annual terms of the hydrological models as compared to GRACE in the range of 1 to 6 weeks. This indicates deficiencies of the hydrological models w.r.t. runoff routing in the river network and/or water retention in lakes and wetlands. As a further result the analysis shows that besides annual and semiannual variations (specific only to some basins) also long-periodic signals with periods in the range of 2.1 to 2.5 years contribute to the periodic water storage variations. These may be related to long-term variations of the continental water storage (e.g. from the El Nino - Southern Oscillation) and comparisons to corresponding climatological indices will be presented.
U21C-0618
Seasonal and Interannual Variability of the Red Sea
The Red Sea is almost completely enclosed. It can exchange water with the open ocean only through the strait of Bab el Mandab that opens into the Gulf of Aden to the south. The seasonal exchange of water through that strait occurs through a multi-layer flow pattern that tends to make the Red Sea colder and saltier, and thus more dense, during the summer; and warmer and fresher, and so less dense, during the winter. These density changes presumably cause seasonal steric variations in sea surface height. Here, we isolate and interpret this steric signal by combining sea surface height observations from satellite altimetery (JASON-1 and ENVISAT radar altimetry; and ICESat laser altimetry) and tide gauge data, with estimates of Red Sea mass variability from GRACE. The results are compared with surface temperature observations from Aqua to separate the effects of surface heating from those of the exchange through the Bab el Mandab.
U21C-0619
Evaluation of GRACE data filters from a hydrological perspective
At this time, about four years of monthly global gravity field solutions from the satellite gravity mission GRACE (Gravity Recovery And Climate Experiment) are available for the scientific public. Transformed into time series of global maps of surface mass anomalies, the data can be used to quantify mass redistributions close to the Earth surface and to contribute significantly to an improved modeling of the underlying processes. Due to the current processing strategies applied by the Science Data Systems for the GRACE gravity recovery, mass variations caused by the continental water cycle dominate the GRACE data series. This makes hydrological model improvement a first application. However, because of correlated errors of the GRACE-only gravity models visible as a striping fragmentation in spatial maps of the GRACE derived gravity data, appropriate filter techniques have to be applied before any analysis. Questions repeatedly posed in this context are: (1) Which filter has sufficient de-striping (i.e. decorrelation) properties? (2) What is the resulting impact on the signal amplitude (i.e. damping) and phase lags? (3) Which filter is optimal for which scale, location or shape of the area to be analyzed? (4) Which filter is preserving local signal properties, thus optimal for which source of mass variations? This study focuses on the third and fourth question by means of a comparative analysis of several filter types from a hydrological perspective, i.e., for surface mass variations by continental water storage changes at the scale of river basins. To this end six published external methods for the derivation of regionally averaged water mass variations from GRACE gravity data were evaluated. These comprise isotropic and anisotropic filters respectively decorrelation methods that use specific constraints on the signal-noise properties of the GRACE and the to-be detected process signal. To evaluate the filter properties, we compute time series of water mass variations from GRACE GFZ-RL04 data and from the WaterGAP Global Hydrology Model (WGHM) for 22 of the world's largest river basin systematically by varying the relevant parameter settings of each filter tool. Applying a criterion of correspondence between modeled and measured time series we obtain a comprehensive assessment of the individual methods. Dependencies on the geographic location, basin shape and signal characteristics relevant for hydrological applications are analyzed and results are confirmed by alternative global hydrological models.
U21C-0620
High Resolution Recovery of Amazon Basin Water Storage Change Using Line-Of-Sight (LOS) Gravity Difference Data from GRACE
GRACE Level 1B data have been analyzed and processed to recover continental water storage in a regional solution, by first estimating in situ Line-Of-Sight (LOS) gravity differences simultaneously with the relative position and velocity vectors of the twin GRACE satellites. This new approach has been validated using a simulation study over the Amazon basin (with three different regularization methods to stabilize the downward continuation solutions) and it is demonstrated that the method achieves an improved spatial resolution as compared to some of the other GRACE processing techniques, including global spherical harmonic solutions, and regional solution using in situ geopotential differences. When compared with a fine-scale (20 km full-wavelength) hydrologic model of the Amazon basin, which accounts for the modeling of surface, subsurface, channel and floodplain stores and fluxes, the GRACE solutions show no discernible time lags with respect to the hydrologic model, resolving the controversy of the allegedly observed 1–2 month lags between GRACE and a number of global hydrologic models. The comparison also enables quantification of the respective contributors of the Amazon Basin water flow dynamics.
U21C-0621
GRACE Water Storage Estimates in Finland and the Effect of Baltic Sea Level Variation
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.
U21C-0622
Laurentia Glacial Isostatic Adjustment Observed From GRACE and Satellite Altimetry
The use of satellite radar altimetry to detect solid Earth deformation signals such as the Glacial Isostatic Adjustment (GIA) has been demonstrated by Lee et al. (2007) observing the Laurentia GIA signals using TOPEX (1992–2002) over the Hudson Bay land region. GRACE gravimetry, despite of its short data record (2002–present), confirmed the presence of two Pleistocene ice domes near the east and west of Hudson Bay manifested in the incomplete GIA in Laurentia (Tamisiea et al., 2007). In this study, we densify land uplift measurements over the Hudson Bay land region by developing 10-Hz stackfile from TOPEX (1992-2002), and by combining altimetry measurements and long-term water level records (Kuo et al., 2007) for data over small lakes. Here we use GRACE observations of Hudson Bay land region uplift, including the use of spherical harmonic and regional solutions (spherical wavelet solutions directly using L1B KBR data), radial GIA models (ICE4G, ICE5G, BIFROST), and 3-D laterally heterogeneous GIA model (RF3S20), and satellite altimetry observed deformation rates, to study potential constraints on the Pleistocene GIA process over Laurentia, including ice modeling and Earth rheology.
U21C-0623
Antarctic ice sheet mass balance estimation by combing GRACE, ICESat and in situ data
An accurate knowledge of Antarctic ice sheet mass balance is one of the important problems for the studies of global sea level change and climate change. One of the advantages to use GRACE gravity field solutions for the study of Antarctic ice sheet mass balance is that GRACE can reveal total mass balance over Antarctica, which is closely related to the global sea level changes, e.g. Velicogna and Wahr (2006). Because of the expanding of the time span of the released data sets and the improvement of the data quality, we can obtain more reliable interannual mass trend with GRACE data. However, the estimated total Antarctic ice sheet mass balance still has large uncertainty because of the large difference of the PGR estimation with several ice models and the difference of GRACE solutions between the 3 data centers. To decrease such uncertainty and to obtain more reliable ice sheet mass trend, we have to validate the estimated values of GRACE mass trends and the PGR predictions. It is important to investigate the smaller regional scale mass trends under considering the spatial distribution of the Antarctic mass trend because of the different geographical and geological feature by each region. Thus, in this study, we firstly investigated three prominent mass trend areas in Antarctica. We discussed the main sources of the each mass trend and estimated ice sheet mass components by combing the results of in situ and other satellite observed data, e.g. snow-stakes data, continuous GPS results and ICESat data. We also investigated other small mass trend areas. Because of the large area, they give relatively large contribution to the total Antarctic mass trend even if the trend is small. Thus, considering the contribution of the mass trends of each small region to the total Antarctic ice sheet mass variation, we finally discussed how much the total Antarctic ice sheet mass estimation is improved and the uncertainty is decrease by investigating the small scale mass variation in detail.
U21C-0624
Seasonal Variability of Mass in the Arctic Ocean Using GRACE and In Situ Bottom Pressure Measurements
The Ocean Bottom Pressure (OBP) measured by the Gravity Recovery and Climate Experiment (GRACE) was recently confirmed to be accurate for the Arctic Ocean Basin by Morison et al. (2007). They showed a high correlation between GRACE and in situ Arctic Bottom Pressure Recorder (ABPR) measurements near the North Pole, and showed that the multiyear trends in bottom pressure were consistent with the ocean circulation reverting to a pre-1990s anticyclonic pattern. Here we examine seasonal changes in bottom pressure. Both GRACE (2002-2007) and ABPR (2005-2007) pressure measurements at the North Pole show a seasonal peak-to-peak variability in the Arctic Ocean of about 6-10 cm of water equivalent with a maximum occurring during the summer and a minimum during the winter. In addition to the North Pole, annual cycles are also displayed by three years of in situ OBP measurements from the Beaufort Gyre, from south of the Aleutian Islands, and from the Fram Strait. The OBP time-series from the four locations (ABPR at the North Pole included) were treated equally, de-tided, and averaged to the same time resolution. The OBP time series from the Fram Strait, the North Pole and the Beaufort Gyre were all positively correlated at a 99% significance level, with correlation coefficients from 0.61 between North Pole and Beaufort Gyre to 0.8 between the Fram Strait and the Beaufort Gyre. The annual cycle, as determined by the first harmonic fitted to each time series, is nearly in phase between these three locations, indicating a mass maximum during the summer and a minimum during the winter. The OBP near the Aleutian Islands was not significantly correlated with any of the other OBP measurements in the Arctic Basin. Seasonal forcing by the atmosphere is investigated. Empirical Orthogonal Functions of sea level pressure, geostrophic winds and GRACE OBP have been used to analyze the response of the OBP to the seasonal atmospheric forcing and the spatial scale at which such relation occurs. The principal component time-series of the leading mode for the geostrophic winds (PCw1) reveals the annual cycle and explains 25% of the variance of the wind field. The leading EOF mode of the GRACE OBP (PCp1) in the Arctic Basin shows a strong annual cycle, which explains 36% of the total variance. Reference: Morison, J., J. Wahr, R. Kwok, and C. Peralta-Ferriz (2007), Recent trends in Arctic Ocean mass distribution revealed by GRACE, Geophys. Res. Lett., 34, L07602, doi:10.1029/2006GL029016.
U21C-0625
GRACE Solutions for the Gravity Field over Central Europe Compared to the Surface Field as Recorded by the GGP Network.
As the number of different solutions from the GRACE satellite gravity project evolves, we can make more meaningful comparisons between the satellite-derived field and the surface field as recorded by superconducting gravimeters. On the GRACE side, we use CSR Level 2 products RL01 and the recent RL04 solutions, GFZ RL04 solutions, and the CNES/GRGS 10-day solutions, all for the time periods these are available. On the GGP side, we take advantage of the 10 years of SG data since July 1997 from 6-8 ground stations in Europe, allowing for the change in the network configuration as stations begin and end recording. Only data since 2002 can be compared directly to GRACE. Our primary measure of variability is the first principal component of the EOF analysis of all the fields. Unsurprisingly, the seasonal components for all the comparisons are similar in phase, but different in amplitude, to the predictions from a global hydrology model (GLDAS), provided allowance is made for the location of the SG stations above or below the soil moisture horizon that controls the gravity signature. We use detailed modeling at the Strasbourg station, as well as published results for Moxa and Membach, to confirm the gravity effect of hydrology. Good agreement is found between the GGP and the CNES/GRGS 10-day solutions, indicating the higher temporal resolution of this satellite solution is valid for our limited geographical area. We also synthesize the gravity field over the sub-group of GGP stations in N.E. Asia to see how the GRACE variability compares to that for the European array and to assess future ground validation using new GGP stations in that part of the world.
U21C-0626
Results from the GRACE Prime Mission: Monitoring Water Comparison of GRACE Monthly Estimates with Surface Gravity Variations at North American Sites
One of the objectives of the GRACE mission is the monitoring of temporal changes in water mass distribution on the continents. To a first approximation, variations in the mean water content and the corresponding gravity signal can be expressed as a seasonal variation and a trend. We compare GRACE seasonal gravity variations with seasonal surface gravity variations at the Canadian Absolute Gravity Site (CAGS), Cantley (Quebec) and on Vancouver Island, BC. GRACE trends are compared with trends from long-series annual absolute gravity measurements at several sites in the continental interior of North America. In this comparison the monthly GRACE gravity values were derived from the CSR-RL04 spherical harmonic monthly models for the period of April 2002 to June 2007. Both least-squares fitting of the harmonic coefficient series and non-isotropic Gaussian filtering are used to extract the gravity signal from these models. The GRACE values represent estimates of mean gravity extending out to about 300 - 450 km from the comparison sites. For the comparison of seasonal variations at CAGS during the GRACE mission, we have used surface gravity variations provided by a GWR TT70 superconducting gravimeter at Cantley. Surface gravity values on Vancouver Island for comparison with GRACE were estimated at 3-month intervals by averaging absolute gravity measurements at four sites observed using absolute gravimeter FG5-106 (Natural Resources Canada). For the comparison of trends, we used surface gravity measurements carried out annually from 1993 to the present using free-fall absolute gravimeter FG5-106 and, occasionally, FG5-102 (NOAA). Some measurements made using JILA instruments were used before 1992. In the trend comparisons we assume that postglacial rebound is the primary process causing the long-term gravity change and use a theoretical, g-dot to h-dot, ratio of -0.15 μGal / mm and the free-air gradient to remove the vertical motion effect from the surface gravity. Significantly larger seasonal variations are observed at CAGS compared to the variations seen by GRACE, probably as a result of topographic effects. A closer fit of seasonal variations between GRACE data and surface gravity is obtained on Vancouver Island where the mean of four relatively closely grouped sites is used. The comparison of trends between GRACE and long-term, annual absolute gravity data in the continental interior shows remarkably good agreement over the GRACE mission period (2002 - 2007). It is clear from the absolute gravity data that at most sites significantly different five-year trends would be observed by GRACE at different epochs. The inter-annual variations do not appear to be explained by surface soil moisture variations. Deeper sources for the mass variations are suggested.
U21C-0627
Error Estimates for GRACE Data Assimilation Into Ocean Models
Using GRACE data to constrain ocean general circulation models requires quantitative estimates of the errors in GRACE estimates of ocean bottom pressure. We attempt a spatial mapping of these errors by comparing several GRACE data products and bottom pressure simulations from an ocean model. Uncertainties in the spatial mean and in the regional mass anomalies about that mean are considered separately. The resultant error maps, when zonally-averaged, are comparable to the calibrated errors provided by the GRACE processing centers. Noticeable differences are the larger errors at high latitudes and near continental regions with high amplitude hydrological seasonal cycles. The error estimates also depend on which de-aliasing background model is used when processing the GRACE data. Implications for ocean modeling and data assimilation are discussed.
U21C-0628
Antarctic Ocean Tides from GRACE Intersatellite Tracking Data and Hydrodynamic Assimilation
Long-wavelength components of the oceanic tides surrounding Antarctica are estimated from over three years of GRACE satellite-to-satellite ranging measurements. An inversion is performed for the major constituents M2, O1, and S2, parameterized as localized average mass anomalies relative to a prior tidal model. Satellite state adjustments are made simultaneously. These long-wavelength anomalies are then assimilated into a high-resolution regional hydrodynamic tidal model. Comparisons to independent "ground truth" data, previously collected by King and Padman, show that assimilation of the GRACE inversions results in improved accuracy, for all three constituents.
U21C-0629
Preliminary Analysis of Arctic Ocean Tides using GRACE Spacecraft Acceleration Data.
Arctic ocean tidal solutions are not constrained by altimetry data because missions such as TOPEX/POSEIDON do not extend to high latitudes. The resulting errors in tidal models alias into the monthly GRACE gravity field solutions at all latitudes. Fortunately, it is possible to use the GRACE inter-satellite ranging data to solve for these tides directly. Five years of GRACE inter-satellite acceleration data are inverted to solve for the amplitude and phase of major solar and lunar tides in the Arctic ocean using a mascon approach. The resulting tidal amplitudes are compared to existing tidal models.
U21C-0630
Annual, Seasonal, and Secular Changes in Time-Variable Gravity from GRACE
The NASA/DLR GRACE mission, launched in 2002, has now operated for more than five years, producing monthly and ten-day snapshots of the variations of the gravity field of the Earth. The available solutions, either from spherical harmonics or from mascons, allow us new insights into the variations of surface gravity on the Earth at annual, inter-annual, and secular time scales. Our baseline time series, based on GGM02C, NCEP Atmospheric Gravity with IB, and GOT00 tides now is extended to July 2007, spanning four+ years, and we analyze both mascon and spherical harmonic solutions from this time series with respect to global hydrology variations. Our 4degx4deg mascon solutions are extended to cover all continental regions of the globe. Comparisons with hydrology (land-surface) models can offer insights into how these models might be improved. We compare our baseline time series, with new time series that include an updated Goddard Ocean Tide (GOT) model, ECMWF- 3hr atmosphere de-aliasing data, and the MOG-2D ocean dealiasing product. Finally, we intercompare the spherical harmonic solutions at low degree from GRACE from the various product centers (e.g., GFZ, CSR, GRGS), and look for secular signals in both the GSFC mascon and spherical harmonic solutions, taking care to compare the results for secular gravity field change with independent solutions developed over 25 years of independent tracking to geodetic satellites by Satellite Laser Ranging (SLR) and DORIS.
U21C-0631
Low-Frequency Ocean Bottom Pressure Signals in the North Pacific
Ocean Bottom Pressure (OBP) from two numerical models, the Gravity Recovery and Climate Experiment (GRACE), and from steric-corrected altimetry are examined in the North Pacific. Much previous work has been done on the seasonal cycle of OBP in the North Pacific; we will focus on the low-frequency signal that appears as a trend from January 2004 to December 2006. OBP from the two models increased significantly in only one area: the Northwest Pacific, north of the Kuroshio Extension Current (KEC). The two measurements (GRACE and steric-corrected altimetry) also show an increase in the same area, but with a magnitude roughly 2 to 3 times higher. GRACE and steric-corrected altimetry also show a significant increase in the eastern part of the basin off the coast of North America of nearly the same magnitude: both data show a similar rate of increase. The change in the Northwest Pacific is linked to a slow-down in the KEC over the 3-year period. We demonstrate that the trend observed off the coast of North America is not caused by leakage of hydrology into the GRACE measurements, but may be related to gravitational changes caused by changes in the nearby land water storage. Such dynamics are not included in ocean models. GRACE and steric-corrected altimetry are completely independent observations of OBP. Based on the fact that these two measurements agree so well, we conclude that OBP has significant low-frequency variations in the North Pacific and that these signals are underestimated by numerical models.