Gravity Recovery And Climate Experiment Products Research Articles

Land water storage variability over West Africa estimated by Gravity Recovery and Climate Experiment (GRACE) and land surface models

Land water storage plays a fundamental role in the West African water cycle and has an important impact on climate and on the natural resources of this region. However, measurements of land water storage are scarce at regional and global scales and especially in poorly instrumented endorheic regions, such as most of the Sahel, where little useful information can be derived from river flow measurements and basin water budgets. The Gravity Recovery and Climate Experiment (GRACE) satellite mission provides an accurate measurement of the terrestrial gravity field variations from which land water storage variations can be derived. However, their retrieval is not straightforward, and different methods are employed, which results in different water storage GRACE products. On the other hand, water storage can be estimated by land surface modeling forced with observed or satellite‐based boundary conditions, but such estimates can be highly model dependent. In this study, land water storage by six GRACE products and soil moisture estimations by nine land surface models (run within the framework of the African Monsoon Multidisciplinary Analysis Land Surface Intercomparison Project (ALMIP)) are evaluated over West Africa, with a particular focus on the Sahelian area. The water storage spatial distribution, including zonal transects, its seasonal cycle, and its and interannual variability, are analyzed for the years 2003–2007. Despite the nonnegligible differences among the various GRACE products and among the different models, a generally good agreement between satellite and model estimates is found over the West Africa study region. In particular, GRACE data are shown to reproduce well the water storage interannual variability over the Sahel for the 5 year study period. The comparison between satellite estimates and ALMIP results leads to the identification of processes needing improvement in the land surface models. In particular, our results point out the importance of correctly simulating slow water reservoirs as well as evapotranspiration during the dry season for accurate soil moisture modeling over West Africa.

GRACE Hydrological estimates for small basins: Evaluating processing approaches on the High Plains Aquifer, USA

The Gravity Recovery and Climate Experiment (GRACE) satellites provide observations of water storage variation at regional scales. However, when focusing on a region of interest, limited spatial resolution and noise contamination can cause estimation bias and spatial leakage, problems that are exacerbated as the region of interest approaches the GRACE resolution limit of a few hundred km. Reliable estimates of water storage variations in small basins require compromises between competing needs for noise suppression and spatial resolution. The objective of this study was to quantitatively investigate processing methods and their impacts on bias, leakage, GRACE noise reduction, and estimated total error, allowing solution of the trade‐offs. Among the methods tested is a recently developed concentration algorithm called spatiospectral localization, which optimizes the basin shape description, taking into account limited spatial resolution. This method is particularly suited to retrieval of basin‐scale water storage variations and is effective for small basins. To increase confidence in derived methods, water storage variations were calculated for both CSR (Center for Space Research) and GRGS (Groupe de Recherche de Géodésie Spatiale) GRACE products, which employ different processing strategies. The processing techniques were tested on the intensively monitored High Plains Aquifer (450,000 km2 area), where application of the appropriate optimal processing method allowed retrieval of water storage variations over a portion of the aquifer as small as ∼200,000 km2.

Glacial isostatic adjustment and nonstationary signals observed by GRACE

Changes in hydrologic surface loads, glacier mass balance, and glacial isostatic adjustment (GIA) have been observed using data from the Gravity Recovery and Climate Experiment (GRACE) mission. In some cases, the estimates have been made by calculating a combination of the linear rate of change of the time series and periodic seasonal variations of GRACE estimates, yet the geophysical phenomena are often not stationary in nature or are dominated by other nonstationary signals. We investigate the variation in linear rate estimates that arise when selecting different time intervals of GRACE solutions and show that more accurate estimates of stationary signals such as GIA can be obtained after the removal of model‐based hydrologic effects. We focus on North America, where numerical hydrological models exist, and East Antarctica, where such models are not readily available. The root mean square of vertical velocities in North America are reduced by ∼20% in a comparison of GRACE‐ and GPS‐derived uplift rates when the GRACE products are corrected for hydrological effects using the GLDAS model. The correlation between the rate estimates of the two techniques increases from 0.58 to 0.73. While acknowledging that the GLDAS model does not model all aspects of the hydrological cycle, it is sufficiently accurate to demonstrate the importance of accounting for hydrological effects before estimating linear trends from GRACE signals. We also show from a comparison of predicted GIA models and observed GPS uplift rates that the positive anomaly seen in Enderby Land, East Antarctica, is not a stationary signal related to GIA.

Signal and error in mass change inferences from GRACE: the case of Antarctica

SUMMARY This paper examines mass changes from Gravity Recovery and Climate Experiment (GRACE) monthly spherical harmonic gravity field solutions, focusing on the application to the Antarctic ice sheet. Regional integration approaches are investigated and compared with an alternative technique of fitting predefined mass change patterns. The main thrust of this new analysis is to better define error sources and error bounds on mass changes of the Antarctic ice sheet. Toward this purpose, we examine the release-4 GRACE products by the GeoForschungsZentrum Potsdam. Although errors related to the correction for glacial isostatic adjustment are significant, both leakage and GRACE error effects are also important. In particular, the details of how mass change interior to the Antarctic continent is weighted and scaled is crucially linked to the end-result mass trend solution. Accounting for error correlations in the GRACE monthly solutions, it is shown that the corresponding errors in Antarctic mass changes are about two times larger than predicted by uncorrelated error propagation. Our ice mass trend estimate for the grounded ice sheet in the period from 08/2002 to 01/2008 is (−109 ± 48) Gt yr −1 , equivalent to a mean eustatic sea level rise of (+0.30 ± 0.13) mm yr −1 .W e also provide estimates for 16 individual drainage basins. Mass losses concentrate in the coastal regions of West Antarctica and on the Antarctic Peninsula with together (−105 ± 35) Gt yr −1 .T he Amundsen Sea Sector alone contributes (−60 ± 8) Gt yr −1 . Although these findings agree with independent observations, GRACE trends deduced from a short time-series come with the cautioning footnote that the strength of interannual variations remains undetermined. We finally discuss directions of methodological improvements.

Basin‐scale water‐balance estimates of terrestrial water storage variations from ECMWF operational forecast analysis

In recent publications, a new basin‐scale dataset of monthly variations in terrestrial water storage (BSWB) was derived for the ERA40 time period (1958–2002) using an atmospheric‐terrestrial water‐balance approach (Seneviratne et al., 2004; Hirschi et al., 2006). Here, we test the feasibility of using ECMWF operational forecast analyses – available for the recent time period in near real time – instead of reanalysis data for the derivation of these estimates. Our results suggest that the moisture flux convergence from the ECMWF operational forecast analysis is generally consistent with ERA40 in the investigated regions, including 35 mid‐latitude river basins and domains. For ten domains with recent streamflow measurements, water‐balance estimates of monthly terrestrial water storage variations derived using the ECMWF operational forecast data are compared with estimates from the Gravity Recovery and Climate Experiment (GRACE). In general the atmospheric‐terrestrial water‐balance estimates show more geographical detail than the analyzed standard resolution GRACE products.

Seasonal variation of ocean bottom pressure derived from Gravity Recovery and Climate Experiment (GRACE): Local validation and global patterns

The Gravity Recovery and Climate Experiment (GRACE) processing centers at the GeoForschungsZentrum Potsdam (GFZ) and the University of Texas Center for Space Research (UTCSR) provide time series of monthly gravity field solutions covering the period since mission launch in March 2002. Although the achieved accuracy still remains an order of magnitude below the mission's baseline goal, these time series have successfully been used to study terrestrial phenomena such as water storage variations. Over the oceans, the monthly gravity field solutions can be converted into estimates of the fluctuating ocean bottom pressure (OBP), which is the sum of atmospheric and oceanic mass variations. The GRACE products may be validated against in situ OBP observations which are available from a ground truth site in the tropical northwest Atlantic Ocean. Large differences are observed between the in situ and GRACE‐derived OBP which are investigated by comparing the tidal and nontidal ocean models used at GFZ and UTCSR for dealiasing short‐term (<2 months) mass variations from satellite measurements. Results show that the barotropic nontidal and tide models need improvement at periods shorter than 1 day and longer than 2 weeks. On a global scale the monthly OBP fields from GRACE generally overestimate the variability compared to ocean general circulation models, especially in tropical regions. This may be attributed to continuing deficiencies in GRACE data processing. Nevertheless, there is some initial evidence that GRACE possesses the potential to observe large‐scale averages of bottom pressure fluctuations.

Can surface pressure be used to remove atmospheric contributions from GRACE data with sufficient accuracy to recover hydrological signals?

The Gravity Recovery and Climate Experiment (GRACE) satellite mission will resolve temporal variations in gravity orders of magnitude more accurately and with considerably higher resolution than any existing satellite. Effects of atmospheric mass over land will be removed prior to estimating the gravitational field, using surface pressure fields generated by global weather forecast centers. To recover the continental hydrological signal with an accuracy of 1 cm of equivalent water thickness down to scales of a few hundred kilometers, atmospheric pressure must be known to an accuracy of 1 mbar or better. We estimate errors in analyzed pressure fields and the impact of those errors on GRACE surface mass estimates by comparing analyzed fields with barometric surface pressure measurements in the United States and North Africa/Arabian peninsula. We consider (1) the error in 30‐day averages of the pressure field, significant because the final GRACE product will average measurements collected over 30‐day intervals, and (2) the short‐period error in the pressure fields which would be aliased by GRACE orbital passes. Because the GRACE results will average surface mass over scales of several hundred kilometers, we assess the pressure field accuracy averaged over those same spatial scales. The atmospheric error over the 30‐day averaging period, which will map directly into GRACE data, is generally <0.5 mbar. Consequently, analyzed pressure fields will be adequate to remove the atmospheric contribution from GRACE hydrological estimates to subcentimeter levels. However, the short‐period error in the pressure field, which would alias into GRACE data, could potentially contribute errors equivalent to 1 cm of water thickness. We also show that given sufficiently dense barometric coverage, an adequate surface pressure field can be constructed from surface pressure measurements alone.