Recent Accomplishments of the GRACE Mission II Posters
Presiding: B Tapley, University of Texas; C Reigber, GeoForschungsZentrum Potsdam
G23B-01 1330h
GRACE estimates as a constraint on non-tidal oceanic contribution to polar wobble
Contributions of ocean bottom pressure and oceanic current to polar wobble are evaluated by using two oceanic data assimilation products. One product comes from Scripps Institution of Oceanography (ECCO), and the other is the Simple Ocean Data Assimilation (SODA) developed at University of Maryland. The results show that seasonal fluctuations in these two assimilated ocean angular momentum (OAM) models agree with each other better along the Greenwich meridian than along the 90-aE meridian. Furthermore, seasonal OAM changes for ECCO are much closer to non-atmospheric residual, than those for SODA. However, seasonal OAM changes for SODA along the Greenwich meridian compares better than those for ECCO to non-atmospheric-hydrologic residual, in which annual and semiannual signals of land hydrologic angular momentum of a climate model, are considered, and annual signal is also compatible with the GRACE-derived land one along the 90-aE meridian. It is likely that annual ECCO change along the 90-aE meridian is overestimated according to the angular momentum conservation law in the Earth system. Consequently, the oceanic role in polar wobble should be further examined. Keywords: Oceanic Angular Momentum (OAM), polar wobble, Earth Rotation
G23B-02 1330h
GRACE Gravity and Airborne Surveys in the Arctic: How They Agree, Differ and Complement Each Other
In the past dozen years large areas of the Arctic have been surveyed using accurate airborne gravity techniques for the most part by the Naval Research Laboratory (NRL). Precise NRL surveys conducted in the late 1990s include numerous long (>1500 km) lines extending from the Svalbard area over the pole to the Canada Basin. These surveys validate - rather well - the GRACE mean fields (e.g., the UTCSR GGM02S) inasmuch as the GRACE and aero-gravity here match each other in the waveband 350 to 3000 km. These late-90s NRL surveys are precise ( ~ 2mGal) and detailed as they were flown at low ( ~ 300 m) elevations. Now that the GRACE data have been validated, we can in turn use these data to evaluate the quality of earlier airborne data collected during times of poorer GPS positioning. In fact, differences between GRACE and low-pass filtered airborne and surface gravity can be used to identify areas where airborne surface gravity are likely be substantially in error. We show examples of such places over southeastern Greenland and the Canadian Archipelago. Ultimately, a high-accuracy Arctic gravity field and geoid can be produced by combining the long-wavelength component of GRACE gravity with the short-wavelength airborne and surface gravity. However, by first using the GRACE data to correct long-wavelength problems in the airborne and surface measurements, a better quality gravity field and geoid can be created. We show the corrected gravity field and its impact on the geoid.
G23B-03 1330h
Tools and Data Services From the NASA Earth Satellite Observations for Remote Sensing Commercial Applications
Several commercial applications of remote sensing data, such as water resources management, environmental monitoring, climate prediction, agriculture, forestry, and preparation for and mitigation of extreme weather events, require access to vast amounts of archived high quality data, and software tools and services for data manipulation and information extraction. Using these data requires detailed understanding of the their internal structure and physical implementation for data reduction, combination, and production. This time-consuming task must be undertaken before the core investigation can begin and is an especially difficult challenge when science objectives require users to deal with large multi-sensor data sets of different formats, structures, and resolutions. In addressing these issues, the Goddard Space Flight Center (GSFC) Earth Sciences (GES), Data and Information Service Center (DISC) Distributed Active Archive Center (DAAC) has made great progress in facilitating science and applications research, by developing innovative tools and data services applied to the Earth sciences atmospheric and climate data. The GES/DISC/DAAC has successfully implemented and maintained a long-term climate satellite data archive and developed tools and services for a variety of atmospheric science missions, including instruments from AIRS, AVHRR, MODIS, SeaWiFS, SORCE, TOMS, TOVS, TRMM, and UARS and Aura, providing researchers with excellent opportunities to acquire accurate and continuous atmospheric measurements. Because these various missions generate an ever increasing amount of data and products as a result of more sophisticated sensors and new science algorithms, the main challenge for data centers like the GES/DISC/DAAC is to guide users through the variety of data sets and products, provide tools to visualize and reduce the volume of the data, and secure uninterrupted and reliable access to the data and related products. This presentation will describe the effort at the GES/DISC/DAAC to build a bridge between multi-sensor data and the effective scientific use of the data, with an emphasis on the heritage satellite observations and science products for climate applications. The intent is to inform users of the existence of this large collection of data and products, suggest starting points for cross-platform science projects and data mining activities, and provide data services and tools information. More information is available through the GES/DISC/DAAC site http://disc.gsfc.nasa.gov.
http://disc.gsfc.nasa.gov
G23B-04 1330h
Degradation of Grace Monthly Geopotentials in 2004 Explained
Through the first 8 months of 2004 the Grace ground tracks have declined in density with the approach of the 61-revs/4-day orbit resonance in September. At the same time the precision of the monthly Geopotentials has also declined significantly. When an NxN band limited Geopotential is recovered from data on a near circular resonant orbit of R-revs /D-days, the limit N must be < 2R for each significant Geopotential frequency to have unique representation in the repeating information waves every D days. In or near "ideal" resonances (N<2R) the ratio of Geopotential harmonics to repeating wave phases is on average ~ 4/1. When N > 2R (as in Grace recoveries of 120x120 fields in or near September 2004) each wave is represented by about 4 (~120/61) frequencies and the ratio of wave phases to harmonics is close to 1/1, a compression of information which seriously degrades the resulting solution. We present a theory for general recoveries of band-limited fields from resonant orbits where R < 2N following the work of Colombo in the 1980s on the "ideal" block diagonal case (R > 2N). The theory applied to Grace low-low intersatellite range-rate data in its R/D = 61/4 orbit shows: (i), the need for a priori information to condition all Geopotential harmonics in a 120x120 solution and (ii), even with conditioning a degradation of degree-precision of between 1 and 3 orders of magnitude over an "ideal" recovery. Clearly, hydrologic time variations from Grace tracking near such "deficient resonances" will be severely compromised.
G23B-05 1330h
Mean Dynamic Topography Based on the Grace GGM02 Model and Altimeter MSS: Can we Capture Subgyre Structures by use of Adaptive Filters?
A high resolution Mean Dynamic Ocean Topography (DOT) has been constructed by differencing of the GRACE Gravity Model 02 (GGM02), based on 363 days of data, from a multi-satellite Mean Sea Surface (GSFCMSS00) both mapped into a 0.25 x 0.25 degree grid. This DOT exhibits correlated noise, especially in the form of quasi-meridional striations, caused mostly by correlated errors in the GRACE geoid. This noise can be filtered out to make feasible the recovery of Gyre-scale structures at resolutions of ~400 km with the use of Guassian smoothing, but the mesoscale subgyre structures are lost due to excessive smoothing. However, we found that a adaptive empirical orthogonal method based on EOF and Singular Spectrum Analysis (SSA), both utilizing the space covariance structure of the data set, is more efficient in removing this kind of noise. The main advantages of this filter are: it works well with short data sets; it does not lose boundary points; it does not smooth out large peaks; and it is able to efficiently separate signal from noise, which can be further analysed. The resultant DT is able to recover subgyre-scale geostrophic circulation associated to Western Boundary Currents (Gulf Stream, Kuroshio, etc), and is consistent with estimates obtained from a high-resolution Boyer-Levitus (BL) climatology. Surprisingly, in the South Atlantic (SA) the Brazil Current system and the SA Subtropical Gyre appears to be more consistent with the double gyre schematics of Tsuchyia (1985) than with the popular one presented in many papers by Stramma and collaborators. In contrast with Stramma, it exhibits a slender Brazil Current/Return Current cell and a signature of a Subtropical South Atlantic Countercurrent around 30 S. This result is consistent with several depth- referenced DOTs obtained from the BL climatology.
G23B-06 1330h
Fuzzy Modeling of Continental Water Storage Changes Observed by GRACE
The low-low satellite-to-satellite (SST) tracking mission, Gravity Recovery and Climate Experiment (GRACE), provides scientists an efficient and cost-effective way to map the Earth's static and monthly temporal gravity fields with unprecedented accuracy and resolution. One of the major objectives of GRACE is to measure the climate-sensitive signals generated by mass redistributions on Earth at spatial scales greater than several hundred km and temporal scales longer than 30 days. Studies including non-isotropic filtering of GRACE signals and alternate processing of GRACE data have enabled enhancement of temporal and spatial resolutions. Fuzzy logic based methods have been widely used by various disciplines for improving model prediction, control, classification etc. Its ability of providing linguistic description of the relations between the model components has made it a valuable tool for potential model improvements. In this study, fuzzy inference systems whose parameters are optimized by mathematical optimization algorithms, have been used to recover monthly or sub-monthly mean water storage anomalies (MWSA) in South America from the gravity variations observed by GRACE. To this end, regularly gridded MWSA was computed by averaging daily water storage anomalies derived from NCEP (National Centers for Environmental Prediction) daily mean water storage (MWS) data. We tested the fuzzy logic algorithm on the GRACE Level 2 (L2) data products and the data products generated by processing Level 1B (L1B) data based on the energy conservation methods. Results from the obtained fuzzy model were tested with independent data that was not used for estimation of the model parameters. Performance of the resulting model was compared with those of other models previously used for detection of MWSA from GRACE observations.