Skip to main content
main-content

Über dieses Buch

Our planet is currently experiencing substantial changes due to natural phen- ena and direct or indirect human interactions. Observations from space are the only means to monitor and quantify these changes on a global and long-term p- spective. Continuous time series of a large set of Earth system parameters are needed in order to better understand the processes causing these changes, as well as their interactions. This knowledge is needed to build comprehensive Earth s- tem models used for analysis and prediction of the changing Earth. Geodesy and geophysics contribute to the understanding of system Earth through the observation of global parameter sets in space and time, such as tectonic motion, Earth surface deformation, sea level changes and gravity, magnetic and atmospheric elds. In the framework of the German geoscience research and development p- gramme GEOTECHNOLOGIEN, research projects related to the theme “Observing the Earth System from Space” have been funded within two consecutive phases since 2002, both covering 3 years. The projects address data analysis and model development using the satellite missions CHAMP, GRACE, GOCE and comp- mentary ground or airborne observations. The results of the rst phase projects have been published in the Springer book, titled “Observation of the Earth System from Space”, edited by Flury, Rummel, Reigber, Rothacher, Boedecker and Schreiber in 2006. The present book, titled “System Earth via Geodetic-Geophysical Space Techniques” summarizes in 40 scienti c papers the results of eight coordinated research projects funded in the second phase of this programme (2005–2008).

Inhaltsverzeichnis

Frontmatter

CHAMP and GRACE

Frontmatter

More Accurate and Faster Available CHAMP and GRACE Gravity Fields for the User Community

CHAMP and GRACE gravity field models are based on satellite-to-satellite tracking in high-low and, in the case of GRACE, low-low mode. The products derived from exploitation of these instrument data showed already a quantum leap compared to models derived at the end of last century. But, especially for GRACE, the gravity fields did not reach the accuracy predicted before launch. Also the CHAMP models did not show long-wavelength time-variable gravity field signals, except for the very low degrees. Therefore several algorithms and methods applied during gravity field determination process have been improved and new CHAMP and GRACE EIGEN (European Improved Gravity field of the Earth by New techniques) satellite-only static and time-variable as well as high resolution combination models have been derived. Additionally, so called “base products” necessary for other CHAMP and GRACE mission experiments, such as the Rapid Science Orbits (RSO) for radio occultation or magnetic field data analysis or Satellite Laser Ranging (SLR) orbit predictions as support to the International Laser Ranging Service (ILRS) have been improved in terms of accuracy and submission time to users. All products are available at the Information System and Data Center (ISDC) which has been completely redesigned. The overall high interest in CHAMP and GRACE products is manifested in nearly 2000 registered ISDC users.

Frank Flechtner

The CHAMP/GRACE User Portal ISDC

The Information System and Data Center (ISDC) portal is the door to geoscience data, information and knowledge, which are provided by national and international projects of the GFZ Potsdam, such as the satellite projects CHAMP and GRACE. In addition to such data management processes as data ingestion, catalog building and data archiving, another important part of the data lifecycle is the provision of user interfaces for the disclosure, discovery and re-use of data. This chapter describes the ontology driven metadata concept, the system architecture including user interfaces and the backend software for the operational services of the ISDC portal.

Bernd Ritschel, Lutz Gericke, Ronny Kopischke, Vivien Mende

Improvements for the CHAMP and GRACE Observation Model

In this chapter the theory and details of the implementation and validation of the GPS carrier phase wind-up correction and the GPS attitude model are presented. It is shown, that a correction of the carrier phase data due to the wind-up effect improves the GPS orbit fit together with the integer ambiguity fixing by 1–2 cm. It is also demonstrated, that changing the definition of the X-axis of the Block IIR satellites (towards Sun as for Block IIA) has no significant influence on orbits and clocks when integer ambiguity fixing is applied, since the difference due to the different axes definitions is completely absorbed by floating L3 ambiguities. The phase wind-up correction was also applied for one week of data for the Low Earth Orbiters CHAMP, GRACE-A and TerraSAR-X. The orbit improvement, measured by Satellite Laser Ranging (SLR) residuals is noticeable and amounts to 3 mm globally. The description of the GPS attitude model, including shadow crossing and post-shadow manoeuvre, noon and midnight turns is given in detail. It is demonstrated, that incorrect attitude modelling can cause yaw angle errors exceeding even the size of one full rotation of the satellite during the shadow crossing.

Grzegorz Michalak, Rolf König

The Release 04 CHAMP and GRACE EIGEN Gravity Field Models

In this article we highlight the advances in gravity field recovery with CHAMP and GRACE, leading to the new GFZ release 04 (RL04) EIGEN (European Improved Gravity field of the Earth by New techniques) models. RL04 consists of time series of monthly CHAMP and GRACE gravity models, pure weekly GRACE solutions and combined static fields from satellite-only and terrestrial data. Additionally a new mean CHAMP-only gravity field model has been generated. It becomes obvious that the improvements in the RL04 background modelling, processing standards and strategies have led to significant improvements in the reprocessed gravity field models. These new RL04 EIGEN models, available for nearly the whole CHAMP and GRACE mission periods, provide an important data base to monitor mass transport and mass distribution phenomena in the system Earth, such as the continental hydrological cycle, ice mass loss in Antarctica and Greenland, ocean mass changes or the ocean surface topography.

Frank Flechtner, Christoph Dahle, Karl Hans Neumayer, Rolf König, Christoph Förste

Orbit Predictions for CHAMP and GRACE

The orbit prediction system of GFZ is an indispensable tool for Satellite Laser Ranging (SLR) stations tracking the satellites CHAMP, GRACE-A/B, and TerraSAR-X, for providing orbit information for initial processing of mission data, and for the purpose of mission operations. The system is presented regarding its structure and quality performance. Outside the proper orbit determination the pre- and post-processing parts of the system have steadily been developed to become more robust for automated processing. Various changes in modelling the orbit forces have led to improved quality of the predicted orbits which is monitored regularly. A quality analysis shows an increasing accuracy in terms of along-track errors or time bias of predicted orbits. The improvement is related to both the decreasing solar activity and to the usage of better models and parameterizations in orbit determination.

Krzysztof Snopek, Daniel König, Rolf König

Rapid Science Orbits for CHAMP and GRACE Radio Occultation Data Analysis

The Rapid Science Orbits (RSOs) for the GPS constellation and for Low Earth Orbiting (LEO) satellites (namely CHAMP, GRACE A/B, SAC-C, TerraSAR-X) are generated with very high reliability on a daily basis with 1 day latency to support radio occultation measurements and other on-board experiments. The RSOs are dynamic orbits computed in a two step approach. Separately estimated GPS orbits and clocks are fixed and used thenafter in the LEO orbit determination process. The quality of the GPS orbits is monitored by comparison to the IGS rapid orbit products as well as by orbital fits of Satellite Laser Ranging (SLR) measurements to GPS-5 and -6. Results of the validation allow to conclude, that in results of recent modelling improvements the accuracy of the GPS orbits is 5 cm in all directions (radial, cross- and along-track). The accuracy of the LEO orbits monitored by SLR amounts to 4–5 cm. There exists a 3–4 cm negative bias in the SLR residuals for the GPS orbits, for most of the LEOs considered here the negative bias is on 1 cm level. The origin of the bias is unclear. The Rapid Science Orbits are publicly available also for other scientific applications.

Grzegorz Michalak, Rolf König

Parallelization and High Performance Computation for Accelerated CHAMP and GRACE Data Analysis

The work package WP210 “parallelization and high performance computing” of the Geotechnologien II project was aimed at improving the overall performance of the Earth Parameter and Orbit System (EPOS) software package used in gravity field processing at the Helmholtz Centre Potsdam GFZ German Research Center for Geosciences, and to interconnect various tasks in an optimal way. The two aspects of this optimization are, on the one hand, to minimize processing time, and, on the other, to solve for as many parameters as possible. In this chapter, we will focus our attention to three important sub-problems. First we show how processing can be simplified and, therefore, accelerated via a more convenient treatment of sender and receiver clock parameters, as they emerge in GPS processing. Secondly, an improved block-parallel algorithm for the computation of normal equations from observation equations, as they occur in gravity field adjustment, is presented. Thirdly, we show how it is possible to combine the existing software into a processing chain that admits the treatment of satellite arcs of arbitrary length in a consistent manner.

Karl Hans Neumayer

GRACE

Frontmatter

Improved GRACE Level-1 and Level-2 Products and Their Validation by Ocean Bottom Pressure

The GRACE mission, launched in 2002, is the first realization of the satellite-to-satellite tracking (SST) concept in the low-low mode. These innovative observations, in combination with high-low GPS SST and accelerometer data, are the basis for dramatic improvements in the field of static and time-variable gravity field determination within the first years of the mission. Nevertheless, mid of 2005 the quality of these models was still significantly below that projected by simulation studies before launch (approximately a factor of 18 for monthly models). This has a significant effect on all higher-level scientific products derived from GRACE gravity field solutions such as ice melting in the Polar Regions, derivation of surface and deep ocean currents or estimation of ocean mass for sea level rise analysis. Therefore three principle causes have been investigated: the quality of the instrument data and the processing strategy to derive calibrated instrument from raw data, deficiencies in the background models, such as the short-term non-tidal atmosphere and ocean mass variations, and possibly wrong or insufficient parameterization of the K-band range-rate or accelerometer observations. Finally, a validation of the gravity models with globally distributed in-situ ocean bottom pressure (OBP) data was initiated. In this chapter the main results of this project are summarized which then were the basis for a consistent reprocessing of GRACE data leading to more consistent and accurate gravity field products.

Frank Flechtner

The GRACE Gravity Sensor System

This chapter is dedicated to the GRACE Gravity Sensor System. In the following, a brief description of the individual components of the sensor system is given, followed by an overview of the interaction of the sensor system with the environment of the GRACE satellites and a brief description of the gravitational and non-gravitational forces acting on the satellites. As an example, the sensor performance of the accelerometer is estimated by real data analysis. It is demonstrated that the sensor performance agrees with its specification and that the derived data processing model enhancements currently yield no amelioration of the derived gravity field models quality.

Björn Frommknecht, Anja Schlicht

Numerical Simulations of Short-Term Non-tidal Ocean Mass Anomalies

Short-term non-tidal ocean mass anomalies are currently reduced within the standard GRACE gravity field processing by means of the Ocean Model for Circulation and Tides (OMCT). Numerical sensitivity tests performed while defining the model configuration to be used for the atmosphere-ocean de-aliasing product (AOD) RL04 include the consideration of the European Centre for Medium-Range Weather Forecasts (ECMWF) short-term forecasts including accumulated wind stresses, the consideration of atmospheric and continental freshwater fluxes, and the treatment of time-variations of the total oceanic mass. Analyses of simulated ocean mass anomalies suggest that benefits of accumulated wind stresses do not outweigh errors contained in the forecasts with respect to the corresponding analyses. The impact of freshwater fluxes is generally small on ocean mass anomalies. Continental freshwater fluxes are in particular dominant on monthly time-scales and longer, allowing to neglect them for AOD purposes. Consequently, the total ocean mass is artificially held constant at every time-step in the operational simulations. Reasonable seasonal variations could have been only obtained after a yearly de-trending of the data, which cannot be performed in an operational setup.

Henryk Dobslaw, Maik Thomas

Improved Non-tidal Atmospheric and Oceanic De-aliasing for GRACE and SLR Satellites

Non-tidal high-frequency atmospheric and oceanic mass variation models are routinely generated at GFZ Potsdam as so-called GRACE Atmosphere and Ocean De-aliasing Level-1B (AOD1B) products. As the barotropic ocean model PPHA used in release 0 (RL00) and 1 (RL01) has shown some deficiencies the latest versions of AOD1B are based on the baroclinic Ocean Model for Circulation and Tides (OMCT). In this chapter, we summarize the configuration and improvements of OMCT for operational AOD1B RL04 generation. As the temporal resolution of AOD1B products is 6 h and therefore S2 air tide signals may not be accounted for properly, some experiments have been made to investigate the influence of 3-hourly meteorological forcing on the resulting GRACE orbits and gravity fields. It turned out that the time-invariant atmospheric tide model used to correct interpolation errors is sufficiently accurate and even the error caused by a complete neglect of the atmospheric tides is below the current GRACE error level. Finally, the AOD1B RL04 model has been reprocessed back to 1976 for a consistent processing of SLR and subsequent combination with GRACE data. First results of LAGEOS derived C20 values with those from GRACE show higher correlations for the annual amplitudes but also semi-annual signals of unknown nature.

Frank Flechtner, Maik Thomas, Henryk Dobslaw

Global Gravity Fields from Simulated Level-1 GRACE Data

The GRACE satellites deliver high quality GPS code and phase, inter-satellite range and range-rate, non-gravitational acceleration, and star camera observations that can be used to estimate the static and time variable gravity field of the Earth with unprecedented accuracy. Nevertheless, the baseline accuracy that was determined in a pre-launch simulation study could not yet be reached. To find out possible reasons and to give recommendations for an improved data processing, another simulation study using the software, standards and processing strategy actually applied at GFZ in the routine processing of GRACE data is performed. The results point to inaccuracies in present ocean tide models. Additionally, it was found that the accelerometer noise cannot be absorbed sufficiently by the instrument parameters estimated so far and a shortening of the arcs seems to be necessary. Finally, an observed bias in the C

20

-coefficient of the GRACE gravity field models could be related to a GPS antenna phase centre bias in along-track direction.

Ulrich Meyer, Björn Frommknecht, Frank Flechtner

ITG-GRACE: Global Static and Temporal Gravity Field Models from GRACE Data

More than 4 years of GRACE data were used to determine the gravity field model ITG-Grace03 s. The solution consists of three parts: a static high resolution model up to a spherical harmonic degree of 180, temporal variations up to degree 40 and the full variance-covariance matrix for the static solution. The temporal gravity field variations are parameterized by continuous basis functions in the time domain. The physical model of the gravity field recovery technique is based on Newton’s equation of motion, formulated as a boundary value problem in the form of a Fredholm type integral equation. The principal characteristic of this method is the use of short arcs of the satellite’s orbit in order to avoid the accumulation of modeling errors and a rigorous consideration of correlations between the range observations in the subsequent adjustment procedure.

Torsten Mayer-Gürr, Annette Eicker, Enrico Kurtenbach, Karl-Heinz Ilk

Validation of GRACE Gravity Fields by In-Situ Data of Ocean Bottom Pressure

GRACE observes the temporally changing gravity field of the Earth with unprecedented accuracy. Compared to the gravity signals of the continental hydrological cycle, local ocean mass variability reflecting ocean current, density and sea level changes are a challenge for the GRACE mission. Hence, validation of GRACE with in-situ observations of ocean bottom pressure is critical to evaluate the capability of GRACE to observe oceanic mass redistribution. Here, GRACE data is compared with in-situ ocean bottom pressure at a hundred sites located in all of the world’s oceans. The advances made by recent GRACE product releases are shown, and gravity fields provided from different data centres are compared. In some regions, particularly at high-latitude sites with comparatively strong ocean bottom pressure variability and dense satellite coverage, GRACE captures oceanic variability quite well. This is a robust feature for all gravity field solutions. In contrast, some other regions show remarkably large differences between different GRACE solutions, suggesting that discrepancies in de-aliasing models play a major role in defining the skill of GRACE to realistically observe local oceanic mass variability.

Andreas Macrander, Carmen Böning, Olaf Boebel, Jens Schröter

Antarctic Circumpolar Current Transport Variability in GRACE Gravity Solutions and Numerical Ocean Model Simulations

The Antarctic Circumpolar Current (ACC) is the main contributor to the inter-basin mass exchange in the ocean carrying a mean total transport of about 134 Sv (1 Sv = 10

6

m

3

/s) of water around the globe. A large part of its transport is found to be related to the prevailing westerly winds over the Southern Ocean. The Southern Annular Mode (SAM) explains 20–30% of the zonal wind component’s variability and enforces (at least to a certain extent) the variability of the oceanic transport. In this study, the representation of the ACC transport variability in the GRACE time-variable gravity solutions is investigated. Monthly GRACE gravity solutions over the ocean are commonly connected to ocean bottom pressure (OBP) fluctuations. Due to the connection between OBP gradients and geostrophic transport, a part of the ACC transport variability can be explained by gradients of OBP anomalies. Model simulations with the Finite Element Sea Ice-Ocean Model (FESOM) feature variations of about 2–3 Sv on annual and semi-annual periods; more than 50% of the total ACC transport variability is explained by OBP variability. GRACE-derived transport anomalies show variations at semi-annual and annual periods of about 6–9 Sv. Variations detected by GRACE coincide with model results with correlations of about 0.75. Atmospheric variability as described by SAM is found to be related to FESOM- and GRACE-derived OBP on short time scales up to annual. The spatial pattern of SAM is reflected in GRACE OBP by showing an annular band of highly negative correlation around Antarctica south of the southern boundary of the ACC. Model simulations with FESOM confirm that these spatial patterns are related to

f

/

H

contours in the Southern Ocean.

Carmen Böning, Ralph Timmermann, Sergey Danilov, Jens Schröter

GOCE

Frontmatter

Gravity and Steady-State Ocean Circulation Explorer GOCE

GOCE has been launched on March 17, 2009. It is the first satellite of ESA’s Living Planet Programme. It is aiming at a better understanding of the Earth system. The mission objective of GOCE is the determination of the Earth’s gravity field and geoid with high accuracy and maximum spatial resolution. The geoid, combined with the actual mean ocean surface as derived from twenty years of satellite radar altimetry, will yield global dynamic ocean topography. It will serve ocean circulation and ocean transport studies and sea level research. GOCE geoid heights will allow conversion of GPS heights to high precision heights above sea level. Gravity anomalies from GOCE will be used for gravity-to-density inversion and in particular for studies of the Earth’s lithosphere and upper mantle.

GOCE is the first satellite carrying a gravitational gradiometer. In essence the spacecraft together with all its sensors can be regarded as a satellite gravimeter. The mission is unique and novel in several ways. Precise GPS-tracking with a newly developed European receiver is providing the orbit with cm-precision. Orbit analysis will give the long wavelength part of the gravity field and be complementary to the high resolution gravimetric data of the gradiometer. The spacecraft, in an extremely low orbit of only 255 km, is kept free of drag influence by ion-thrusters and is guided smoothly around the Earth by magneto torquers. The gradiometer and star sensors deliver the data necessary for drag-free and angular control. The high demand on the purity of the gravimetric signal is achieved via extreme levels of thermal control and material stiffness.

In the context of the German Geotechnology-Programme methods have been developed for optimal sensor and data analysis, for combination with complementary data sets and for calibration and validation.

Reiner Rummel, Thomas Gruber

GOCE Data Analysis: From Calibrated Measurements to the Global Earth Gravity Field

The goal of this chapter is to describe an in-situ approach to determine a global Earth gravity model and its variance/covariance information on the basis of calibrated measurements from the GOCE mission. As the main characteristics of this procedure, the GOCE data are processed

sequentially

on a parallel computer system,

iteratively

via application of the method of preconditioned conjugate gradient multiple adjustment (PCGMA), and in situ via development of the functionals at the actual location and orientation of the gradiometer. We will further explain the adaption of the unknown stochastic model, determined by estimating decorrelation filters and variance components with respect to the GOCE observation types (i.e. SST, SGG, and regularizing prior information).

Jan Martin Brockmann, Boris Kargoll, Ina Krasbutter, Wolf-Dieter Schuh, Martin Wermuth

GOCE and Its Use for a High-Resolution Global Gravity Combination Model

The recently launched GOCE mission will set a new milestone in gravity field determination. Due to the high accuracy of its gradiometry observations, the resulting satellite-only Earth gravity models will have a resolution of up to degree/order 250, with an expected accuracy of 1–2 cm in terms of geoid heights. In order to increase this resolution up to at least 360, a combination with surface data is necessary. In the following article the strategies for this combination developed at GFZ, Helmholtz-Zentrum Potsdam, are outlined and discussed. Basically two approaches for the combination exist; the first uses complete and block-diagonal normal equations, while the other uses complete normal equations only. In this article, both approaches are explained with their pros and cons, and the GFZ-strategy is presented at its current stage for the combination of GOCE with GRACE and surface data.

Richard Shako, Christoph Förste, Oleg Abrikosov, Jürgen Kusche

Spectral Approaches to Solving the Polar Gap Problem

Due to its sun-synchronous orbit, GOCE (Gravity field recovery and steady-state Ocean Circulation Explorer) observations do not cover the polar areas of the Earth. The impact of the limited spatial observation geometry on gravity field recovery is referred to as the polar gap problem. This chapter gives an overview on both spatial and spectral approaches to overcome the polar gap problem. Furthermore we confront strategies based on GOCE data only with methods augmenting additional external gravity field information. Concerning the latter, we present the combination of the GOCE normal equations system with spectral a priori information in terms of tailored regularization. With regard to a GOCE-only solution, we adopt band-limited base functions, commonly referred to as Slepian functions that are optimally concentrated within the spherical belt of GOCE data availability. In terms of regional gravity field determination this approach definitively solves the polar gap problem.

Oliver Baur, Jianqing Cai, Nico Sneeuw

Regionally Refined Gravity Field Models from In-Situ Satellite Data

The satellite mission GOCE (Gravity field and steady-state Ocean Circulation Explorer) will enable the determination of the Earth’s gravity field with unprecedented accuracy, especially regarding the high-frequency part of the gravity field spectrum. To exploit the full potential of the mission, it is advantageous to develop methods to extract as much information out of the given signal as possible. In the approach presented here a global gravity field represented by a spherical harmonic expansion up to a moderate degree is derived in a first step and then refined by regionally adapted high resolution refinements being parameterized by splines as space localizing basis functions. These radial basis functions are designed to reflect the spectral characteristics of the gravity field to be modeled. Another important aspect in the regional gravity field analysis approach is the downward continuation process. In this context, a regionally adapted regularization will be introduced, which assigns different regularization matrices to geographical areas with varying signal content. Regularization parameters individually determined for each region take into account the varying frequency behavior, allowing to extract additional information out of a given data set. If desired, regional solutions with global coverage can be combined to a global solution using quadrature methods. The approach is demonstrated by a simulation scenario that combines a global GRACE solution as reference field with regional refinements calculated from GOCE observations.

Annette Eicker, Torsten Mayer-Gürr, Karl-Heinz Ilk, Enrico Kurtenbach

Quality Evaluation of GOCE Gradients

Before entering the final gravity field processing, the quality of the GOCE gradients has to be assessed. Here, two procedures are presented, the mutual comparison and analysis of observed gradients in cross-overs and the application of terrestrial gravity data which are upward continued and transformed into reference gradients for the GOCE observations.

The cross-over (XO) approach is an independent relative validation method, i.e., a procedure using GOCE data only. XO analysis allows checking the gravity gradients at the accuracy level expected for GOCE. The developed near real-time validation module shall also be applied during the measurement phase of the GOCE mission, where dedicated quality reports will be provided. Apart from monitoring the general quality of the GOCE gradients, the determination of calibration parameters like trend and periodic components is possible. The general XO idea and some essential features are presented in this chapter.

For external calibration and validation, regional gravity field data over well-surveyed areas (in Europe) can be upward continued to gradients at GOCE altitude, serving as reference gradients for the GOCE data. Those reference gradients in the GOCE observation frame can then again be used for estimating (regional) error model parameters like scale factors. Here, the basic approach for the use of the terrestrial data is briefly described and the potential accuracy level of the computed reference gradients in the spectral domain presented.

Jürgen Müller, Focke Jarecki, Insa Wolf, Phillip Brieden

Validation of Satellite Gravity Field Models by Regional Terrestrial Data Sets

Within the next few years, improved high-resolution global gravity field models are anticipated from the GOCE mission. The expected accuracies are about 1–2 cm in terms of geoid undulations and 1 mGal for gravity, both at a resolution of about 100 km. Then, from a combination of the GOCE based global gravity field models (expected to be available up to spherical harmonic degree and order 250) with regional terrestrial data sets, an accuracy of about 1 cm is expected for the complete geoid spectrum. In this context, accurate and independent terrestrial data sets are essential for the combination process as well as for the validation of the results. Therefore, a regional validation and combination experiment was carried out in Germany as a work package within the framework of the GOCE-GRAND II project. The main goals of this undertaking were the preparation of high-quality validated terrestrial data sets, which can be used for a future external validation of the GOCE products as well as for the combination with the GOCE data. Within extensive measurement campaigns, significant efforts were put into absolute gravity observations with an A-10 instrument at field stations as spot-checks of the existing data base, and into astronomically determined vertical deflections using the digital transportable zenith camera system TZK2-D. In this contribution, gravity data, GPS and levelling control points, astrogeodetic vertical deflections and gravimetric quasigeoid models are utilized as terrestrial data sets for internal cross-validations as well as for the external validation of global satellite gravity field models.

Johannes Ihde, Herbert Wilmes, Jan Müller, Heiner Denker, Christian Voigt, Michael Hosse

Comparison of GRACE and Model-Based Estimates of Bottom Pressure Variations Against In Situ Bottom Pressure Measurements

Results from several numerical ocean models were used in combination with available ocean bottom pressure data to quantitatively examine the skill of ocean models in simulating fast and slow bottom pressure variations and to test the quality of GRACE monthly fields of bottom pressure variations. The comparison between model simulations and bottom pressure data does reveal a substantial agreement between models and pressure measurements on high frequencies, but also some clear differences on longer time scales (> 1 year) that need to be corrected in order to improve estimates of the barotropic circulation. We also find a good agreement between monthly GRACE solutions and ECCO/GECCO syntheses that encourage us now to use the GRACE fields as constraints in ocean syntheses efforts. Differences of the order of a few centimeters appear consistent with previously estimated uncertainties provided by Quinn and Ponte (2008). There appears to be a large potential for assimilating GRACE data into ocean circulation models and thereby correct the seasonally varying barotropic circulation in the models, but results also highlight remaining uncertainties in the GRACE data.

Detlef Stammer, Armin Köhl, Vanya Romanova, Frank Siegismund

SEAVAR

Frontmatter

Sea Level Variations – Prospects from the Past to the Present (SEAVAR)

Climate change and climatic fluctuations are a natural phenomenon and have occurred over different temporal scales during the Earth’s history. In addition to this natural variability, the question of human-induced changes of climate has gained increasing public awareness. This interest is caused by indications of recent global warming, which has been related to increasing portions of anthropogenic greenhouse gases in the atmosphere. The importance of sea level when studying climate change is, that it responds as a highly sensitive indicator of climate change, the main causal connections being the mass exchange between land ice and ocean water and the thermal expansion of the ocean water.

Tilo Schöne, Jens Schröter

Radar Altimetry Derived Sea Level Anomalies – The Benefit of New Orbits and Harmonization

Most of today’s sea level studies are using the recent and high quality data of JASON-1 and TOPEX/Poseidon. Previous missions, like, e.g., ERS-1 or GEOSAT, show significant higher noise and errors. One of the main error sources is the orbit determination. In this study new GEOSAT orbits have been derived using new models and orbit determination strategies. With the new orbits the accuracy in terms of cross-over RMS is reduced from 15 to 9 cm RMS, which is almost the level of ERS-2. In addition, the orbit accuracy becomes more uniform for both the Geodetic Mission and Exact Repeat Mission period. The effects of recomputed satellite orbits on sea-level anomalies are shown.

Tilo Schöne, Saskia Esselborn, Sergei Rudenko, Jean-Claude Raimondo

Combining GEOSAT and TOPEX/Poseidon Data by Means of Data Assimilation

In order to assess the sea level rise for periods longer than the continuous observations by TOPEX/Poseidon two assimilation experiments were performed. Both use the GEOSAT mission data additionally and bridge the gap between these observations in accordance with standard ocean observations. The first experiment (GETO) showed that without continuous records of the sea level no reliable estimates can be made. Therefore additional observations are used in the second experiment (GETORC), that are provided by sea level time series at tide gauges. From these data global maps of sea level anomalies were constructed using an EOF technique. GETORC then provides the answer to the total sea level rise and to its temporally and spatially varying contributions. Ocean warming accounts for one third of global sea level rise by steric expansion. Half of it is confined to the upper 700 m of the ocean. The other half of oceanic thermosteric expansion is found in the deep ocean below. In addition to warming of the ocean we find an inflow of fresh water with amplitude close to 2 mm/year.

Manfred Wenzel, Jens Schröter

Reanalysis of GPS Data at Tide Gauges and the Combination for the IGS TIGA Pilot Project

The purposes and algorithms of the reanalysis of GPS data at tide gauge benchmarks are described. GPS data from the global network of 403 stations from March 1998 till December 2007 have been reprocessed at GFZ to derive a weekly time series of station coordinates within the IGS GPS Tide Gauge Benchmark Monitoring Pilot Project (TIGA). The combination procedure and some results of the combination and comparison of the solutions derived at six TIGA analysis centres are provided.

Sergei Rudenko, Daniela Thaller, Gerd Gendt, Michael Dähnn, Tilo Schöne

Sea Level Rise in North Atlantic Derived from Gap Filled Tide Gauge Stations of the PSMSL Data Set

A reconstruction of the sea level rise in the North Atlantic from the tide gauge station data is presented. As the time series contain numerous gaps these were first filled by applying the Multi-Channel-Singular-Signal-Analysis (MSSA). The method is based on spatial and temporal covariances. For reconstruction we observed the time span from 1950 to 2006 to receive consistent time series for that period. The station data are then interpolated to the full North Atlantic Ocean. To achieve this, a sequence of sea level analysis performed by the Inverse Finite Element Ocean model (IFEOM) was calibrated against the reconstructed gauges. We present the trend estimate for the period from 1950 to 1994. The results show 1.18 mm/year sea level rise for this period. About 27% are explained by steric expansion of the ocean while the remaining 73% are due to inflow of water from land or surrounding oceans.

Heiko Reinhardt, Dimitry Sidorenko, Manfred Wenzel, Jens Schröter

Using ARGO, GRACE and Altimetry Data to Assess the Quasi Stationary North Atlantic Circulation

We analyse the North Atlantic circulation by combining data from ARGO profiling buoys and satellite altimetry measurements into an inverse finite-element ocean circulation model. The model solution is consistent with ocean dynamics and approximate conservation of temperature and salinity. The ocean circulation is analysed separately for the years 2005 and 2006. Temperature and salinity fields found as solution are close to ARGO data and correspond to a large-scale ocean circulation with a surface elevation close to the altimeter measurements. Numerical experiments carried out with and without use of the altimetric data are discussed. Including altimetry is shown to improve the model solutions, in particular, producing more realistic heat transports. The analysed temperature and salinity fields that minimize the model/data misfit are discussed. It is found that both types of observations are to a large extent complementary to each other.

Falk Richter, Dimitry Sidorenko, Sergey Danilov, Jens Schröter

A 15-Year Reconstruction of Sea Level Anomalies Using Radar Altimetry and GPS-Corrected Tide Gauge Data

A reconstruction of sea level anomalies (SLA) using tide gauges and satellite radar altimetry data for the 1987–2001 period is presented. The time series of a set of 18 tide gauges from the TIGA network, for which GPS vertical trends were available, are corrected for land movement. The comparison with a reconstruction using glacial isostatic adjustment (GIA) model data, as well as with altimetry data, yields a better regional resolution of SLAs in the North Atlantic and North Pacific. The lack of eligible tide gauges makes for a slightly noisy yet stable reconstruction. A comparably low rate of sea level rise of 0.4 mm/year (compared to 3.6 mm/year from uncorrected tide gauge data), is calculated for the 1987–2001 period. In contrast, for the 1995–2001 period, comparison with the trend from altimetry data yields good agreement at 2.2 mm/year vs. 2.4 mm/year (tide gauges: 3.1 mm/year).

Nana Schön, Saskia Esselborn, Tilo Schöne

TIVAGAM

Frontmatter

Continental Water Storage Variations from GRACE Time-Variable Gravity Data

Time-variable gravity fields from the GRACE satellite mission are a unique source of information to quantify spatio-temporal variations of total continental water storage for large areas. To make optimum use of these data for hydrological applications, specific processing strategies have to be applied in terms of data filtering, error budgeting, data combination and evaluation, and assimilation techniques for

hydrological model

hydrological model

s. Methods developed in the project

TIVAGAM

TIVAGAM

are of general use for understanding and modelling dynamic processes in the Earth system.

Andreas Güntner

Surface Mass VariabilitySurface mass variability from GRACE and Hydrological Models Hydrological model : Characteristic PeriodsPeriods characteristic and the Reconstruction of Significant SignalsReconstruction of significant signals

In order to analyse spatio-temporal variations of surface mass anomalies induced by hydrological mass redistributions at the Earth’s surface we use products from the Gravity Recovery and Climate Experiment (GRACE) satellite mission as well as global hydrological models. As a novelty we identify dominant periodic patterns that are not restricted to the fundamental annual frequency and its overtones, using a method that combines conventional Principal Components Analysis (PCA) with a determination of sine waves of arbitrary periods from the principal components. We assess the significance of the derived spectra taking into account correlated errors of the GRACE data by means of a Monte-Carlo technique. This allows us to create filtered GRACE time series including only the significant terms, which serve for basin-specific calibration of hydrological models with respect to the dominant periodic water storage variations.

Svetozar Petrovic, Roland Braun, Franz Barthelmes, Johann Wünsch, Jürgen Kusche, Rico Hengst

Time-Space Multiscale AnalysisTime-Space Multiscale Analysis Multiscale analysis and Its Application to GRACE and Hydrology Data

We present two concepts of a

multiresolution analysis

Multiresolution analysis

<See>

Siehe

Multiscale analysis</See>

of temporal and spatial variations of the Earth’s

gravitational potential.

Gravitational potential.

First we apply a separated wavelet analysis using Euclidean wavelets in the time and spherical wavelets in the space domain and, second, we realize a tensor product wavelet analysis using Legendre and spherical wavelets for the time and space domain, respectively. Based on the results of the multiresolution analysis we compute the correlation coefficients between satellite and hydrology data which help us to develop a filter for extracting an improved hydrology model from the satellite data of GRACE (Gravity Recovery and Climate Experiment). Such an extraction is finally realized based on the results of the tensor product analysis.

Willi Freeden, Helga Nutz, Kerstin Wolf

Mass Variation Signals in GRACE Products and in Crustal Deformations crustal deformation from GPS: A Comparison

Geophysical surface mass variations are reflected both in gravity field variations and in load deformations of the

solid Earth

solid Earth

solid Earth

. These two signatures may be observed by

GRACE

GRACE

and by

GPS

GPS

, respectively. This article reports about a comparison between both. Concerning GPS-derived deformations, a meaningful geophysical interpretation requires both homogeneously processed observations and a stable realization of the terrestrial reference system. Here we use results from a reprocessing of a global GPS network with consistent use of the latest processing and modelling strategies. This reprocessing includes the estimation of low-degree deformation terms. We directly compare them to respective GRACE results and find good agreement. Our main results concern the comparison of site displacement time series obtained from GPS, on the one hand, and from GRACE gravity variations converted to load deformations, on the other hand. We do this comparison both for the GRACE background models of short-term variations and for the final monthly GRACE solutions. For vertical deformations, we find good agreement. In contrast, the agreement is poor for the horizontal directions. The differences between GPS and GRACE contain some components which appear to have large-scale correlated patterns in space and seasonal patterns in time. More detailed analyses indicate that residual errors in the GPS solutions are likely the dominant cause of these differences. Analysing internal deformations of regional subnetworks is a way to circumvent some of the large-scale systematics of the GPS solution. Indeed,

regional analyses

regional analyses

show reasonable agreement between GPS and GRACE even in the horizontal components. Overall, our results demonstrate the progress and challenges of combining independent satellite geodetic observations within the Global Geodetic Observing System.

Martin Horwath, Axel Rülke, Mathias Fritsche, Reinhard Dietrich

Monthly and Daily Variations of Continental Water Storage and Flows

Monthly and daily variations of continental water storage and river discharge were calculated on a global scale with the WaterGAP

Global Hydrological Model

Global Hydrological Model

. In order to improve model results, two major enhancements were added to the model structure. First, a

dynamic flow velocity

dynamic flow velocity

approach was introduced to replace the standard version of a globally constant flow velocity of 1 m/s. Model results were compared to GRACE

water storage variations

water storage variations

and observed

river discharge

river discharge

. The success of the dynamic velocity approach varied among basins and depended on the chosen value for the river roughness parameter. Second, a new

reservoir operation

reservoir operation

algorithm was applied for 487 reservoirs worldwide with a total storage capacity of approximately 4,000 km

3

. Simulation of reservoir storage changes (but not of basin-wide total water storage variations) and of river discharge was significantly improved. Two different approaches for model

calibration

calibration

are combined to obtain a best global-scale estimate of continental water storages and flows.

Kristina Fiedler, Petra Döll

Calibration of a Global Hydrological Modelglobal hydrological model with GRACE Data

Water mass variations from the GRACE satellite mission are an unprecedented data set for validation and calibration of large-scale hydrological models. We develop an efficient multi-objective

calibration

calibration

framework to incorporate time series of monthly measured

river discharge

river discharge

and GRACE water storage variations into the parameter tuning process of the WaterGAP Global Hydrology Model (

WGHM

WGHM

). The calibration approach is applicable to large river basins worldwide. For the example of the

Amazon

Amazon

river basin, simulation results improved for both objectives after calibration. The improvement is shown to be due to decreased flow velocities and, thus, larger and longer-lasting

water storage

water storage

in the surface water network of the Amazon. The results highlight the beneficial nature of GRACE for advancing hydrological models and for understanding water storage variations in the continental water cycle.

Susanna Werth, Andreas Güntner

NRT-RO

Frontmatter

Near-Real-Time Provision and Usage of Global Atmospheric Data from CHAMP and GRACE (NRT-RO): Motivation and Introduction

Global and precise atmospheric satellite-based measurements with high temporal and spatial resolution are especially suited to characterize global change related atmospheric variations and to improve global numerical weather forecasts. Atmospheric research cannot be imagined without satellite measurements since the sixties, when the meteorological use of satellite data started. Especially data from passive multi-spectral radiometers contributed substantially to the understanding of global atmospheric processes. But these data from, e.g., MSU (Microwave Sounding Unit) and AMSU (Advanced Microwave Sounding Unit) are influenced by instrument and orbit changes, calibration problems, instrument drifts, and insufficient vertical resolution.

Jens Wickert

Global Atmospheric Data from CHAMP and GRACE-A: Overview and Results

The German geoscience satellite CHAMP (CHAllenging Minisatellite Payload) provides global atmospheric measurements almost continuously since early 2001. It currently generates a unique and operationally available long-term set of GPS radio occultation (RO) data. RO data from the US-German GRACE-A satellite (Gravity Recovery and Climate Experiment) are in addition operationally available since May 22, 2006. Data and analysis results from both satellites are provided to the international scientific community and stimulated several activities to improve GPS RO data analysis and the application in atmospheric research and for global weather forecasts. The data are currently in use by more than 40 research groups worldwide. A near-real time data provision from CHAMP and GRACE was installed and is demonstrated by GFZ within the GEOTECHNOLOGIEN research project NRT-RO (Near-Real-Time Radio Occultation) funded by the German Federal Ministry of Education and Research (BMBF). An average delay between measurement aboard the satellites and provision of corresponding analysis results below 2 h is operationally reached. This successful research activity was one of the preconditions for the beginning of the operational GPS RO data use to improve global weather forecasts in 2006 by Met Office and ECMWF (European Centre for Medium-Range Weather Forecasts). This result is a milestone and a breakthrough for the acceptance of the innovative GPS RO data in Numerical Weather Prediction (NWP).

Jens Wickert, Georg Beyerle, Carsten Falck, Sean B. Healy, Stefan Heise, Wolfgang Köhler, Grzegorz Michalak, Dave Offiler, Detlef Pingel, Markus Ramatschi, Markus Rothacher, Torsten Schmidt

Near-Real Time Satellite Orbit Determination for GPS Radio Occultation with CHAMP and GRACE

Precise and rapidly available orbits of GPS and Low-Earth-Orbiting (LEO) satellites are the prerequisite for processing of the radio occultation data from CHAMP, GRACE and other LEOs performing occultation measurements. For efficient occultation data assimilation by the weather prediction systems a 3 h timeline is required. In 2002 GFZ has started to generate orbits at fixed 3 h intervals with a mean latency of 2.2 h, so-called Ultra-rapid Science Orbits (USOs). The resulting delay of the occultation products was in the range 3–5 h, sufficient for building an operational radio occultation system and for performing data assimilation studies in weather centers. To better meet the 3 h timeline, a new Near-Real Time (NRT) orbit processing system was developed to generate GPS and LEO orbits every 1.5 h with low latency, typically less than 0.5 h. This reduces the average delay for occultation products to 1 h 45 min. The NRT system consists of three independent chains generating LEO orbits with latencies ranging from 13 to 30 min and accuracies in the range 6–10 cm validated by Satellite Laser Ranging (SLR).

Grzegorz Michalak, Rolf König

The Operational Processing System for GPS Radio Occultation Data from CHAMP and GRACE

In this chapter, a description of the CHAMP and GRACE atmospheric processing system for radio occultation data at GFZ Potsdam is given. The generation of radio occultation products, as e.g. atmospheric excess phases and vertical profiles of refractivity or temperature is a complex process. Besides the scientific challenge the design and installation of an automatic data processing system is also of great importance. This system must be able to process the different input data from external data sources, coordinate the different data streams and scientific software modules, and feeds the results into the data center automatically. Caused by different user demands the CHAMP and GRACE Atmospheric Processor is divided into two parts: A near-real time processing mode makes radio occultation analysis results available on average 2 h after measurements. In the standard processing mode quality checked profiles of atmospheric parameters are available with a latency of about 2 days.

Torsten Schmidt, Jens Wickert, Grzegorz Michalak

Assimilation of CHAMP and GRACE-A Radio Occultation Data in the GME Global Meteorological Model of the German Weather Service

The assimilation of GPS radio occultations within the three-dimensional variational data assimilation system of the German Weather Service requires GPS radio occultation bending angle forward operators. To optimize the forward operator setup, different one- and three-dimensional bending angle forward operators are evaluated. The innovation statistics for radio occultation data from the CHAMP, GRACE-A and FORMOSAT-3/COSMIC satellites are compared with estimates based on the background and observation errors specified in the assimilation scheme. Numerical experiments are performed to assess the impact of assimilated radio occultation data on the weather forecast scores.

Detlef Pingel, Andreas Rhodin, Werner Wergen, Mariella Tomassini, Michael Gorbunov, Jens Wickert

MAGFIELD

Frontmatter

The Earth’s Magnetic Field at the CHAMP Satellite Epoch

Since the Carl-Friedrich Gauss epoch, the Earth’s magnetic field has changed dramatically, its dipole moment decreasing by some 10%. This single observation has raised a large number of questions about a future possible polarity transition, and consequently the possible effects on the Earth’s environment. Here, the progress made over the last few years, when the advent of the satellite era brought a major breakthrough in our understanding of the geomagnetic field at large scales, is summarized. Obtaining good data from observations is a prerequisite, so ground-based and satellite measurements are described. By properly combining these data, information about the characteristics of the geomagnetic field can be retrieved. So, recent global geomagnetic field models based on spherical harmonic analyses are generally discussed. The GRIMM model, as a powerful tool to study the internal geomagnetic field, is discussed in more details. An important and ongoing topic is also the description of the lithospheric contributions, so a large part of this chapter is dedicated to the large and small scales of this field. An outlook to expected future results and unresolved questions is provided in the final part of this chapter.

Mioara Mandea, Matthias Holschneider, Vincent Lesur, Hermann Lühr

GGOS-D

Frontmatter

Integration of Space Geodetic Techniques as the Basis for a Global Geodetic-Geophysical Observing System (GGOS-D): An Overview

The objective of the GGOS-D project was the investigation of the technological, methodological and information-technological realization of a global geo-detic-geophysical observing system, especially the integration and combination of the space geodetic techniques. In the course of this project a data management and information system has been developed and implemented, very consistent long time series for GPS, VLBI and SLR were generated based on common state-of-the-art standards for modeling and parameterization, and these series were combined to compute a Terrestrial and a Celestial Reference Frame with very high accuracy and consistency. These reference frames were subsequently used as the basis to produce time series of geodetic and geophysical parameters like site coordinates, Earth orientation parameters (EOPs), troposphere parameters and low-degree gravity field coefficients. Finally, these time series were compared, validation and interpreted in order to improve our knowledge of the Earth system behavior.

Markus Rothacher, Hermann Drewes, Axel Nothnagel, Bernd Richter

GGOS-D Data Management – From Data to Knowledge

The main objective within the Joint Project “Integration of Space Geodetic Techniques as the Basis for a Global Geodetic-Geophysical Observing System (GGOS-D)” is the investigation of the technological, methodological, and IT-based realization of a global geodetic-geophysical observing system. The main emphasis is put on the development and implementation of the systems for data acquisition and transfer as well as the generation of consistent and integrated geodetically observed time series. Because the generation of such time series is embedded in a highly complex system of measuring and processing procedures the contributions for the data and information system of GGOS-D play a central role to coordinate the data and information flow between the project partners as well as to guarantee the documentation and availability of standardized and consistent data series and to provide high level tools to work with the data. The proposal of the BKG “IT Contributions to the Realization of an Operational Global Geodetic-Geophysical Observing System” takes care of this central role of the data and information system within GGOS-D.

Wolfgang Schwegmann, Bernd Richter

GGOS-D Consistent, High-Accuracy Technique-Specific Solutions

Consistent and homogeneous long-time series of the space geodetic techniquesGlobal Positioning System (GPS), Satellite Laser Ranging (SLR), andVery Long Baseline Interferometry (VLBI) provide the basis for thecombination efforts of GGOS-D. For a consistent combination, thedefinition of common standards for parameterization and modeling isessential. These standards and the technique-specific processingoptions of all individual GPS, SLR, and VLBI solutions as well asthe combined SLR and VLBI solutions are discussed.

Peter Steigenberger, Thomas Artz, Sarah Böckmann, Rainer Kelm, Rolf König, Barbara Meisel, Horst Müller, Axel Nothnagel, Sergei Rudenko, Volker Tesmer, Daniela Thaller

GGOS-D Global Terrestrial Reference Frame

The GGOS-D global terrestrial reference frame has been computed from a combination of homogeneously processed VLBI, SLR and GPS observation time series. A major focus was on the analysis of station position time series, investigations regarding the handling of non-linear station motions, and the development of refined combination methods. The terrestrial reference frame (station positions and velocities) has been estimated simultaneously with the Earth orientation parameters and the celestial reference frame (quasar coordinates).

Detlef Angermann, Hermann Drewes, Michael Gerstl, Barbara Meisel, Manuela Seitz, Daniela Thaller

GGOS-D Consistent and Combined Time Series of Geodetic/Geophyical Parameters

The generation of the GGOS-D global terrestrial reference frame is based on VLBI, SLR, and GPS observations. The respective observation blocks, analysed as individual units, depend on the technique and cover either full weeks, full days (GPS and SLR) or observing sessions of 24 h duration (VLBI). From these observation units, time series of parameters have been inferred and studies of the quality of the results have been carried out for the identification of deficits in the analyses. In this paper, we describe examples of time series of site coordinates, Earth orientation, and atmosphere parameters as well as peculiarities in the behaviour of these parameters.

A. Nothnagel, T. Artz, S. Böckmann, N. Panafidina, M. Rothacher, M. Seitz, P. Steigenberger, V. Tesmer, D. Thaller

GGOS-D Integration with Low Earth Orbiters

Within GGOS-D the attempts of determining a terrestrial reference frame have been enriched by solutions coming from the integrated approach where the whole GPS constellation in combination with the Low Earth Orbiters CHAMP and GRACE are processed together by dynamic satellite orbit adjustment delivering all relevant parameters within one step. A characterization is given concerning the satellites involved, the data used, and the parameterization. There is clear evidence that the integrated approach produces much more accurate results than a comparable two-step processing as revealed by residuals concerning the Low Earth Orbiters as well as at the example of the dynamic origin. The dynamic origin time series with daily resolution is presented currently exhibiting a precision in the range of a centimetre.

Daniel König, Rolf König

Backmatter

Weitere Informationen

Premium Partner

    Bildnachweise