ALBERT

Description: In situ ocean bottom pressure (OBP) obtained from 154 different locations irregularly scattered over the globe is carefully processed to isolate signals related to the ocean general circulation and large‐scale sea level changes. Comparison against a global numerical ocean model experiment indicates poor correspondence for periods below 24 hr, possibly related to residual tidal signals and small timing errors in the atmospheric forcing applied to the ocean model. Correspondence increases rapidly for periods between 3 and 10 days, where wind‐driven dynamics are already well understood and consequently well implemented into numerical models. Coherence decreases again for periods around 30 days and longer, where processes not implemented into ocean general circulation models as barystatic sea level changes become more important. Correspondence between in situ data and satellite‐based OBP as obtained from the Gravity Recovery and Climate Experiment (GRACE) German Research Centre for Geosciences RL05a gravity fields critically depends on the postprocessing of Level‐2 Stokes coefficients that also includes the selection of appropriate averaging regions for the GRACE‐based mass anomalies. The assessment of other available GRACE Level‐2 products indicates even better fit of more recent solutions as ITSG‐Grace2016 and the Center for Space Research and Jet Propulsion Laboratory RL05 mascons. In view of the strong high‐frequency component of OBP, however, a higher temporal resolution of the oceanic GRACE products would be rather advantageous.

Description: 〈span〉〈div〉SUMMARY〈/div〉Temporal aliasing errors induced by high-frequency tidal and non-tidal mass variability in the Earth system are among the three most important error sources that limit the accuracy of present-day surface mass estimates from satellite gravimetry. By means of end-to-end simulations, we demonstrate that the Kalman Smoother approach developed by Kurtenbach 〈span〉et al.〈/span〉 effectively captures non-tidal submonthly variability, and thereby reduces temporal aliasing errors way beyond the level of simply subtracting the standard dealiasing model AOD1B. Validation against 〈span〉in situ〈/span〉 ocean bottom pressure observations confirms that the Kalman Smoother solutions published together with the ITSG-Grace2016 monthly gravity fields contain high-frequency signal over the oceans not predicted by AOD1B. The daily gravity fields therefore reduce aliasing artefacts in the monthly gravity fields, and at the same time provide observational evidence on submonthly bottom pressure variability presently not reflected in state-of-the-art numerical ocean circulation models. It is thus recommended to include a Kalman Smoother approach into any standard GRACE processing scheme. For a hypothetical double-pair configuration currently under consideration as a future mass change mission, we find that the benefit of the Kalman Smoother is much smaller thanks to the increased number of observations taken at different inclinations, which lead to generally reduced aliasing errors and much more isotropic spatial error correlations. We also reassess the idea of pre-eliminating low-resolution daily gravity fields and find large distortions in the monthly mean gravity solution at spatial wavelengths around the cut-off-degree of the daily fields. We thus recommend further study for any satellite gravity mission concept that critically relies on such pre-elimination schemes for reaching its science objectives.〈/span〉

Description: 〈span〉〈div〉Summary〈/div〉Temporal aliasing errors induced by high-frequency tidal and non-tidal mass variability in the Earth system are among the three most important error sources that limit the accuracy of present-day surface mass estimates from satellite gravimetry. By means of end-to-end simulations, we demonstrate that the Kalman Smoother approach developed by Kurtenbach et al. (2012) effectively captures non-tidal submonthly variability, and thereby reduces temporal aliasing errors way beyond the level of simply subtracting the standard dealiasing model AOD1B. Validation against in situ ocean bottom pressure observations confirms that the Kalman Smoother solutions published together with the ITSG-Grace2016 monthly gravity fields contain high-frequency signal over the oceans not predicted by AOD1B. The daily gravity fields therefore reduce aliasing artefacts in the monthly gravity fields, and at the same time provide observational evidence on submonthly bottom pressure variability presently not reflected in state-of-the-art numerical ocean circulation models. It is thus recommended to include a Kalman Smoother approach into any standard GRACE processing scheme. For a hypothetical double-pair configuration currently under consideration as a future mass change mission, we find that the benefit of the Kalman Smoother is much smaller thanks to the increased number of observations taken at different inclinations, which lead to generally reduced aliasing errors and much more isotropic spatial error correlations. We also reassess the idea of pre-eliminating low-resolution daily gravity fields and find large distortions in the monthly-mean gravity solution at spatial wavelengths around the cutoff-degree of the daily fields. We thus recommend further study for any satellite gravity mission concept that critically relies on such pre-elimination schemes for reaching its science objectives.〈/span〉

Description: To enable the use of GRACE and GRACE-FO earth observation data for rapid monitoring applications, the Horizon2020 funded EGSIEM (European Gravity Service for Improved Emergency Management) project has established a demonstrator for a near real-time (NRT) gravity field service. The service aims to increase the temporal resolution of mass transport products from one month to one day and to reduce the latency from currently two months to five days. This allows the monitoring of hydrological extreme events as they occur, in contrast to a ‘confirmation after occurrence’ as is the situation today. The service will be jointly run by GFZ (German Research Centre for Geosciences) and Graz University of Technology, with each analysis center providing an independent solution. On-line validation will be performed by the University of Luxembourg using GNSS loading. A six-month long operational test run of the service starting in April 2017 is planned, in case GRACE Quick-Look data (provided by JPL) is still available. Within this time period, daily gravity field solutions serve as input to the EGSIEM Hydrological Service, which derives flood and drought indicators to be used within DLR’s Center for Satellite Based Crisis Information and the Global Flood Awareness System (GloFAS). This contribution highlights the current status of the NRT service and the results of the preparation phase. The performance of the NRT mass transport products will be shown by comparison with independent GNSS loading and ocean bottom pressure data as well as as catchment aggregated values for hydrological extreme Events.

Description: Earth observation satellites yield a wealth of data for scientific, operational and commercial exploitation. However, the redistribution of mass in the system Earth is not yet part of the standard inventory of Earth Observation (EO) data products to date. It is derived from the Gravity Recovery and Climate Experiment (GRACE) mission and its Follow-On mission (GRACE-FO). Among many other applications, mass redistribution provides fundamental insights into the global water cycle. Changes in continental water storage impact the regional water budget and can, in extreme cases, result in floods and droughts that often claim a high toll on infrastructure, economy and human lives. The initiative for a European Gravity Service for Improved Emergency Management (EGSIEM) established three different prototype services to promote the unique value of mass redistribution products for Earth Observation in general and for early-warning systems in particular. The first prototype service is a scientific combination service to derive improved mass redistribution products from the combined knowledge of the European GRACE analysis centres. Secondly, the timeliness and reliability of such products is a primary concern for any early-warning system and therefore EGSIEM established a prototype for a near real-time service that provides dedicated gravity field information with a maximum latency of five days . Third, EGSIEM established a prototype of a hydrological / early warning service that derives wetness indices as indicators of hydrological extremes and assessed their potential for timely scheduling of high-resolution optical/radar satellites for follow-up observations in case of evolving hydrological extreme events.

Description: The GRACE and GRACE-FO satellites observe the redistribution of mass in terrestrial water storage, ice sheets, oceans, atmosphere, and solid Earth. Because GRACE data is typically accumulated into monthly-mean gravity fields, an a priori background model, namely the Atmosphere and Ocean Dealiasing Level 1B (AOD1B) product, is applied to remove non-tidal variability that would otherwise alias into the monthly solutions. The main disadvantage of AOD1B RL06 compared to its previous release is that it does not simulate the dynamics beneath the Antarctic ice shelves, which can have a strong influence on global ocean circulation. The primary motivation for this work is the development of the new release of AOD1B, but the performed model experiments can also provide useful insight into the influence processes in the Southern Ocean have on global ocean dynamics. To be able to test various model experiments, as well as to compare GRACE gravity field solutions, validation against in situ measured ocean bottom pressure (OBP) is used. The validation is somewhat better suited for submonthly variability of the ocean models than for long-term signals measured by GRACE because the in situ time series are affected by the errors in trend and drift removal on longer temporal scales. The difference between the pointwise in situ and the area-averaging GRACE measuring technique also influences the comparison. It is shown that post-processing choices can severely impact the results of the validation of GRACE fields, so if different solutions are compared, their post-processing needs to be identical. Validation against in situ OBP is used to compare the EGSIEM combined GRACE solution with its five contributing datasets. It is shown that the combined solution is very close to the leading CSR RL05 and ITSG-Grace2016 solutions, outperforming the others. To investigate whether GRACE is able to detect submonthly signals, the ITSG-Grace2016 daily Kalman solution, from which the submonthly atmospheric and oceanic variability has been removed with AOD1B RL05, is validated against in situ OBP. The results show that GRACE successfully captures some submonthly variability that is not predicted by the incorporated dealiasing model. As a first step towards AOD1B RL07, the dynamics beneath the Antarctic ice shelves are implemented into the model used as the oceanic part of AOD1B, the Max Planck Institute Ocean Model (MPIOM). The bathymetry is modified to include the areas under the ice shelves and two new model experiments are performed: in one those regions are treated as open ocean, while in the other the atmospheric forcing is modified to simulate the ice shelves. The changes caused by such modifications are not limited only to the Southern Ocean, but also affect the Northern Atlantic, confirming the role the Weddell Sea has on the meridional overturning circulation. While surface changes exceed the typical variability only in a few regions, the differences at the bottom of the ocean are larger. The changes caused by ice shelf forcing are of the same order of magnitude in the vicinity of the ice shelves, but much smaller globally. A comparison with the GLORYS2v4 ocean reanalysis shows that the new model experiments are closer to the reanalysis, especially in the regions where the original MPIOM experiment performs the worst. The analysis of OBP variability points out some possible issues that need to be fixed before publishing the new AOD1B release. Validation against in situ OBP, however, shows that the modifications are without a doubt in the right direction: the new model experiment has increased relative explained variances in the 1 - 3 days band by approximately 5 % throughout Pacific, and by more than 10 % in the Antarctic Circumpolar Current region.

Description: Die GRACE- und GRACE-FO-Satelliten beobachten die Umverteilung von Masse in terrestrischen Wasserspeichern, Eisdecken, Ozeanen, Atmosphäre und fester Erde. Da GRACE-Daten in der Regel in monatlichen gemittelten Schwerefeldern gesammelt werden, wird ein a priori Hintergrundmodell Atmosphere and Ocean Dealiasing Level 1B (AOD1B) angewendet, um nicht-gezeitenbedingte Schwankungen zu beseitigen, die andernfalls zu einem Alias-Effekt in den monatlichen Lösungen führen würden. Der Hauptnachteil von AOD1B RL06 im Vergleich zu seiner Vorgängerversion besteht darin, dass die Dynamik unter den Antarktis-Eisschelfs, die einen starken Einfluss auf die globale Ozeanzirkulation haben kann, nicht simuliert wird. Die Hauptmotivation für diese Arbeit ist die Entwicklung des neuen Releases von AOD1B, die durchgeführten Modellexperimente können jedoch auch nützliche Einblicke in den Einfluss von Prozessen im Südpolarmeer auf die globale Ozeandynamik liefern. Um verschiedene Modellexperimente testen und GRACE-Schwerefeldlösungen vergleichen zu können, wird eine Validierung gegen den in situ gemessenen Meeresbodendruck (OBP) durchgeführt. Die Validierung ist für die submonatliche Variabilität der Ozeanmodelle etwas besser geeignet als für die von GRACE gemessenen Langzeitsignale, da die In-situ-Zeitreihen auf längeren Zeitskalen von Trend- und Driftentfernungenfehlern beeinflusst werden. Der Unterschied zwischen der punktweisen in situ- und der flächenmittelnden GRACE-Messtechnik beeinflusst auch den Vergleich. Es wird gezeigt, dass die Auswahl der Postprozessierung die Validierungsergebnisse von GRACE-Feldern erheblich beeinflussen kann. Wenn also verschiedene Lösungen verglichen werden, muss die Postprozessierung identisch sein. Die Validierung gegen In-situ-OBP wird verwendet, um die in EGSIEM Projekt kombinierte GRACE-Lösung mit ihren fünf beitragenden Datensätzen zu vergleichen. Es wird gezeigt, dass die kombinierte Lösung den führenden CSR RL05- und ITSG-Grace2016-Lösungen sehr nahe kommt und die anderen übertrifft. Um zu untersuchen, ob GRACE in der Lage ist, submonatliche Signale zu erkennen, wird die tägliche ITSG-Grace2016-Kalman-Lösung, aus der die submonatliche atmosphärische und ozeanische Variabilität mit AOD1B RL05 entfernt wurde, gegen in situ OBP validiert. Die Ergebnisse zeigen, dass GRACE gewisse submonatliche Variabilitäten erfolgreich erfasst, die vom integrierten Dealiasing-Modell nicht vorhergesagt werden. Als erster Schritt in Richtung AOD1B RL07 wird die Dynamik unter den Antarktis-Eisschelfs in das Max Planck Institute Ocean Model (MPIOM) implementiert, das als ozeanischer Teil von AOD1B verwendet wird. Die Bathymetrie wird modifiziert, um die Bereiche unter den Eisschelfen einzubeziehen, und es werden zwei neue Modellexperimente durchgeführt: In einem Experiment werden diese als offener Ozean behandelt, während im anderen der atmosphärische Antrieb modifiziert wird, um die Eisschelfe zu simulieren. Die durch solche Modifikationen verursachten Veränderungen beschränken sich nicht nur auf das Südpolarmeer, sondern betreffen auch den Nordatlantik, was die Rolle des Weddellmeeres für die meridionale Umwälzzirkulation bestätigt. Während die Oberflächenveränderungen die typische Variabilität nur in wenigen Regionen überschreiten, sind die Unterschiede am Meeresboden größer. Die Veränderungen, die durch den Eisschelfantrieb verursacht werden, sind in der Nähe der Eisschelfs in der gleichen Größenordnung, global jedoch viel geringer. Ein Vergleich mit der Ozean-Reanalyse GLORYS2v4 zeigt, dass die neuen Modellexperimente näher an der Reanalyse liegen, insbesondere in den Regionen, in denen das ursprüngliche MPIOM-Experiment am schlechtesten abschneidet. Die Analyse der OBP-Variabilität zeigt einige mögliche Probleme auf, die vor der Veröffentlichung der neuen Version des AOD1B-Produkts behoben werden müssen. Die Validierung mit In-situ-OBP zeigt jedoch, dass die Modifikationen zweifellos in die richtige Richtung weisen: Das neue Modellexperiment hat die relativen erklärten Varianzen im Bereich von 1 - 3 Tagen im gesamten pazifischen Raum um ungefähr 5 % erhöht und um mehr als 10 % in der Region der Antarktische Zirkumpolarströmung.