Beschreibung: Global continental hydrological models use atmospheric weather data like precipitation and evaporation to force the simulation of continental water storage variations and river discharge. The predominant dependency of the modelled hydrological results from the incoming recipitation-evaporation budget is especially obvious when calculating global geodetic parameters such as Earth rotation excitation and gravity field changes. Many geodetically oriented hydrological studies are based on forcing data from the ERA-40 re-analysis of the European Centre for Medium-Range Weather Forecasts (ECMWF). In 2006 ECMWF started to develop a new interim re-analysis system derived from the latest version of their operational system. The ERA-Interim re-analysis starts 1989 and is now available until 2005. Using an improved assimilation background model and additional observation data several of the problems experienced in ERA-40 have been eliminated or significantly reduced in ERA-Interim, most notably the too-strong tropical oceanic precipitation from the early 1990s onwards. Nevertheless precipitation over tropical continental regions like in Africa is still higher than the estimates from the Global Precipitation Climate Centre (GPCC). The differences between ERA-40 and ERA-Interim forcing data significantly change hydrological Earth rotation excitation. The hydrological land surface discharge model LSDM was used to determine these differences in polar motion, length of day and low degree gravity coefficients. The detected biases indicate that the overall continental water storage is reduced, and part of the water masses are shifted between continents and seasons. The trends in excitation time series due to unbalanced precipitation-evaporation budgets vanish, whereas the seasonal timing of regional water storage events remains almost unaffected. Additional results from regional studies like in the Nile basin help to analyse the quality of the new ERA-Interim data and to classify their benefits for geodetic Earth system models.

Beschreibung: Operational global mass transport data of the atmosphere and the oceans are widely used for studies of earth rotation excitation and gravity field simulations and are essential for GRACE dealising purposes, too. Seasonal and short periodic variations are also caused by continental water mass redistributions. In order to account for the continental hydrology processes as well and to close the global water cycle, continental water mass storage fields and fluxes are needed in the same operational manner as for the atmosphere and ocean. To simulate continental water mass redistributions a land surface scheme (SLS) is combined with a hydrological discharge model (HDM). The SLS generates components of the land water budget like soil moisture, snow accumulation and evaporation as well as surface fluxes like runoff and drainage. The latter is applied as input data for the HDM simulating the lateral water flow. Both models work on daily time steps. The new extended model combination (LSXM+HDXM) for the operational use, provides daily variations of the global water mass storage and the corresponding water fluxes as well as the hydrological angular momentum functions and low degree gravity coefficients in near real time. Since both, LSXM+HDXM and OMCT, are consistently forced with operational analyses from ECMWF, the complete data set of atmospheric, oceanic and hydrologic mass variations allows a realistic representation of mass transports in the global hydrological cycle.

Beschreibung: Angular momentum forecasts for up to 10 days into the future, modeled from predicted states of the atmosphere, ocean and continental hydrosphere, are combined with the operational IERS EOP prediction bulletin A to reduce the prediction error in the very first day and to improve the subsequent 90-day prediction by exploitation of the revised initial state and trend information. EAM functions derived from ECMWF short-range forecasts and corresponding LSDM and OMCT simulations can account for high-frequency mass variations within the geophysical fluids for up to 7 days into the future primarily limited by the accuracy of the forecasted atmospheric wind fields. Including these wide-band stochastic signals into the first days of the 90-day statistical IERS predictions reduces the mean absolute prediction error even for predictions beyond day 10, especially for polar motion, where the presently used prediction approach does not include geophysical fluids data directly. In a hindcast experiment using 1 year of daily predictions from May 2011 till July 2012, the mean prediction error in polar motion, compared to bulletin A, is reduced by 32, 12, and 3 % for prediction days 10, 30, and 90, respectively. In average, the prediction error for medium-range forecasts (30–90 days) is reduced by 1.3–1.7 mas. Even for UT1-UTC, where AAM forecasts are already included in IERS bulletin A, we obtain slight improvements of up to 5 % (up to 0.5 ms) after day 10 due to the additional consideration of oceanic angular momentum forecasts. The improved 90-day predictions can be generated operationally on a daily basis directly after the publication of the related IERS bulletin A product finals2000A.daily.

Beschreibung: Daily effective angular momentum functions from atmosphere, oceans, and continental hydrosphere that are consistent in terms of global mass conservation among the sub-systems are obtained from atmospheric data the most recent ECMWF re-analysis ERA Interim and corresponding simulations with the hydrological model LSDM and the ocean model OMCT covering 1989 - 2008. Correlations between simulations and geodetic excitation functions based on the EOP C04 polar motion series are generally improving when considering oceanic and even continental effects in addition to the atmosphere, with correlation coefficients that exceed values of 0.8 during the most recent years. While contributions to the annual wobble are found to be of similar amplitude and phase as in previous studies, both seasonal averaged and inter-annual variations are able to capture the main characteristics of individual peaks in the corresponding geodetic excitation functions. By decomposing the simulated global angular momentum functions into their regional contributions, atmospheric and oceanic pressure and current distributions in accordance with continental water storage variations are shown to be of similar importance for polar motion excitation on seasonal time-scales, whereas continental water flow contributions to the relative angular momentum of the Earth have been found to be three orders of magnitude lower than the corresponding effect of water storage changes. The data-sets discussed here are publicly available via the restructured Geophysical Fluids Center of the IERS.

Beschreibung: Consistent simulations of mass, momentum, and heat fluxes within and exchanges between the subsystems atmosphere, oceans, and continental hydrosphere are generated daily by means of an operational processing system consisting of the Ocean Model for Circulation and Tides and the Land Surface and Discharge Model forced with global operational weather data provided by the European Centre for Medium-Range Weather Forecasts (ECMWF). Mass conservation within the modelling system is ensured by coupling via continental freshwater fluxes and by introducing an additional ocean layer compensating annual total mass imbalances. Daily updated atmospheric, oceanic and hydrological effective angular momentum functions are publicly available with a few days latency only, enabling quasi-real time interpretation of observed Earth rotation variations. Exemplarily, recent variability patterns of angular momentum as well as relevant underlying physical processes related to mass transports in the atmosphere-hydrosphere system will be presented. By replacing ECMWF’s analyses with its medium-range forecasts, the processing system also allows shortterm predictions of Earth rotation parameters. As demonstrated in a hind-cast experiment using 10-day atmospheric forecasts from 2003 - 2008, short-term polar motion prediction errors can be reduced by 26%, what is mainly a consequence of taking into account short-term dynamics of geophysical fluids not considered in the current prediction approach of IERS bulletin A. Modelled forecasts reach relative explained variances between 40% and 80%, whereas bulletin A forecasts can explain only up to 40% of the observed polar motion variance within the first 10 prediction days.

Beschreibung: Angular momentum functions based on forecasted model states can predict high-frequency mass variations within the geophysical fluids for 5-7 days into the future. In a hindcast experiment 10-day forecasts of angular momentum functions, modeled from predicted states of the atmosphere, ocean and continental hydrosphere, are used to incorporate wide-band stochastic signals into the statistically derived IERS bulletin A predictions. The combination approach concatenates the rapid solution of the past 90 days from bulletin A with one week of modeled forecasts and the trend and bias corrected 90-day prediction of bulletin A. We analyzed, how the improvements in the first days of the predicted EOPs by the model forecasts manifest itself in a decreased prediction error even at prediction days 30 or 60. Comparing the predictions with IERS bulletin A and C04 the reduction of the mean absolute error is significant in polar motion whereas UT1-UTC does not benefit much since the atmospheric excitation forecasts are already included in bulletin A.

Beschreibung: The impact of continental water mass redistributions on Earth rotation is deduced from stand-alone runs with the Hydrological Discharge Model (HDM) forced by ERA40 re-analyses as well as by the unconstrained atmospheric climate model ECHAM5. The HDM is attached in three different approaches to the atmospheric forcing models. First, ECHAM5 and its embedded land surface model generates directly runoff and drainage appropriate for the subsequent processing with HDM, like it is realized in the dynamically coupled model system ECOCTH, too. Second, an intermediate Simplified Land Surface scheme (SLS) is used to separate ERA40 precipitation into runoff, drainage, and evaporation. Third, precipitation and evaporation are used as input for the Land Surface Discharge Model (LSDM), which estimates runoff and drainage internally for its HDM-like discharge scheme. The individual models are validated by observed river discharges. The induced rotational variations represent mainly the different forcing from precipitation-evaporation and trends from inconsistent mass fluxes. The dynamical coupling of atmosphere and ocean has only a subordinated influence.

Beschreibung: Simulation results from the hydrological land surface discharge model LSDM and its deviation from observations are dominated by characteristics of the precipitation forcing fields. In order to improve LSDM model results, the errors indroduced by the mismodelled fresh water income, like operational ECMWF precipitation and evaporation, should be well-understood. Overestimated precipitation in global ECMWF fields is a known problem but there exist no quantitative correction patterns to bring the atmospheric freshwater budget into global balance as it is particularly important for the determination of Earth rotation excitation functions. As the LSDM model can provide the general relationship between the atmospheric freshwater income and the continental water storage variations hydrological mass variations detected with GRACE can also be associated with global precipitation fields on seasonal timescales. Comparisons of LSDM and GRACE water storage variations lead to seasonal correction patterns for ECMWF precipitation fields that are also contrasted with global precipiation estimations from the Global Precipitation Climate Centre. The differences in precipitation are related to seasonal signals in hydrological Earth rotation excitation functions.

Beschreibung: High-resolution load-induced crustal deformations calculated from numerical models are tested for their ability to predict hydrologically-induced station height variability, as they are known to be large enough to affect epoch-wise parameters obtained from the analysis of global geodetic networks. Loading contributions due to terrestrial water storage as given by global hydrological models are calculated on a 0.5° global regular grid with daily temporal resolution. Apart from the dominant seasonal variations, the hydrological loading signal discloses also rapid changes exceeding 1 mm in several regions that can be associated with major precipitation events and river floods. Locally strong loading signals with exceptionally high amplitudes, in many cases even with nonseasonal nature, occur along the major river channels. Only high-resolution loading calculations considering also the water mass anomalies stored in the model river flow can resolve the correct amplitudes in the surrounded area up to 100 km distance. The comparison of the modeled hydrological surface deformation with GPS station time series shows that high-resolution hydrological loading estimates based on global-scale models are able to explain a considerable fraction (up to 54%) of the observed vertical station movements caused by continental water storage variations.