Estimating integrated water vapor trends from VLBI, GPS,and numerical weather models: sensitivity totropospheric parameterization

©2018. American Geophysical UnionIn this study, we estimate integrated water vapor (IWV) trends from very long baseline interferometry (VLBI) and global navigation satellite systems (GNSS) data analysis, as well as from numerical weather models (NWMs). We study the impact of modeling and parameterization of the tropospheric delay from VLBI on IWV trends. We address the impact of the meteorological data source utilized to model the hydrostatic delay and the thermal deformation of antennas, as well as the mapping functions employed to project zenith delays to arbitrary directions. To do so, we derive a new mapping function, called Potsdam mapping functions based on NWM data and a new empirical model, GFZ‐PT. GFZ‐PT differs from previous realizations as it describes diurnal and subdiurnal in addition to long‐wavelength variations, it provides harmonic functions of ray tracing‐derived gradients, and it features robustly estimated rates. We find that alternating the mapping functions in VLBI data analysis yields no statistically significant differences in the IWV rates, whereas alternating the meteorological data source distorts the trends significantly. Moreover, we explore methods to extract IWV given a NWM. The rigorously estimated IWV rates from the different VLBI setups, GNSS, and ERA‐Interim are intercompared, and a good agreement is found. We find a quite good agreement comparing ERA‐Interim to VLBI and GNSS, separately, at the level of 75%.DFG, 255986470, GGOS-SIM-2: Simulation des "Global Geodetic Observing System

Atmospheric Contributions to Global Ocean Tides for Satellite Gravimetry

To mitigate temporal aliasing effects in monthly mean global gravity fields from the GRACE and GRACE‐FO satellite tandem missions, both tidal and non‐tidal background models describing high‐frequency mass variability in atmosphere and oceans are needed. To quantify tides in the atmosphere, we exploit the higher spatial (31 km) and temporal (1 hr) resolution provided by the latest atmospheric ECMWF reanalysis, ERA5. The oceanic response to atmospheric tides is subsequently modeled with the general ocean circulation model MPIOM (in a recently revised TP10L40 configuration that includes the feedback of self‐attraction and loading to the momentum equations and has an improved bathymetry around Antarctica) as well as the shallow water model TiME (employing a much higher spatial resolution and more elaborate tidal dissipation than MPIOM). Both ocean models consider jointly the effects of atmospheric pressure variations and surface wind stress. We present the characteristics of 16 waves beating at frequencies in the 1–6 cpd band and find that TiME typically outperforms the corresponding results from MPIOM and also FES2014b as measured from comparisons with tide gauge data. Moreover, we note improvements in GRACE‐FO laser ranging interferometer range‐acceleration pre‐fit residuals when employing the ocean tide solutions from TiME, in particular, for the S1 spectral line with most notable improvements around Australia, India, and the northern part of South America

Inter-Comparison of UT1-UTC from 24-Hour, Intensives, and VGOS Sessions during CONT17

This work focuses on the assessment of UT1-UTC estimates from various types of sessions during the CONT17 campaign. We chose the CONT17 campaign as it provides 15 days of continuous, high-quality VLBI data from two legacy networks (S/X band), i.e., Legacy-1 (IVS) and Legacy-2 (VLBA) (having different network geometry and are non-overlapping), two types of Intensive sessions, i.e., IVS and Russian Intensives, and five days of new-generation, broadband VGOS sessions. This work also investigates different approaches to optimally compare dUT1 from Intensives with respect to the 24 h sessions given the different parameterization adopted for analyzing Intensives and different session lengths. One approach includes the estimation of dUT1 from pseudo Intensives, which are created from the 24 h sessions having their epochs synchronized with respect to the Intensive sessions. Besides, we assessed the quality of the dUT1 estimated from VGOS sessions at daily and sub-daily resolution. The study suggests that a different approach should be adopted when comparing the dUT1 from the Intensives, i.e., comparison of dUT1 value at the mean epoch of an Intensive session. The initial results regarding the VGOS sessions show that the dUT1 estimated from VGOS shows good agreement with the legacy network despite featuring fewer observations and stations. In the case of sub-daily dUT1 from VGOS sessions, we found that estimating dUT1 with 6 h resolution is superior to other sub-daily resolutions. Moreover, we introduced a new concept of sub-daily dUT1-tie to improve the estimation of dUT1 from the Intensive sessions. We observed an improvement of up to 20% with respect to the dUT1 from the 24 h sessions.The publication costs are supported within the funding programme “Open Access Publikationskosten” Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)—Project Number 491075472. K.B. is funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)– Project-ID 434617780—SFB 1464 (TerraQ). S.B. was partially supported by Generalitat Valenciana (SEJIGENT/2021/001), the European Union—NextGenerationEU (ZAMBRANO 21-04) and Ministerio de Ciencia e Innovación (Spanish Project PID2020-119383GB-I00)

Investigating the Relationship Between Length of Day and El-Niño Using Wavelet Coherence Method

The relationship between the length of day (LOD) and El-Niño Southern Oscillation (ENSO) has been well studied since the 1980s. LOD is the negative time-derivative of UT1-UTC, which is directly proportional to Earth Rotation Angle (ERA), one of the Earth Orientation Parameters (EOP). The EOP can be determined using Very Long Baseline Interferometry (VLBI), which is a space geodetic technique. In addition, satellite techniques such as the Global Navigation Satellite System (GNSS), Satellite Laser Ranging (SLR), Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) can provide Earth Rotation Parameters, i.e., polar motion and LOD. ENSO is a climate phenomenon occurring over the tropical eastern Pacific Ocean that mainly affects the tropics and the subtropics. Extreme ENSO events can cause extreme weather like flooding and droughts in many parts of the world. In this work, we investigated the effect of ENSO on the LOD from January 1979 to April 2022 using the wavelet coherence method. This method computes the coherence between the two non-stationary time-series in the time-frequency domain using the real-valued Morlet wavelet. We used the Multivariate ENSO index version 2 (MEI v.2) which is the most robust series as the climate index for the ENSO, and LOD time-series from IERS (EOP 14 C04 (IAU2000A)). We also used Oceanic Niño and Southern Oscillation index in this study for comparison. The results show strong coherence of 0.7 to 0.9 at major ENSO events for the periods 2–4 years between LOD and MEI.v2.Kyriakos Balidakis is funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)—Project-ID 434617780—SFB 1464 (TerraQ). Santiago Belda was partially supported by Generalitat Valenciana (SEJIGENT/2021/001), the European Union—NextGenerationEU (ZAMBRANO 21-04) and Ministerio de Ciencia e Innovación (Spanish Project PID2020-119383GB-I00). Chaiyaporn Kitpracha acknowledges funding from Deutscher Akademischer Austauschdienst (DAAD) under grant number 91650950

Water-Vapour Monitoring from Ground-Based GNSS Observations in Northwestern Argentina

The Central Andes in northwestern Argentina are characterized by steep topographic and climatic gradients. The humid foreland areas at 1 km asl elevation rapidly rise to over 5 km in the eastern Cordillera, and they form an orographic rainfall barrier on the eastern windward side. This topographic setting combined with seasonal moisture transport through the South American monsoon system leads to intense rainstorms with cascading effects such as landsliding and flooding. In order to better quantify the dynamics of water vapour transport, we use high-temporal-resolution global navigation satellite system (GNSS) remote sensing techniques. We are particularly interested in better understanding the dynamics of high-magnitude storms with high water vapour amounts that have destructive effects on human infrastructure. We used an existing GNSS station network with 12 years of time series data, and we installed two new ground stations along the climatic gradient and collected GNSS time series data for three years. For several stations we calculated the GNSS signal delay gradient to determine water vapour transport direction. Our statistical analysis combines in situ rainfall measurements and ERA5 reanalysis data to reveal the water vapour transport mechanism for the study area. The results show a strong relationship between altitude and the water vapour content, as well as between the transportation pathways and the topography.Fil: Antonoglou, Nikolaos. German Research Centre for Geosciences; Alemania. Universitat Potsdam; AlemaniaFil: Balidakis, Kyriakos. German Research Centre for Geosciences; AlemaniaFil: Wickert, Jens. Technishe Universitat Berlin; Alemania. German Research Centre for Geosciences; AlemaniaFil: Dick, Galina. German Research Centre for Geosciences; AlemaniaFil: de la Torre, Alejandro. Universidad Austral; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Bookhagen, Bodo. Universitat Potsdam; Alemani

Inter-Comparison of UT1-UTC from 24-Hour, Intensives, and VGOS Sessions during CONT17

This work focuses on the assessment of UT1-UTC estimates from various types of sessions during the CONT17 campaign. We chose the CONT17 campaign as it provides 15 days of continuous, high-quality VLBI data from two legacy networks (S/X band), i.e., Legacy-1 (IVS) and Legacy-2 (VLBA) (having different network geometry and are non-overlapping), two types of Intensive sessions, i.e., IVS and Russian Intensives, and five days of new-generation, broadband VGOS sessions. This work also investigates different approaches to optimally compare dUT1 from Intensives with respect to the 24 h sessions given the different parameterization adopted for analyzing Intensives and different session lengths. One approach includes the estimation of dUT1 from pseudo Intensives, which are created from the 24 h sessions having their epochs synchronized with respect to the Intensive sessions. Besides, we assessed the quality of the dUT1 estimated from VGOS sessions at daily and sub-daily resolution. The study suggests that a different approach should be adopted when comparing the dUT1 from the Intensives, i.e., comparison of dUT1 value at the mean epoch of an Intensive session. The initial results regarding the VGOS sessions show that the dUT1 estimated from VGOS shows good agreement with the legacy network despite featuring fewer observations and stations. In the case of sub-daily dUT1 from VGOS sessions, we found that estimating dUT1 with 6 h resolution is superior to other sub-daily resolutions. Moreover, we introduced a new concept of sub-daily dUT1-tie to improve the estimation of dUT1 from the Intensive sessions. We observed an improvement of up to 20% with respect to the dUT1 from the 24 h sessions.DFG, 491075472, Open-Access-Publikationskosten / 2022-2024 / Deutsches GeoForschungsZentrum GFZ / Potsdam-Institut für Klimafolgenforschung (PIK)DFG, 434617780, SFB 1464: Relativistische und quanten-basierte Geodäsie (TerraQ

Improving the vertical modeling of tropospheric delay

Accurate tropospheric delays from Numerical Weather Models (NWM) are an important input to space geodetic techniques, especially for precise real-time Global Navigation Satellite Systems, which are indispensable to earthquake and tsunami early warning systems as well as weather forecasting. The NWM-based tropospheric delays are currently provided either site-specific with a limited spatial coverage, or on two-dimensional grids close to the Earth surface, which cannot be used for high altitudes. We introduce a new method of representing NWM-derived tropospheric zenith hydrostatic and wet delays. A large volume of NWM-derived data is parameterized with surface values and additional two or three coefficients for their vertical scaling to heights up to 14 km. A precision of 1–2 mm is achieved for reconstructing delays to the NWM-determined delays at any altitudes. The method can efficiently deliver NWM-derived tropospheric delays to a broader community of space geodetic techniques.DFG, 434617780, SFB 1464: Relativistische und quanten-basierte Geodäsie (TerraQ

Sensitivity of GNSS tropospheric gradients to processing options

An analysis of processing settings impacts on estimated tropospheric gradients is presented. The study is based on the benchmark data set collected within the COST GNSS4SWEC action with observations from 430 Global Navigation Satellite Systems (GNSS) reference stations in central Europe for May and June 2013. Tropospheric gradients were estimated in eight different variants of GNSS data processing using precise point positioning (PPP) with the G-Nut/Tefnut software. The impacts of the gradient mapping function, elevation cut-off angle, GNSS constellation, observation elevation-dependent weighting and real-time versus post-processing mode were assessed by comparing the variants by each to other and by evaluating them with respect to tropospheric gradients derived from two numerical weather models (NWMs). Tropospheric gradients estimated in post-processing GNSS solutions using final products were in good agreement with NWM outputs. The quality of high-resolution gradients estimated in (near-)real-time PPP analysis still remains a challenging task due to the quality of the real-time orbit and clock corrections. Comparisons of GNSS and NWM gradients suggest the 3 degrees elevation angle cut-off and GPS+GLONASS constellation for obtaining optimal gradient estimates provided precise models for antenna-phase centre offsets and variations, and tropospheric mapping functions are applied for low-elevation observations. Finally, systematic errors can affect the gradient components solely due to the use of different gradient mapping functions, and still depending on observation elevation-dependent weighting. A latitudinal tilting of the troposphere in a global scale causes a systematic difference of up to 0.3 mm in the north-gradient component, while large local gradients, usually pointing in a direction of increasing humidity, can cause differences of up to 1.0 mm (or even more in extreme cases) in any component depending on the actual direction of the gradient. Although the Bar-Sever gradient mapping function provided slightly better results in some aspects, it is not possible to give any strong recommendation on the gradient mapping function selection.Web of Science37344642

Evaluation of VLBI Observations with Sensitivity and Robustness Analyses

Very Long Baseline Interferometry (VLBI) plays an indispensable role in the realization of global terrestrial and celestial reference frames and in the determination of the full set of the Earth Orientation Parameters (EOP). The main goal of this research is to assess the quality of the VLBI observations based on the sensitivity and robustness criteria. Sensitivity is defined as the minimum displacement value that can be detected in coordinate unknowns. Robustness describes the deformation strength induced by the maximum undetectable errors with the internal reliability analysis. The location of a VLBI station and the total weights of the observations at the station are most important for the sensitivity analysis. Furthermore, the total observation number of a radio source and the quality of the observations are important for the sensitivity levels of the radio sources. According to the robustness analysis of station coordinates, the worst robustness values are caused by atmospheric delay effects with high temporal and spatial variability. During CONT14, it is determined that FORTLEZA, WESTFORD, and TSUKUB32 have robustness values changing between 0.8 and 1.3 mm, which are significantly worse in comparison to the other stations. The radio sources 0506-612, NRAO150, and 3C345 have worse sensitivity levels compared to other radio sources. It can be concluded that the sensitivity and robustness analysis are reliable measures to obtain high accuracy VLBI solutions

On the development and impact of propagation delay and geophysical loading on space geodetic technique data analysis

This study contributes to the effort of space geodesy to reach the 1 mm accuracy level on a global scale. This stringent requirement is vital to realize a global reference system upon which phenomena such as sea-level rise can be reliably monitored. The study deals with two interrelated challenges: modeling geophysical loads imposed on the crust of the Earth and quantifying the atmospheric propagation delay of signals employed by space geodetic techniques, namely VLBI, SLR, GNSS, and DORIS. If not adequately modeled, both geophysical loading and propagation delay corrupt space geodetic data analysis results, thus distorting the implied reference frame and compromising the physical interpretation of other parameters. The explicit purpose of this work is to understand how these effects propagate into the parameters estimated within the geodetic adjustment, and to develop models that alleviate geodetic results from these effects. To achieve this goal, the scientific framework was divided into two contributions to be understood and enhanced: the theory governing the effects of geophysical loading and atmospheric propagation, and the space geodetic technique data analysis pipeline, largely using VLBI as a test-bed. In essence, the research conducted here includes: (i) the development of software capable of realistically simulating VLBI, SLR, GNSS, and DORIS observations within a Monte Carlo framework, (ii) the homogenization of in~situ meteorological data recorded at VLBI and SLR stations, (iii) the development of ray-traced delays, mapping functions and higher-order gradients for all four space geodetic techniques, (iv) the comprehensive investigation of inter-frequency and inter-system atmospheric ties, (v) the development of models to describe the displacement induced by mass redistribution within Earth's fluid envelope including the atmosphere, the ocean, and the continental hydrology, (vi) the development of empirical models to describe the signal propagation delay (GFZ-PT) and the non-tidal geophysical loading displacement (EGLM), and (vii) the study of the impact of the atmospheric refraction and non-tidal geophysical loading models in space geodetic data analysis on station coordinates, the terrestrial reference frame, the Earth orientation, and the integrated water vapour trends. A number of developments were carried out herein for the first time, for example, the simulation of space geodetic measurements based on ray-traced delays, the study of systematic errors on the reference frame induced by not properly accounting for the orbital altitude of the satellites in the calculation of atmospheric refraction corrections, and the assessment of the probability of successful laser ranges based on integrated cloud fraction along the ray path. It was found that microwave and optical atmospheric gradients are starkly different both spatially and temporally, and cannot be scaled to fit each other. Failing to account for non-tidal geophysical loading and atmospheric asymmetries induces a scale bias in the SLR reference frame as well as a spurious geocenter motion predominantly along the Z-axis. Employing a VLBI-tailored atmospheric refraction model to reduce DORIS observations displaces stations in the radial component thus inducing a large scale bias in the implied frame. Employing homogeneous in lieu of raw meteorological data in VLBI data analysis reduces the scatter of station coordinates and improves the baseline length repeatability. Employing the mapping functions developed herein in lieu of VMF1 yields an overall improvement in VLBI data analysis. Applying the geophysical loading models developed herein reduces the response of almost all station coordinate and baseline length series at seasonal and synoptic timescales. Based on the investigations carried out herein, differences in Earth orientation induced by the quality of the atmospheric refraction and geophysical loading models - or their very application for the latter - are not statistically significant in the framework of the modern VLBI system. Nevertheless, to fulfill the 1 mm requirement, proper treatment of geophysical loading and atmospheric refraction is a necessity