ALBERT

Description: The system of oceanic flows constituting the Atlantic Meridional Overturning Circulation (AMOC) moves heat and other properties to the subpolar North Atlantic, controlling regional climate, weather, sea levels, and ecosystems. Climate models suggest a potential AMOC slowdown towards the end of the 21〈sup〉st〈/sup〉 century due to anthropogenic forcing, which would accelerate coastal sea level rise along the western boundary and dramatically increase coastal flood risk. While the slowdown has not been observed to date, we show here that the AMOC-induced intrinsic changes in gyre-scale heat content, superimposed on the global mean sea level rise, are already influencing the frequency of floods along the United States southeastern seaboard. For the South Atlantic Bight and Gulf of Mexico coasts, using observations and an ocean state estimate, we have established a strong link between coastal sea level, the associated flood frequency, and gyre-scale dynamic sea level and oceanic heat content variability, which are largely controlled by AMOC-driven ocean heat convergence. We find that ocean heat convergence, being the primary driver for interannual sea level changes in the subtropical North Atlantic, has led to an exceptional gyre-scale warming and associated dynamic sea level rise since 2010, accounting for 30-50% of flood days in 2015-2020. The results of this study highlight the importance of accounting for natural, large-scale sea level variability in order to improve coastal sea level projections and to better assess coastal flood risk.

Description: For the purpose of orbit validation, currently, many low Earth orbit (LEO) satellites of Earth observation missions are equipped with laser retro-reflectors. These Satellites Laser Ranging (SLR) observations exhibit a great potential to improve the accuracy of the SLR-based terrestrial reference frame, which is only realized by spherical geodetic satellites at present. How to weigh the SLR observations collected by different LEO satellites is always a troublesome issue when processing these observations. The purpose of this study is to optimize the integration of multi-LEO satellite SLR solutions by adjusting the weights of the observations using the Helmert Variance component estimation (VCE). The SLR observations of seven LEO satellites from three typical LEO missions: GRACE-Follow-On, Swarm, and Sentinel-3, are processed. The results indicate that the coordinate accuracy of SLR stations is significantly improved by 21% after applying VCE, while the improvement for Earth pole coordinates can reach 34%. We find that classifying the observations by satellite-station pair in the VCE processing outperforms the satellite-specific or station-specific observations classification. We also investigate the impact of range bias parameterization on the VCE performance. In addition, considering a steep rise in the computational time when using VCE, we evaluate the feasibility of a simplified VCE method in SLR observation processing. The result indicates that the simplified VCE solution can present the same performance as the traditional VCE solution, but the computational efficiency is significantly improved by thirty times.

Description: Atmospheric Precipitable Water Vapor (PWV) information from Global Navigation Satellite Systems (GNSS) has been proven to be of a high value for meteorology due to its distinguished characteristics such as high accuracy, high temporal and spatial resolution, global coverage and 24/7 availability. In this study, real-time PWVs were retrieved from GNSS observations over 66 GNSS stations in USA for a period of 10 years from 2010 to 2020. The IGS ultra-fast orbit products were used to estimate Zenith Total Delay (ZTD), and a global Zenith Hydrostatic Delay (ZHD) model and a global weighted mean temperature (T〈sub〉m〈/sub〉) model, both of which do not require meteorological parameters (surface pressure or surface temperature), were used to calculated ZHD and T〈sub〉m〈/sub〉. Then, the PWV together with rainfall data for the period from 2010 to 2018 were used to construct a simulated real-time rainfall forecast model, which included three components: PWV value, PWV increase, and PWV maximum increase rate. Finally, the accuracy of the model was evaluated using data for the period from 2019 to 2020 and compared with a model that was constructed based on the high-accuracy PWV derived from GNSS. The results showed that the accuracy of the real-time model is consistent with that of the high-accuracy model, and both of them can achieve good forecast effect.

Description: GSeisRT software developed by Wuhan University is dedicated to real-time GNSS data processing, and it is free and open for the science community under mutual agreement. GSeisRT consists of a server end and a client end. GSeisRT server can use a regional GNSS network to estimate satellite clocks and phase biases independently or access IGS (International GNSS Service) real-time clock and orbit products to estimate satellite phase biases, which is more flexible and compatible with different data scenarios. Then GSeisRT client can realize multi-GNSS (GPS/GLONASS/GALILEO/QZSS/BeiDou) precise point positioning with ambiguity resolution (PPP-AR) and achieve centimeter-level to sub-centimeter-level precision in real time based on the products of GSeisRT server or IGS real-time service. The acquisition of real-time position can be applied to scenarios such as earthquake displacement monitoring, which is necessary and socially significant. In addition, GSeisRT will provide a lite version of the client with a user interface that can estimate and display the ionospheric delay of multi-GNSS satellites in real time. Considering user privacy and data security, GSeisRT could not only support the processing of public data streams but also be deployed in a private operating environment to process proprietary data streams, then users' data security and autonomy can be guaranteed.

Description: China’s first interplanetary mission to Mars, Tianwen-1, consists of an obiter, a lander and a rover (Zhurong). It was lunched on 23 July 2020, entered Mars obit on 10 February 2021 and landed on Mars on 15 May 2021. This paper will overview the main scientific objectives and the major research advances of Tianwen-1, in particular from the Zhurong rover landed at the southern Utopia Planitia. The Zhurong rover has performed in-situ investigations of the geomorphological features, surface and subsurface geology, magnetic field and environments along its traverse to the south of the landing area. The short-wave infrared spectral data showed the presence of hydrated sulfate/silica materials on the Amazonian terrain at the landing site. These hydrated minerals and associated duricrust suggest that a more active Amazonian hydrosphere for Mars than previously thought. A detailed subsurface image constructed from the rover radar data shows a ~70-m-thick, multi-layered structure below a less than 10-m-thick regolith. The structural layering suggests the occurrence of episodic water-involved resurfacing in Utopia Planitia during the Late Hesperian to Amazonian epochs, although there is no direct evidence for the presence of liquid water today in the upper 80 m. The Zhurong rover also made the first ground magnetic vector measurements at 16 sites along the ~1-km track. This in-situ observation reveals an extremely weak magnetic field, much smaller than that was predicted from the orbit, in contrast to the large crustal magnetic field measured by InSight in Elysium Planitia.

Description: Precipitation change is critical for the Three-River Headwaters (TRH) region, which serves downstream communities in East Asia. The spring (March-April-May) precipitation over the TRH region shows an increasing trend from 1979 to 2018, as revealed by the China gridded precipitation product (CN05.1). However, the physical processes responsible for this precipitation change are still unclear. This study investigated the characteristics of spring precipitation and water budget over the TRH region using the ERA5 global reanalysis and the Weather Research and Forecast (WRF) model. The WRF version employed in this study includes online calculations of the atmospheric water budget and an ET-tagging procedure to trace evapotranspired water in the atmosphere. Both ERA5 and WRF reproduce the spring precipitation increase. Moreover, WRFD02 (3 km) reduces the wet bias by around 60% and 77% compared to WRFD01 (9 km) and ERA5 (30 km). Both ERA5 and WRF demonstrate that the increase of spring precipitation is dominated by moisture convergence, especially the atmospheric water fluxes from the southern boundary. The enhanced moisture inflow is sustained by enhanced mass flux while the enhanced moisture outflow is sustained by increased moisture. The ET-tagging results further demonstrate the weakened precipitation recycling process because of the significant increase of precipitation produced by external moisture. Compared to ERA5, the reduced wet bias with WRF is attributed to a better spatial resolution of orographic barrier effects, which reduce the southerly water fluxes. The results highlight the potential of regional climate downscaling to better represent the atmospheric water budget in complex terrain.

Description: Due to high nonlinearity, the accurate prediction of the Antarctic circumpolar current (ACC) transport is still challenging. Using an eddy-permitting ocean model and the conditional nonlinear optimal perturbation approach, the predictability related to the ACC transport sudden shift through the Drake Passage (DP) is investigated by exploring the optimal precursor (OPR) and the optimally growing initial error (OGIE). The sudden shift in ACC transport is defined as a fluctuation exceeding two standard deviations (~16 Sv; 1 Sv = 10〈sup〉6〈/sup〉 m〈sup〉3 〈/sup〉s〈sup〉–1〈/sup〉) within 30 days. The OPRs and OGIEs for four different cases exhibit similar spatial patterns and primarily exist in the middle DP (58°S–62°S, 72°W–64°W) at the depth of 1000–3000 m. It implies that vigorous ACC variations are sensitive to the initial perturbations there. Moreover, the OPRs and OGIEs both undergo significant evolutions and trigger anomalous dipolar eddy-like circulations, although the signals evolving from the OPRs are stronger. Further investigation suggests density components determine such perturbation evolution processes and baroclinic instability plays a critical role. Observing system simulation experiments suggest that improving initial condition qualities in the sensitive area near the middle DP (especially in the deep layers) can effectively enhance the ACC transport prediction skills. By revealing spatial structures and growth mechanisms of the OPRs and OGIEs related to the short-term ACC transport prediction, this study highlights the importance of deep perturbations near the middle DP and further provides scientific guidance on designing observing networks in the DP.

Description: Two identical high-sensitivity triaxial Rover fluxgate magnetometers (RoMag) are mounted on Zhurong Rover to detect the surface magnetic field on Mars. Although a rover magnetic compensation procedure was conducted to remove the magnetic interferences pre-launch [Du et al., 2020], due to the different state of the payloads and electric power system such as the solar panel, an along-track calibration of the magnetometer is necessary to obtain a more accurate Martian magnetic field. With efforts from the Zhurong Rover engineering team, two methods, mast yaw rotations, and Rover yaw rotations were utilized separately to determine the Martian horizontal magnetic components. Results show that the Martian horizontal magnetic components determined by the two methods are in good agreement, with the root mean square deviation ~ 2.0 nT. The vertical component was also constrained through the pitch movements of the mast when assuming the interferences field distributes like a dipole field. A linear correlation between magnetic field measurements and the solar array currents was derived to calibrate the body field during the regular exploration. We conclude that more accurate measurements could be made when applying the calibration results in the magnetic survey on the surface of Mars.

Description: Gravitational field is a mysterious physical phenomenon that has attracted much attention of scientists around the world. In physical geodesy, the gravitational field is mathematically represented by its potential which is typically described by spherical harmonics and each potential value corresponds to an equipotential surface. From this perspective, the gravitational field is deemed to be a host of equipotential surfaces. The geometrical features of the equipotential surfaces are of great importance for in-depth understanding of the structure of gravitational field, which is currently the key motivation of numerous theoretical studies of gravitational field (e.g., probing into the gravitational field’s high-order structure via gravitational tensors, gravitational derivatives, etc).Complement to the equipotential surfaces, plumb lines provide us a different perspective to view the gravitational field. In our previous research, we reformulate the gravitational field in terms of a potential flow; the gravitational vector field is mapped onto a potential-flow vector field, in which the plumb line and the stream line are equivalent to each other (Yin & Sneeuw, 2021). In this study, we further investigate the geometrical feature of a gravitational plumb line by utilizing the fundamental equations of the potential flow. Specifically, we formulate the Lagarangian derivatives of a potential-flow stream line (equivalent to a gravitational plumb line) to arbitrary orders and then derive the curvature and the torsion. The two geometrical quantities, though expressed from gradient tensors, are independent of coordinate systems owing to their scalar form. We expect them to have a good practical application in exploration geophysics.