ALBERT

Description: The impacts of the Arctic stratospheric polar vortex (SPV) changes on wintertime frontogenesis in the northern middle latitudes are analyzed. Both composite analysis and model simulations reveal that the intensity and frequency of frontogenesis over Siberia, the Mediterranean and southern North Atlantic during weak SPV years are significantly larger than those during strong SPV years, while the frontogenesis over the northern parts of the North Atlantic and North Pacific Oceans are weaker and less occur during weak SPV years. These features are more noticeable in middle January and February. The contributions of resultant deformation changes to frontogenesis intensity changes over most regions of the middle latitudes are larger than those of horizontal divergence changes, and the contribution of stretching deformation and shearing deformation is almost the same with each other. The changes in frontogenesis intensity are attributed to changes in tropospheric circulation and temperature gradient associated with SPV changes. Potential vorticity (PV) anomalies in the upper troposphere and lower stratosphere (UTLS) caused by the weakened and shifted SPV towards Siberia lead to tropospheric cyclonic flows, favoring more cold-air mass transport towards mid-latitude Siberia. Meanwhile, more high-PV air towards Siberia results in steeper tropospheric isentropes during weak SPV years. Consequently, both temperature gradient and frontogenesis over Siberia are enhanced. More southward transport of cold-air mass due to the equatorward shift of the polar jet stream induced by weak SPV enhances frontogenesis over the southern North Atlantic.

Description: Integrated processing of high-rate GNSS and accelerometer data can overcome the disadvantage of each individual sensors and to increase the quality of derived co-seismic displacement. However, the contribution of accelerometer is usually underestimated by estimating baseline shifts epoch-wise which in fact happened very rarely. To take full advantage of both sensors, we propose a sliding window based Kalman filter to detect baseline shifts according to the disagreement of GNSS and accelerometer data and to estimate only the detected baseline shifts. The relationship of the window width, minimal detectable baseline shift and the displacement accuracy is investigated. The performance of the proposed approach is demonstrated by datasets collected during Samos Island earthquake (Mw 7.0, 30th October 2020). The results show that the baseline shifts in accelerometers can be precisely detected and estimated according to the very good agreement of the displacement integrated from accelerometer data after applying baseline shift corrections and that estimated from high-rate GNSS. Furthermore, the baseline corrected accelerometer data provides tight and reliable constraints on position and velocity to facilitate correct PPP ambiguity resolution. Thanks to the proposed approach, the complementary of GNSS and accelerometers is fully employed, consequently the co-seismic displacements of the tightly integrated processing are significantly improved.

Description: Radiative transfer is computationally expensive, and it can be so expensive (for instance, in large eddy simulation (LES)) that often it is only possible to solve with the two-stream (one-dimensional) approximation. Here, a numerical method is presented that is fast---potentially as fast as the fluid dynamics solver of LES. Furthermore, it uses not the two-stream approximation, but the fully three-dimensional (3D) radiative transfer equation. This substantial savings in cost and memory is achieved using discontinuous Galerkin (DG) spectral elements for a fast and stable convergence, and hp-adaptive mesh refinement (hp-AMR) to handle sharp gradients near cloud edge, and isotropic (Rayleigh) scattering and anisotropic (Mie) scattering. Numerical examples are presented in two spatial dimensions to allow the calculation of high-resolution solutions as a baseline for comparison. The methods have the potential for use in LES, remote sensing, and other applications.

Description: Recent advances in high-precision real-time applications of the Global Navigation Satellite System (GNSS) have expanded to monitoring and early detection of natural hazards. The real-time Precise Point Positioning with Regional Augmentation (PPP+RA) technique has become increasingly popular for providing rapid and precise ground displacement information due to the availability of real-time precise GNSS satellite orbit and clock products, as well as high-rate observations. In this contribution, we implemented the PPP+RA technique to the Early-Warning and Rapid Impact Assessment with real-time GNSS in the Mediterranean (EWRICA) project for earth surface displacement monitoring. A real-time GNSS precise positioning system is developed for the project. About 80 stations from the RING network in Italy are used to evaluate the performance the system. It's worth noting that more than half of the receivers in the RING network are LEICA receivers that support GPS only. We use GFZ’s real-time precise satellite orbit and clock products for the data processing. The results show that the daily averaged accuracy of PPP+RA is 1.08 cm, 1.15 cm, and 3.20 cm in the east, north, and up directions, respectively, with a Time To First Fix (TTFF) of 1.1 minutes. Additionally, the system provides short-term relative positioning accuracy within 10 minutes, with a precision of a few millimetres, demonstrating its capability to detect even slight ground movements in real-time.

Description: Real-time satellite clock offset is a crucial element for real-time precise point positioning (RT-PPP). However, the elapsed time for undifferenced (UD) multi-global navigation satellite system (GNSS) real-time satellite clock offset estimation at each epoch is increased with the growth of stations, which may fall short of real-time application requirements. Therefore, a rapid estimation method for UD multi-GNSS real-time satellite clock offset is proposed to improve the computation efficiency, in which both the dimension of the normal equation (NEQ) and the number of redundant observations are calculated before adjustment; if these two values are larger than the predefined thresholds, the elevation mask is gradually increased until they are less than the predefined thresholds. Then, the clock offset estimation is conducted; this method is called clock offset estimation using partial observations. Totals of 50, 60, 70 and 80 stations are applied to perform experiments. Compared to clock offset estimation using all observations, the elapsed times of clock offset estimation using partial observations can be reduced from 6.80 to 3.10 s, 7.93 to 2.97 s, 12.04 to 3.14 s for 60, 70 and 80 stations, respectively. By using the proposed method, the elapsed time of the clock offset estimation at each epoch is less than 5 s. The estimated clock offset accuracy for GPS, BDS-3, Galileo and GLONASS satellites are better than 0.04, 0.05, 0.03 and 0.16 ns when using the partial observations to estimate clock offset with 50, 60, 70 and 80 stations, respectively. For the multi-GNSS kinematic PPP using the estimated clock offset from 50, 60, 70 and 80 stations with partial observations, the positioning accuracy at 95% confidence level in the east, north and up direction are better than 2.70, 2.20 and 5.60 cm, respectively.