ALBERT

Beschreibung: The Bransfield Basin is a young (∼4 Ma) back-arc basin related to the remnant subduction of the Phoenix Plate that once existed along the entire Pacific margin of the Antarctic Peninsula. Based on a recently deployed amphibious seismic network, we use ambient noise tomography to obtain the S-wave velocity structure in the Central Bransfield Basin (CBB). Combining with the stress-field inverted from focal mechanisms, our images reveal that the CBB suffers a significant extension in the northwest-southeast direction. The extension is strongest in the northeastern CBB with associated mantle exhumation and weakens to the southwest with decoupled deformations between the upper crust and lithospheric mantle. Such an along-strike variation of extension can be explained by slab window formation and forearc rotation, which are associated with the Phoenix Plate detachment during the ridge–trench collisions at the southwest of the Hero Fracture Zone.

Beschreibung: The Tibetan Plateau (TP) contains the largest amount of snow outside the polar regions and is the source of many major rivers in Asia. An accurate long-range (i.e. seasonal) meteorological forecast is of great importance for this region. SEAS5 provides global long-range meteorological forecasts including over the TP. However, SEAS5 assimilating IMS snow data only below 1500m altitude may affect its forecast skill for this region. To investigate the impacts of snow assimilation and dynamical downscaling on temperature and precipitation forecasts, twin ensemble reforecasts initialized with and without snow assimilation above 1500m altitude over the TP for spring and summer 2018 are conducted, and are further downscaled by WRF. The results show that the reforecasts without snow assimilation underestimate daily temperature over the TP, which are consistently improved after snow assimilation in the spring. However, the positive biases between the precipitation reforecasts and satellite observations worsen in the east TP. Compared to the experiment without snow assimilation, the snow assimilation experiment significantly increases temperature and precipitation for the east TP and around the longitude 95ºE. The higher temperature after snow assimilation, in particular the cold bias reduction after initialization, can be attributed to the effects of a more realistic, decreased snowpack, providing favourable conditions for generating more precipitation. Overall, snow assimilation can improve seasonal forecasts through the interaction between land surface and atmosphere. Moreover, the convection-permitting dynamical downscaling further decreases the biases of seasonal forecasts, as more accurate physical processes can be resolved at this scale.

Beschreibung: The Tibetan Plateau (TP) is called the “Third Pole”, which contains the largest amount of snow and glaciers outside the polar regions. As an important and variable feature of the land surface, snow coverage on the TP has great impacts on regional climates. However, the commonly used ERA5 reanalysis in dynamic downscaling largely overestimates the snow depth for the TP. To improve the representation of snow cover in ERA5, a new ERA5 driven downscaling dataset (High Asia Refined analysis version 2, HAR v2) was generated by the Weather Research and Forecasting (WRF) model with the bias-corrected snow depth. This study aims to identify and better understand the impacts of bias-corrected initial snow conditions on dynamical downscaling simulations, by comparing the HAR v2 with a 5-year ERA5 forced WRF simulation without bias correction of initial snow depth (referred to as WRF_ERA5). Results show that the bias correction significantly improves the simulation of air temperature at 2 m (T2), with regional mean cold bias reduced by 0.2-2.4°C, but no significant improvement in precipitation simulation is found. Further comparative analysis reveals that higher snow depth in WRF_ERA5 leads to T2, mean daily precipitation, summer extreme precipitation, and contributions of convective precipitation to summer mean daily precipitation decrease by 0-4°C, 0-60%, 0-40%, and 0-10%, respectively, in most areas of the TP. Overall, this study confirms the importance of snow cover for the TP climate.

Beschreibung: Radiative transfer models are widely applied in climate models to simulate vertical temperature perturbations caused by external radiative forcings. A large part of radiative transfer models is the infrared gaseous spectral transmittance scheme, which quantifies the longwave atmospheric absorption. A rapid infrared gaseous spectral transmittance scheme, called the optimized alternate mapping correlated K-distribution model (OMCKD), is introduced in this paper. To improve the accuracy of our scheme without increasing pseudo-monochromatic calculations, we introduce the optimal iteration method to automatically tune the equivalent absorption coefficients in the cumulative probability subspace. In addition, a new expression weighted by black-body radiation is introduced to calculate the equivalent absorption coefficient. The OMCKD simulates heating rate and radiation flux with errors of less than 0.12 K d-1 and 0.35 W m-2, respectively, below stratopause for standard atmospheric profiles. The OMCKD is also evaluated and compared with the rapid radiative transfer model for general circulation models (RRTMG) in realistic atmospheric profiles. We found that OMCKD can accurately produce heating rates and generally captures radiative forcings associated with large perturbations to the concentrations of main greenhouse gases. Furthermore, the number of pseudo-monochromatic calculations in OMCKD is 11.4% less than that in RRTMG, which indicates less computational cost.

Beschreibung: How the Tibetan Plateau grew farther north from the India-Asia collision boundary has profound implications on the mechanics of continental deformation and accretion. The Hoh-Xil Basin in the northern Tibetan Plateau was once a foreland basin adjacent to the high-elevation proto-plateau until the Early Miocene and was rapidly uplifted since then. Here we collected 363 broadband seismic stations operated from 2007 to 2020, including 226 stations from five linear arrays mainly deployed in two north-south profiles across the core of the Hoh-Xil Basin with an average interstation distance of ~15 km. Based on Rayleigh wave signals extracted from ambient noise cross-correlations, we obtained more than 13000 dispersion curves and constructed a 3-D S-wave velocity (V〈sub〉S〈/sub〉) model using measurements in 6–65 s periods by the direct inversion method. Our model shows significant lateral variations of V〈sub〉S〈/sub〉 in the crust and uppermost mantle from the southern to the northern Tibetan Plateau, which should reflect different melt fractions according to the theoretical seismic velocity-melt fraction relationship. We observe widespread partially molten crust in the northern Tibetan Plateau but only isolated pockets in the south manifested as low-V〈sub〉S〈/sub〉 anomalies in the middle-lower crust. The spatial correlation of these low-V〈sub〉S〈/sub〉 anomalies with strong uppermost mantle low-V〈sub〉S〈/sub〉 anomalies and young crust-/mantle-derived magmatism in the Hoh-Xil Basin suggests that the plateau grew through magmatic intrusion and accretion induced by the asthenospheric upwelling due to lithospheric mantle removal. These findings lead to implications for the role of delamination-driven magmatism in the continental crust growth in collision orogens.

Beschreibung: Electromagnetic ion cyclotron (EMIC) emissions in the Earth’s inner magnetosphere are a significant driver of relativistic electron precipitation into the atmosphere. The amplitudes of EMIC waves often exceed 1 nT, introducing nonlinear effects not included in the diffusive quasi-linear theory. While the nonlinear effects have been previously studied analytically and by test-particle simulations, most studies focus on parallel-propagating monochromatic waves. Here we present results from test-particle simulations of relativistic electrons interacting with EMIC waves of different amplitudes and wave normal angles. Diffusive and advective components of pitch-angle scattering are analyzed, highlighting the important effect of harmonic resonances on both co-streaming and counter-streaming electrons. Backward-in-time simulations are used to obtain the perturbed phase space density distribution and study the loss-cone electron population. It is demonstrated that the net effect of advective and diffusive motion during one quarter-bounce results in near isotropization of the distribution unless the nonlinear phase-trapping effects become dominant. It is further revealed that particles below the fundamental resonance energy experience not only nonresonant scattering caused by the amplitude modulations but also fractional-resonant scattering stemming from nonlinearly perturbed trajectories. Finally, the simple monochromatic wave model with constant obliquity is replaced with a full-wave model based on finite-difference time-domain simulations initialized with waveforms from spacecraft measurements. Differences between the two models are discussed, assessing the limitations of monochromatic models in the analysis of relativistic electron scattering and precipitation.

Beschreibung: To calibrate airborne gravity gradiometers under development in China, it is urgent to build an airborne gravity gradiometer test site. The site’s selection depends on the preknowledge of high-resolution gravity and gradient structures. The residual terrain modelling (RTM) technique is generally applied to recover the short-scale gravity field signals. Limited by the quality and resolution of density models, RTM terrain generally assumes a constant density. This would involve great errors over areas with large density anomalies, such as volcano areas. This study determines a high-resolution gravity field by combining the long-wavelength signals generated by EGM2008, short-scale signals of RTM with constant density, and gravity signals of density anomalies. The density anomalies are obtained through the equivalent source method (EQS) from sparse terrestrial gravity measurements. Compared to the recovery rate of 54.62% using the classical RTM method, the recovery rate increases to 86.22% after involving density anomalies. With this method, the gravity field signals over Wudalianchi Volcano Field (WVF) on the Earth's surface and at a flight height of 100 m above terrain are investigated. The contribution of each part and their attenuation characters are studied. The 5 km*5 km area around Bijiashan and Wohushan volcanos shows a strong gravity signature which is recommended for the test site. This study gives the location of the airborne gravity gradiometer test site which is an essential step in the instruments’ development. Besides, the method in this study will provide an initial foundation methodology for future data processing over the test site.

Beschreibung: River algal blooms are a global environmental problem that is characterized by complex causes, large impact range and long duration. This problem would be further complicated by the operation of large-scale water transfer projects. Here, we combine the Convergent Cross-Mapping (CCM) and machine learning to reveal the causes and predict the occurrence of algal blooms during a 15-year period in the Han River, the largest tributary of the Yangtze River of China and the freshwater source of the central route of the South-to-North Water Diversion Project (SNWDP), a mega infrastructure of China. We show that the random forest, a machine learning model, outperformed the gradient boosting machine model in predicting algal blooms with a 10-day lead time with the application of resampling methods to tackle imbalanced data (i.e., the algal blooms events are relatively rare compared to the non-blooms). We combine CCM causality analysis and machine learning to elucidate that the water temperature in the Han River (HR) and the water levels in HR and the Yangtze River as the dominant factors affecting algal blooms in HR with consistently high nutrient concentrations. Finally, we suggest that the operation of SNWDP weakens the response of algal blooms to the water level variation in HR, which is one of the key driving factors affecting algal blooms before the operation of SNWDP. This study provides important reference for detecting the causes of river algal blooms and exploring the impact of large-scale water transfer projects on riverine environment.

Beschreibung: As a complementary alternative to GPS, the independent BDS-3 technology is of interest for low-cost space missions due to its global service. In this study, we assess the single-frequency real-time orbit determination accuracy using BDS-3-only and test the orbit determination algorithm with actual fight data of new frequencies of B1C and B2a from the Haiyang-2D satellite of China. Compared with the B1C frequency, the accuracy of the B2a code can be effectively improved due to a 10-time higher chipping rate, which makes the combined measurements of the code and the carrier phase to be better than 2 cm. Additionally, the B2a measurements exhibit good performance in terms of the availability and the antenna hemisphere's coverage, which reduce the differences between vertical and horizontal dilution of precision by 70%. Preliminary orbit determination results show that the solutions of the B2a frequency with three-dimensional position and velocity errors of 32 cm and 0.30 mm/s are achieved, and the position errors in the along-track, cross-track, and radial components are improved by 26.3%, 18.8%, and 18.2%, respectively, compared with the B1C. Overall, the achieved performance of the BDS-3 is expected to meet the needs of onboard position knowledge for many low-cost satellites.

Beschreibung: Numerous hydrologic designs require seasonal design flood (SDF) estimates. In SDF estimation, to obtain more reliable SDFs for different time segments within flood season, some studies have established the bivariate distribution of the annual maximum flood (AMF) discharges and timings (dates of occurrence) with single copulas, which implicitly assumes these two flood characteristics to have a monotonic dependence structure (MDS). However, the rationality of this MDS assumption between the AMF discharges and timings has not been investigated. In this study, within the framework of SDF estimation, a mixed-copula function modelling the nonmonotonic dependence structure (NMDS) between the AMF discharges and timings is proposed to characterize the seasonal variation of discharges from increasing to decreasing, and then SDFs under nonstationary conditions are derived using the equivalent reliability method. Compared to the MDS assumption, the NMDS offers a better performance in modelling observed AMDFs (1951~2019) in the Wujiang River Basin. The results suggest that within the whole flood season from May to October, the NMDS-based SDFs in June-July exceed the annual design flood while those in other months are lower than the annual design flood, which allows reducing the flood control storage capacity to improve floodwater utilization in downstream flood management without increasing flood risk. With influences of changing environments on AMF discharges further explored, the seasonal fluctuation in design floods, initially driven by the temporal unevenness of rainfall, is significantly flattened by regulation of reservoirs. These results would be helpful for downstream flood risk management and floodwater resources utilization.