Description: Over the past decades, human-induced climate change has led to a widespread wetting and warming of the Tibetan Plateau (TP), affecting both ecosystems and the carbon cycling therein. Whether the previously observed climate changes stimulate carbon uptake via enhanced photosynthesis or carbon loss via enhanced soil respiration remains unclear. Here we present 14 years of observations of carbon fluxes, meteorological variables and remotely sensed plant cover estimations from a central Tibetan alpine steppe ecosystem at Nam Co, the third largest lake on the TP. Using modified Mann-Kendall trend tests, we found a significant increasing daily net carbon uptake of 0.5 g C m−2 decade−1, which can be explained by a widespread greening at the southern shore of lake Nam Co. The Plateau-wide changes in temperature and precipitation are locally expressed as an increasing diurnal temperature range during winter, higher water availability during spring, higher cloud cover during early summer and less water availability during late summer. While these changes differ over the course of the year, they tend to stimulate plant growth more than microbial respiration, leading to an increased carbon uptake during all seasons. This study indicates that during the 14 years study period, a higher amplitude in winter temperatures and an earlier summer monsoon promote carbon uptake in a central Tibetan alpine steppe ecosystem.

Description: The Tibetan alpine steppe ecosystem covers an area of roughly 800 000 km2 and contains up to 3.3 % soil organic carbon in the uppermost 30 cm, summing up to 1.93 Pg C for the Tibet Autonomous Region only (472 037 km2). With temperatures rising 2 to 3 times faster than the global average, these carbon stocks are at risk of loss due to enhanced soil respiration. The remote location and the harsh environmental conditions on the Tibetan Plateau (TP) make it challenging to derive accurate data on the ecosystem–atmosphere exchange of carbon dioxide (CO2) and water vapor (H2O). Here, we provide the first multiyear data set of CO2 and H2O fluxes from the central Tibetan alpine steppe ecosystem, measured in situ using the eddy covariance technique. The calculated fluxes were rigorously quality checked and carefully corrected for a drift in concentration measurements. The gas analyzer self-heating effect during cold conditions was evaluated using the standard correction procedure and newly revised formulations (Burba et al., 2008; Frank and Massman, 2020). A wind field analysis was conducted to identify influences of adjacent buildings on the turbulence regime and to exclude the disturbed fluxes from subsequent computations. The presented CO2 fluxes were additionally gap filled using a standardized approach. The very low net carbon uptake across the 15-year data set highlights the special vulnerability of the Tibetan alpine steppe ecosystem to become a source of CO2 due to global warming. The data are freely available at https://doi.org/10.5281/zenodo.3733202 (Nieberding et al., 2020a) and https://doi.org/10.11888/Meteoro.tpdc.270333 (Nieberding et al., 2020b) and may help us to better understand the role of the Tibetan alpine steppe in the global carbon–climate feedback.

Description: A statistical study of ion upflow and field-aligned currents (FACs) has been performed in the topside ionosphere of both hemispheres for magnetic quiet and disturbed times by using DMSP satellite observations from 2010–2013. Distributions in MLT/MLat reveal that ion upflow occurrence shows a dawn-dusk asymmetry distribution that matches well with the Region 1 FACs. In addition, there are highest occurrence regions near noon and within the midnight auroral disturbance area, corresponding to dayside cusp and nightside auroral disturbance regions, respectively. Both the ion upflow occurrence and FAC regions expand equatorward to a wider area during disturbed times.

Description: Snow albedo is an essential factor in the land surface energy balance and the water cycle. It is usually parameterized as functions of snow-related variables in land surface models (LSMs). However, comparing with albedo schemes in the CLM and Noah-MP LSMs, the default snow albedo scheme in the widely used Noah LSM shows evident drawbacks in land-atmosphere interactions simulations during an extreme snow process on the complex topographic Tibetan Plateau (TP). We firstly demonstrate that the improved Noah snow albedo scheme includes MODIS albedo products and explicit considers snow depth as an additional factor. It performs well in relation to near-surface meteorological elements estimates during an extreme snow process. Then, we comprehensively evaluate the performance of the improved snow albedo scheme in WRF coupled with Noah LSM in simulating the additional eight heavy snow events on the TP against in-situ observations, MODIS albedo and IMS snow cover products. It reveals that the improved snow albedo scheme significantly outperforms the default Noah scheme in relation to air temperature, albedo and sensible heat flux estimates, by alleviating cold bias estimates, albedo overestimates and sensible heat flux underestimates, respectively. This in turn contributes to more accurate reproductions of snow cover. The averaged RMSE relative reductions (and relative increase in correlation coefficients) for air temperature, albedo, sensible heat flux and snow depth reach 27% (5%), 32% (69%), 13% (17%) and 21% (108%) respectively. These results demonstrate the strong potential of the improved snow albedo parameterization scheme for heavy snow events simulations on the TP.

Description: Groundwater storage anomaly (GWSA) can be estimated at larger scales from Gravity Recovery and Climate Experiment (GRACE), or at small scales from groundwater-level (GWL) observations, but hampered by leakage errors and lacking of reliable aquifer storage coefficient (Sc), respectively. Here, we developed a coordinated forward modeling (CoFM) to reconcile each other by reducing spatial mismatch between GRACE-based and in-situ-based spherical harmonics through iteratively updating the Sc, with a hypothetical experiment and a case study in the North China Plain (NCP). The results demonstrate that CoFM, independent of reliable Sc, can confidently estimate GWSA trend at 0.5° grid scale when GWSA is dominated by GWL in GRACE-like hypothetical experiments. Besides, CoFM can reveal the larger (2.4 times) groundwater depletion rate in the piedmont of NCP relative to that in the east-central sub-plain, with updated Sc comparable to that from 34 pumping test data. This study highlights a practical solution for improved GWSA estimates through reconciling GRACE and in-situ data.

Description: The Tibetan Plateau (TP), the Iranian Plateau (IP), and the Mongolian Plateau (MP) belong to High Asia, their synergy thermal forcing plays a crucial role in Asian monsoon systems and downstream climate. Therefore, to better understand the turbulent heat exchange between land and air and its climatic effects over the three Plateaus, the applicability of the sensible heat flux (SH), the dominant component of the surface heat source and one of the essential factors affecting the Asian monsoon, should be evaluated. In this study, six widely used reanalysis datasets (ERA5, ERA5-Land, CFSR, JRA55, MERRA2, GLDAS) are selected and the monthly reanalyzed SH is assessed against the in-situ observations. The statistical results show that ERA5-Land and JRA55 perform better with low bias and root mean square error as well as a relatively high correlation coefficient, while CFSR has good consistency with the observations under the alpine meadow. In addition, spatial patterns of annual SH among the six datasets are compared. Except for JRA55 and CFSR, other datasets reveal that SH is higher over the northern and western TP than the eastern, a higher SH occurs over the southern MP, and a homogeneous pattern presents over the IP. In general, ERA5-Land has reliable performance, which is suggested to be used in the analysis and simulation of the connection between SH over High Asia and Asian monsoon variability.

Description: Aims. For decades now, researchers have been looking for a way to tie the kinematic and dynamic reference frames. Certain worldwide organizations have looked to using co-location in space, combining various techniques. Given the long list of possible applications of the Global Navigation Satellite System (GNSS), it is worthwhile investigating the connection between the most accurate and stable International Celestial Reference Frame (ICRF) and the Earth-centered Celestial Inertial reference frame (ECI) used in GNSS data processing. Methods. We simulated phase-referencing observations of GNSS satellites and nearby radio source calibrators to realize the connection between the two celestial reference frames. We designed two schemes for observation plans. One scheme is to select the satellite target when it can be observed by the greatest number of stations in order to obtain high-precision positioning. During each scan, we employ four regional networks to simultaneously track four chosen satellites. The alternative scheme is to observe satellite orbits of as many satellites as possible on different daily observations. In addition, to test the two schemes, we used Monte Carlo methods to generate 1000 groups of random errors in the simulation. Results. Finally, we estimate the right ascension and declination offsets (∆α, ∆δ) of GNSS satellites in the ICRF, and then derive frame tie parameters based on those results: three global rotation angles (A1, A2, A3). The celestial angular offset results assessed from the former scheme show that this scheme leads to high precision of namely 1 mas, while the parameters of the frame tie determined from the second scheme can achieve an improved precision of better than 1.3 µas.

Description: Large earthquakes are usually followed by decaying aftershocks. In some cases, a large earthquake is followed by even larger earthquakes. To estimate seismic hazards of earthquake sequence, it’s necessary to make sure whether the mainshock has already happened. There is no effective method yet to distinguish foreshocks from aftershocks. However, statistical analysis may tell us some useful clues. Since 2010, 35 MS ≥ 6.0 earthquakes have occurred in Chinese mainland, of which 6 MS ≥ 6.0 earthquakes have foreshocks with magnitude larger than 5. Based on foreshock traffic light system (Gulia and Wiemer, 2019), b-positive (van der Elst, 2021) and envelope method (Lippiello et al., 2021), we explored the temporal characteristics of b-value in the Gutenberg–Richter law and the envelope of foreshocks with MS ≥ 5.0 by using catalog and waveform of 6 MS ≥ 6.0 earthquake sequences and tried to propose a real-time foreshock classification method. The preliminary studies show that the temporal variation and the envelope may be useful to recognize foreshocks. For example, the evolution of foreshock sequence of 2019 Yangbi MS 6.4 earthquake can be divided into 3 periods. In the first and second period, the foreshock sequence varied like common aftershock sequence, but the seismicity rate unusually accelerated preceding the mainshock. And the study of waveform of MS ≥ 5.0 foreshocks before Yangbi MS 6.4 earthquake shows that the envelope presents an atypical sawtooth profile and the corresponding β-value is smaller than 0.4 which is much lower than the normal value.

Description: Mass redistribution of the atmosphere, oceans, and terrestrial water storage generates crustal displacements which can be predicted by environmental loading models and observed by the Global Positioning System (GPS). In this paper, daily height time series of 235 GPS stations derived from a homogeneously reprocessed Crustal Movement Observation Network of China (CMONOC) and corresponding loading displacements predicted by the Deutsche GeoForschungsZentrum (GFZ) are compared to assess the effects of loading corrections on the nonlinear variations of GPS time series. Results show that the average root mean square (RMS) of vertical displacements due to atmospheric, nontidal oceanic, hydrological, and their combined effects are 3.2, 0.6, 2.7, and 4.0 mm, respectively. Vertical annual signals of loading and GPS are consistent in amplitude but different in phase systematically. The average correlation coefficient between loading and GPS height time series is 0.6. RMS of the GPS height time series are reduced by 20% on average. Moreover, an investigation of 208 CMONOC stations with observing time spans of ~4.6 years shows that environmental loading corrections lead to an overestimation of the GPS velocity uncertainty by about 1.4 times on average. Nevertheless, by using a common mode component filter through principal component analysis, the dilution of velocity precision due to environmental loading corrections can be compensated.

Description: During February 2023, a total of 32 individual distributed acoustic sensing (DAS) systems acted jointly as a global seismic monitoring network. The aim of this Global DAS Month campaign was to coordinate a diverse network of organizations, instruments, and file formats to gain knowledge and move toward the next generation of earthquake monitoring networks. During this campaign, 156 earthquakes of magnitude 5 or larger were reported by the U.S. Geological Survey and contributors shared data for 60 min after each event’s origin time. Participating systems represent a variety of manufacturers, a range of recording parameters, and varying cable emplacement settings (e.g., shallow burial, borehole, subaqueous, and dark fiber). Monitored cable lengths vary between 152 and 120,129 m, with channel spacing between 1 and 49 m. The data has a total size of 6.8 TB, and are available for free download. Organizing and executing the Global DAS Month has produced a unique dataset for further exploration and highlighted areas of further development for the seismological community to address.