<p>GRACE observations revealed that rapid mass loss in the West Antarctic Ice Sheet (WAIS) abruptly paused in 2015, followed by a much lower rate of mass loss ( 21.3&#177;5.7 Gt&#8231;yr<sup>-1</sup>) until the decommissioning of GRACE in 2017. The critical 1-year GRACE inter-mission data gap raises the question of whether the reduced mass loss rate persists. The Swarm gravimetry data, which have a lower resolution, show good agreement with GRACE/GRACE-FO observations during the overlapping period, i.e. high correlation (0.78) and consistent trend estimates. Swarm data efficiently bridge the GRACE/GRACE-FO data gap and reveal that WAIS has returned to the rapid mass loss state ( &#160;161.5&#177;48.4&#160; Gt&#8231;yr<sup>-1</sup>) that prevailed prior to 2015 during the GRACE inter-mission data gap. The changes in precipitation patterns, driven by the climate cycles, further explain and confirm the dramatic shifts in the WAIS mass loss regime implied by the Swarm observations.</p>
Abstract Previous studies suggested that a weakened Gulf Stream may have contributed to the Younger Dryas (YD) cold event, however the physical mechanism of the Gulf Stream weakening is unclear. Here, we utilize paired Mg/Ca and δ 18 O measurements of planktonic foraminifera from the northerly and southerly marine sediments in the middle Atlantic Ocean to reconstruct past changes in tropical surface ocean temperature and salinity over the past 14 ka. We demonstrate that during the YD the sea surface temperature (SST) at the northerly core (0°16′S) was up to 2.1–2.7°C cooler whereas the SST at the southerly core (14°53′S) was up to 0.8–1.6°C warmer than modern temperatures. The δ 18 O, salinity values, accumulation rates, and other hydrological elements at the northerly core all exhibit opposite patterns with those at the southerly core. The antiphased variations of the paleohydrological elements at the northerly and southerly cores suggest that the Atlantic Equatorial Currents shifted southward significantly from its present‐day position during the YD, possibly resulting in a decrease of the Gulf Stream, in turn a cool Northern Hemisphere.
The high spatial-temporal variability of soil moisture necessitates monitoring at a high resolution in order to improve our understanding of Earth system processes. Current large-scale soil moistures inferred from the microwave satellites have limited spatial resolution, typically in the range of tens of kilometers. Recent studies have revealed that synthetic aperture radar (SAR) backscatter exhibits qualitative relationships with soil moisture, suggesting the potential for large-scale high-resolution mapping of soil moisture. Here, we proposed a method for directly estimating soil moisture content based on the Advanced Integral Equation Model (AIEM) and Mironov dielectric model. The approach involves establishing a series of semi-empirical models, independent of preceding surface roughness determination, using two Envisat ASAR alternating polarization (AP) model precision products. We generate a time series of high-resolution soil moisture using Envisat ASAR AP data acquired from 2004 to 2011, with an uncertainty of approximately 0.05 m 3 /m 3 . Our soil moisture retrievals demonstrate very good agreement with European Space Agency (ESA) Climate Change Initiative (CCI) soil moisture products and the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 re-analysis hourly products, even in the absence of synchronous ground measurements. Furthermore, our study reveals good temporal coherence between drought and heavy rainfall events, and SAR-derived soil moisture, which suggests a potential to capture heavy rainfall and drought events. We conclude that SAR-derived soil moisture is a more direct and efficient method in quantifying soil moisture at a high spatial resolution, making it more suitable for watershed scale hydrological studies.
Abstract. Although the knowledge of the gravity of the Earth has improved considerably with CHAMP, GRACE and GOCE satellite missions, the geophysical community has identified the need for the continued monitoring of the time-variable component with the purpose of estimating the hydrological and glaciological yearly cycles and long-term trends. Currently, the GRACE-FO satellites are the sole dedicated provider of these data, while previously the GRACE mission fulfilled that role for 15 years. There is a data gap spanning from July 2017 to May 2018 between the end of the GRACE mission and start the of GRACE-FO, while the Swarm satellites have collected gravimetric data with their GPS receivers since December 2013. We present high-quality gravity field models from Swarm data that constitute an alternative and independent source of gravimetric data, which could help alleviate the consequences of the 10-month gap between GRACE and GRACE-FO, as well as the short gaps in the existing GRACE and GRACE-FO monthly time series. The geodetic community has realized that the combination of different gravity field solutions is superior to any individual model and set up a Combination Service of Time-variable Gravity Fields (COST-G) under the umbrella of the International Gravity Field Service (IGFS), part of the International Association of Geodesy (IAG). We exploit this fact and deliver to the highest quality monthly-independent gravity field models, resulting from the combination of four different gravity field estimation approaches. All solutions are unconstrained and estimated independently from month to month. We tested the added value of including Kinematic Baselines (KBs) in our estimation of Gravity Field Models (GFMs) and conclude that there is no significant improvement. The non-gravitational accelerations measured by the accelerometer on-board Swarm-C were also included in our processing to determine if this would improve the quality of the GFMs, but observed that is only the case when the amplitude of the non-gravitational accelerations is higher than during the current quiet period in solar activity. Using GRACE data for comparison, we demonstrate that the geophysical signal in the Swarm gravity field models is largely restricted to Spherical Harmonic degrees below 12. A 750 km smoothing radius is suitable to retrieve the temporal variations of Earth’s gravity field over land areas since mid-2015 with roughly 4 cm Equivalent Water Height (EWH) agreement with respect to a GRACE-derived parametric model. Over ocean areas, we illustrate that a more intense smoothing with 3000 km radius is necessary to resolve large scale gravity variations, which agree with the aforementioned parametric model under 2 cm EWH, while at these spatial scales the model represents variations with amplitudes between 2 and 3.5 cm EWH. The agreement with GRACE and GRACE-FO over nine selected large basins under analyses is 1.19 cm, 0.60 cm/year and 0.75 in terms of temporal mean, trend and correlation coefficient, respectively.
Abstract The growth mechanism of the Tibetan Plateau, postulated by a number of hypotheses, remains under intense debate. Our analysis of recent satellite‐based gravity model reveals that Tibetan lithosphere has been decoupled and folded. It is further evidenced by the existence of crustal melts and channel flow that have been observed by seismic and magnetotelluric explorations. Based on 3D geodynamic simulations, we elucidate the exact buckling structures in the upper crust and lithospheric mantle: at mixed wavelengths between ∼240 and ∼400 km, the lower crustal viscosity is smaller than ∼10 19 Pa·s, implicating weak lower crustal flow beneath the Plateau. This mixed wavelength is consistent with the result of our inverse gravity modeling. Our results facilitate a new plausible hypothesis that the decoupled lithospheric folding mechanism can explain the growth mechanism of the anomalously thick and wide Tibetan Plateau by conflating our idea and contemporary hypothesized scientific findings.
While the droplet impact dynamics on stationary superhydrophobic surfaces has been extensively studied, the dynamic behaviors of impact droplets on moving superhydrophobic surfaces have received less attention. Here, we report the droplet impact dynamics on a moving superhydrophobic surface. We show that compared to the stationary surface, the moving superhydrophobic surface breaks the symmetry in both droplet spreading and retracting. Specifically, the shear force exerted by the moving surface acting on the impact droplet enlarges the maximum spreading in the moving direction, and thus, the droplet contact time is reduced. The contact time of impact droplets was examined thoroughly under the effects of the droplet impact (normal) and the wall moving (tangential) Weber numbers. We provide a scaling analysis to explain how the contact time depends on the normal and tangential Weber numbers. Our experimental investigation and theoretical analysis provide insight into the droplet impact dynamics on moving superhydrophobic surfaces.
Observing global terrestrial water storage changes (TWSCs) from (inter-)seasonal to (multi-)decade time-scales is very important to understand the Earth as a system under natural and anthropogenic climate change. The primary goal of the Gravity Recovery And Climate Experiment (GRACE) satellite mission (2002–2017) and its follow-on mission (GRACE-FO, 2018–onward) is to provide time-variable gravity fields, which can be converted to TWSCs with ∼ 300 km spatial resolution; however, the one year data gap between GRACE and GRACE-FO represents a critical discontinuity, which cannot be replaced by alternative data or model with the same quality. To fill this gap, we applied time-variable gravity fields (2013–onward) from the Swarm Earth explorer mission with low spatial resolution of ∼ 1500 km. A novel iterative reconstruction approach was formulated based on the independent component analysis (ICA) that combines the GRACE and Swarm fields. The reconstructed TWSC fields of 2003–2018 were compared with a commonly applied reconstruction technique and GRACE-FO TWSC fields, whose results indicate a considerable noise reduction and long-term consistency improvement of the iterative ICA reconstruction technique. They were applied to evaluate trends and seasonal mass changes (of 2003–2018) within the world’s 33 largest river basins.
The delays of radio signals transmitted by global navigation satellite system (GNSS) satellites and induced by neutral atmosphere, which are usually represented by zenith tropospheric delay (ZTD), are required as critical information both for GNSS positioning and navigation and GNSS meteorology. Establishing a stable and reliable ZTD model is one of the interests in GNSS research. In this study, we proposed a regional ZTD model that makes full use of the ZTD calculated from regional GNSS data and the corresponding ZTD estimated by global pressure and temperature 3 (GPT3) model, adopting the artificial neutral network (ANN) to construct the correlation between ZTD derived from GPT3 and GNSS observations. The experiments in Hong Kong using Satellite Positioning Reference Station Network (SatRet) were conducted and three statistical values, i.e., bias, root mean square error (RMSE), and compound relative error (CRE) were adopted for our comparisons. Numerical results showed that the proposed model outperformed the parameter ZTD model (Saastamoinen model) and the empirical ZTD model (GPT3 model), with an approximately 56%/52% and 52%/37% RMSE improvement in the internal and external accuracy verification, respectively. Moreover, the proposed method effectively improved the systematic deviation of GPT3 model and achieved better ZTD estimation in both rainy and rainless conditions.
'/%3e%3cuse xlink:href='%23a' transform='rotate(23.036 .093 25.536)'/%3e%3cuse xlink:href='%23a' transform='rotate(45.87 1.273 16.18)'/%3e%3cuse xlink:href='%23a' transform='rotate(69.945 .996 12.078)'/%3e%3cuse xlink:href='%23a' transform='rotate(20.66 -19.689 31.932)'/%3e%3c/svg%3e)
