We investigate the effect of extremely rough bathymetry on energy dissipation and mixing in a coastal region characterized by small-scale seafloor features penetrating a strongly-stratified density interface of comparable vertical scale. Our data from the non-tidal Baltic Sea include shear microstructure measurements and observations from a broadband echosounder, here used to resolve the extreme variability and intermittency of stratified turbulence in the vicinity of obstacles. Scale analysis and acoustic imaging of small-scale turbulent motions suggest that the underlying mixing mechanisms are related to topographic wake eddies and, to a smaller extent, to breaking internal waves near the bathymetric features. Vertical diffusivities exceed those at a nearby reference station with smooth bathymetry by up to two orders of magnitude. Our study emphasizes the importance of rough small-scale (< 1 km) bathymetric features for energy dissipation and vertical turbulent transport in coastal areas shaped by e.g., glacial, tectonic, or volcanic processes.
Vegetation turnover time (τ) is a central ecosystem property to quantify the global vegetation carbon dynamics. However, our understanding of vegetation dynamics is hampered by the lack of long-term observations of the changes in vegetation biomass. Here we challenge the steady state assumption of τ by using annual changes in vegetation biomass that derived from remote-sensing observations. We evaluate the changes in magnitude, spatial patterns, and uncertainties in vegetation carbon turnover times from 1992 to 2016. We found that the forest ecosystem is close to a steady state at global scale, contrasting with the larger differences between τ under steady state and τ under non-steady state at the grid cell level. The observation that terrestrial ecosystems are not in a steady state locally is deemed crucial when studying vegetation dynamics and the potential response of biomass to disturbance and climatic changes.
It is unclear why two-phase fluid flows in porous media develop a series of fluid displacement patterns. This study treats a two-phase flow system as an open thermodynamical system with a two-phase displacement process that follows the principle of the minimum operating power (MOPR). When different constraints are imposed on the system, the pore-scale interfacial dynamic response to this principle varies significantly, and a series of self-regulation mechanisms exist. These new findings not only explain the physical origins of the diverse fluid displacement patterns and interface reconstruction events but also provide new insights into the interface invasion protocol.
Raindrop Size Distributions (RSDs) samples from 17 flight missions though 6 hurricanes collected by Precipitation Imaging Probe (PIP) during National Oceanic and Atmospheric Administration’s hurricane field program in 2020 are used to study gamma fits of the RSDs in hurricanes. The method of moment (MM) is adopted for solving for the three parameters in gamma distribution. The results show that the usage of lower (higher) moments produces large biases for integral rain variables (IRV) of higher (lower) moments. These biases can be alleviated by extracting the best fits from five groups that use increasing higher orders of moments for MM. An intercept (N0)— slope (λ) relation identified from the fitted gamma distributions captures 92% of the variance of the data, where the majority of remaining 8% can be further captured by including the impact of liquid water content (LWC), as shown in the results from a random forest regression model.
Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) is a space-borne instrument dedicated to monitoring high-energy transients, thereinto Terrestrial Gamma-ray Flashes (TGFs) and Terrestrial Electron Beams (TEBs). We propose a TGF/TEB search algorithm, with which 147 bright TGFs and 4 TEBs are identified during an effective observation time of $\sim$ 9 months. We show that, with gamma-ray and charged particle detectors, GECAM can effectively identify and distinguish TGFs and TEBs, and measure their temporal and spectral properties in detail. Moreover, we find an interesting TEB consisting of two pulses with a separation of $\sim$ 150 ms, which is expected to originate from a lightning process near the geomagnetic footprint. We also find that the GECAM TGF’s lightning-association ratio is $\sim$ 80\% in the east Asia region using the GLD360 lightning network, which is significantly higher than previous observations.
Accurate predictions of fluid flow, mass transport, and reaction rates critically impact the efficiency and reliability of subsurface exploration and sustainable use of subsurface resources. Quantitative dynamical sensing and imaging can play a pivotal role in the ability to make such predictions. Geophysical thermoacoustic technology has the potential to provide the aforementioned capabilities since it builds upon the principle that electromagnetic and mechanical wave fields can be coupled through a thermodynamic process. In this letter, we present laboratory experiments featuring the efficacy of thermoacustic imaging in the monitoring of preferential flow of water in porous media. Our laboratory experimental equipment can be readily packaged in a form factor that fits in a borehole, and the use of multiple acoustic transducers—which can be combined with volumetric coding techniques—has the potential to provide quasi-real-time imaging (0.5 Hertz video rate) of regions in close proximity (a few meters) of an open field well.
Floods are often disastrous due to underestimation of the magnitude of rare events. When the occurrence of floods follows a heavy-tailed distribution the chance of extreme events is sizable. However, identifying heavy-tailed flood behavior is challenging because of limited data records and the lack of physical support for currently used indices. We address these issues by deriving a new index of heavy-tailed flood behavior from a physically-based description of streamflow dynamics. The proposed index, which is embodied by the hydrograph recession exponent, enables inferring heavy-tailed flood behavior from daily flow records. We test the index in a large set of case studies across Germany. Results show its ability to identify cases with either heavy- or nonheavy-tailed flood behavior, and to evaluate the tail heaviness. Remarkably, the results are robust also for decreasing the lengths of data records. The new index thus allows for assessing flood hazards from commonly available data.
In the absence of consistent meteorological data on Mars, the morphology of dunes can be employed to study its atmosphere. Specifically, barchan dunes, which form under approximately unimodal winds, are reliable proxies for the dominant wind direction. Here, we characterize near-surface winds on Mars from the morphology of >106 barchans mapped globally on the planet by a convolutional neural network. Barchan migration is predominantly aligned with the global circulation: northerly at mid-latitudes and cyclonic near the north pole, with the addition of an anti-cyclonic north-polar component that likely originates from winds emerging from the ice cap. Locally, migration directions deviate from regional trends in areas with high topographic roughness. Notably, obstacles <100 km such as impact craters are efficient at deflecting surface winds. Our database, which provides insights into planetary-scale aeolian processes on modern-day Mars, can be used to constrain global circulation models to assist with predictions for future missions.
The strength of CO2 fertilisation is a major uncertainty across terrestrial biosphere models (TBMs) and is suggested to be overestimated without a representation of nitrogen (N) limitation. Here, we compare TBM projections with and without coupled C and N cycling over alternative future scenarios (the Shared Socioeconomic Pathways) to examine how representing N cycling influences CO2 fertilisation as well as the effects of a comprehensive group of physical and socioeconomic global change drivers. Because elevated N deposition and N mineralisation (driven by elevated temperature) have stimulated terrestrial C sequestration over the historical period, a TBM without N cycling must exaggerate the strength of CO2 fertilisation to compensate for these unrepresented N processes and to reproduce the historical terrestrial C sink. As a result, it cannot reliably project the future terrestrial C sink, overestimating CO2 fertilisation as CO2 increases faster than N deposition and temperature in future scenarios.
Nutrients associated with internal waves are known to perturb phytoplankton communities in oligotrophic oceans, but details of the relevant processes and mechanisms are unclear. Here we report insights about the impacts of internal waves on the phytoplankton community based on 154-hour time-series of observations in an oligotrophic basin of the South China Sea. We found that the temporal variations of phytoplankton communities in the upper, middle, and lower layers of the euphotic zone differ. We demonstrated that these changes probably resulted from the perturbation caused by internal waves. These results suggest that the structure of the phytoplankton community in oligotrophic oceans is best described by a three-layer system at steady state and that the perturbation caused by internal waves helps to reveal this structure. We believe that the paradigm of this three-layer structure will provide a new theoretical framework for the study of phytoplankton-based biogeochemical processes in oligotrophic oceans.
Sudden stratospheric warmings (SSWs) are large-scale phenomena characterized by dramatic dynamic disruptions in the stratospheric winter polar regions. Previous studies, especially those employing whole atmosphere models, indicate that SSWs have strong impacts on the circulation of the mesosphere lower thermosphere (MLT) and drive a reversal in the mean meridional circulation (MMC) near 90-125 km altitude. However, the robustness of these effects and the roles of SSW-induced changes in global-scale wave activity to drive the reversal have been difficult to observe simultaneously. This work employs horizontal lower thermospheric (~93-106 km altitude) winds near 10S-40N latitude from the Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI) instrument onboard the Ionospheric Connection Explorer (ICON) to present observational evidence of a prominent MLT MMC reversal associated with the January 2021 major SSW event and to demonstrate connections to semidiurnal tidal activity and possible associations with a ~3-day ultra-fast Kevin wave (UFKW).
Seismological data can provide timely information for slope failure hazard assessments, among which rockfall waveform identification is challenging for its high waveform variations across different events and stations. A rockfall waveform does not have typical body waves as earthquakes do, so researchers have made enormous efforts to explore characteristic function parameters for automatic rockfall waveform detection. With recent advances in deep learning, algorithms can learn to automatically map the input data to target functions. We develop RockNet via multitask and transfer learning; the network consists of a single-station detection model and an association model. The former discriminates rockfall and earthquake waveforms. The latter determines the local occurrences of rockfall and earthquake events by assembling the single-station detection model representations with multiple station recordings. RockNet achieves macro F1 scores of 0.990 and 0.981 in terms of discriminating earthquakes and rockfalls from other events with the single-station detection and association models, respectively.
This study finds that sea level height in Arctic marginal sea in melting season enters an accelerated rise period since the beginning of the 21st century. It is found that precipitation is the dominant factor affecting the change of sea level height in melting season in 1979-1998. Polar vortex and Arctic Oscillation become dominant factors since the accelerated rise period, especially in Norwegian Sea, Barents Sea and Kara Sea. Main reason for the change of dominant factors may be that a clockwise surface wind anomaly in strong polar vortex year became more significant in these regions during the accelerated rise period. The strong wind anomaly affects distribution of sea water through processes such as surface wind stress. Specifically, a polar vortex-wind-sea level height mechanism is strengthened, thus affecting the change of sea level height. CESM2 future scenario simulation results show that sea level height will rise by 0.4m by 2100.
We performed wind tunnel studies of sand–bed collisions with natural sand particles and found an impact angle of 10.5o over a loose bed, and calculated the critical impact velocity (vic ≅ 1.2027 m s-1). The number of splashing particles (Ns) increased linearly with vi, but the coefficient of restitution CoR decreased linearly with vi. The momentum lost through frictional processes αlost was insensitive to vi, with a value of 0.2466. The mean splash velocity increased with vi for vi < 7 m s-1, and gradually reached its maximum value (0.7534 m s-1) at vi = 7 m s-1, whereas decreased slowly with vi for vi> 7 m s-1 and gradually approached a constant (0.6137 m s-1). In addition, we developed a probability distribution model for liftoff velocity. Our results emphasize the crucial role of the impact angle and have significant consequences for modeling sand–bed collisions in a natural environment.
Atmospheric aerosol radiative effects regulate surface air pollution (O3 and PM2.5) via both the aerosol–photolysis effect (APE) and the aerosol–radiation feedback (ARF) on meteorology. Here, we elucidate the roles of APE and ARF on surface O3 and PM2.5 in the heavily polluted megacity, Delhi, India by using a regional model (WRF-Chem) with constraints from available and limited observation. While APE reduces surface O3 (by 6%) and PM2.5 concentrations (by 2.4% via impeding the secondary aerosol formations), ARF contributes to a 17.5% and 2.5% increase in surface PM2.5 and O3, respectively. The synergistic APE and ARF impact contributed to ~1 % of the total concentrations of O3 and PM2.5. Hence, the reduction of PM2.5 may lead to O3 escalation due to weakened APE. Sensitivity experiments indicate the need and effectiveness of reducing VOC emission for the co-benefits of mitigating both O3 and PM2.5 concentrations in Delhi.
Stream networks are highly abundant across Earth’s surface, reflecting the tectonic and climatic history under which they have developed. Recent studies suggest that branching angles are strongly correlated with climatic aridity. However, the impact of tectonic forcing, especially in tectonically active regions, remains ambiguous. Here we analyze the branching angles of major stream networks on the eastern Tibetan Plateau, a region with complex tectonics, variable climate, and diverse landscapes. We find that spatial variations in tectonic uplift (as reflected in channel gradients) shape the branching geometry of stream networks on the steep eastern margin while in the flat interior of the eastern Tibetan Plateau, branching angles are mainly controlled by climatic aridity. This leads to the conclusion that, in the steep margin of the eastern Tibetan Plateau, climatic impacts on branching angles are overprinted by stronger tectonic controls.
This paper reports on the standing whistler waves upstream of Mercury’s quasi-perpendicular bow shock. Using MESSENGER’s magnetometer data, 36 wave events were identified during interplanetary coronal mass ejections (ICMEs). These elliptic or circular polarized waves were characterized by: (1) a constant phase with respect to the shock, (2) propagation along the normal direction to the shock surface, and (3) rapid damping over a few wave periods. We inferred the speed of Mercury’s bow shock as ~31 km/s and a shock width of 1.76 ion inertial length. These events were observed in 20% of the MESSENGER orbits during ICMEs. We conclude that standing whistler wave generations at Mercury are generic to ICME impacts and the low Alfvén Mach number (MA) collisionless shock, and are not affected by the absolute dimensions of its bow shock. Our results further support the theory that these waves are generated by the current in the shock.
Agricultural management strategies are crucial in regulating the soil-atmosphere interaction. The crop landscape is influenced by farmers through different field practices, and further impacts the variations of soil temperature, soil moisture, and field microclimate. To examine how different management strategies affect the soil properties and the aforementioned interaction, two observation systems were installed in an organic-certified (ORG) tea field and a conventional (CONV) tea field in northern Taiwan. The results show that the variation of canopy temperature was more significant in CONV while the difference in soil diurnal temperature range was minor. However, the daily loss rate of soil water content in ORG was two times faster than that in CONV (0.93% d−1 vs. 0.46% d−1). These findings suggest that the appropriate management strategies could assist farmers in adapting to environmental fluctuations and provide quantitative references for assessing soil characteristics under different agricultural applications and climatic conditions.