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Wintertime precipitation, especially snowstorms, significantly impacts people's lives. However, the current forecast skill of wintertime precipitation is still low. Based on data augmentation (DA) and deep learning, we propose a DABU‐Net which improves the Global Forecast System wintertime precipitation forecast over southeastern China. We build three independent models for the forecast lead times of 24, 48, and 72 hr, respectively. After using DABU‐Net, the mean Root Mean Squared Errors (RMSEs) of the wintertime precipitation at the three lead times are reduced by 19.08%, 25.00%, and 22.37%, respectively. The threat scores (TS) are all significantly increased at the thresholds of 1, 5, 10, 15, and 20 mm day−1 for the three lead times. During heavy precipitation days, the RMSEs are decreased by 14% and TS are increased by 7% at the lead times within 48 hr. Therefore, combining DA and deep learning has great prospects in precipitation forecasting. Plain Language Summary: In this paper, we propose a deep learning‐based method to improve the forecast performance of Global Forecast System wintertime precipitation over southeastern China. Due to the imbalanced distribution of precipitation data, we use data from the three other seasons as an augmented data set for wintertime precipitation to train the deep neural network. The results show that the method can reduce the Root Mean Squared Error and improve the TS, a metric of precipitation forecast performance, of the precipitation. In particular, TS at the threshold of 20 mm day−1 are increased by 69.23%, 90.00%, and 100.00% at lead times of 24, 48, and 72 hr. The proposed method performs well during heavy precipitation days at lead times within 48 hr. Combining data augmentation with deep learning provides a successful approach to predicting precipitation. Key Points: A deep learning model based on data augmentation (DA) is proposed to improve the Global Forecast System wintertime precipitation forecastThe deep learning model improves the heavy precipitation forecast at lead times within 48 hrDA plays a critical role in the heavy precipitation forecast [ABSTRACT FROM AUTHOR]

Large‐scale seepage‐faces occur on small‐slope tidal flats. All previous studies assume that the seepage‐face has only one single exit point. Here we show via numerical simulations of tidally‐influenced groundwater flow that, in a two‐dimensional vertical, homogeneous transect of a tidal flat with gentle beach slope of 1‰, multiple seepage‐faces may occur with at most four unsaturated beach surface segments which separate four seepage‐faces. Salinity‐variation induced density‐dependent flow leads to this complex phenomenon. While the seepage‐faces are the groundwater discharging zones on the beach surface, the unsaturated zones are the recharging zones. The whole aquifer beneath the tidal flat is almost occupied by seawater and forms a wall blocking the horizontal seaward discharge of inland fresh groundwater so that inland freshwater discharges mainly occur near the high tide mark. This is in great contrast with the traditional results that inland freshwater discharge occurs mainly near low tide mark. Plain Language Summary: Tidal flat with very gentle‐slopes distributes extensively around the world, for example, the coastline of Bohai, the Yellow Sea of China, and Willapa bay of USA. Due to small slopes, the intertidal zone can be as wide as several kilometers, leading to large‐scale seepage‐faces (saturated beach surface with groundwater efflux) during ebbing and low tides. All previous studies assume that the seepage‐face has only at most one segment, the upper end of which is the exit point of water table and the other end is the intersection point of seawater and beach surface. Here, we found that in a tidal flat with gentle beach slope of 1‰, multiple seepage‐faces may occur with at most 4 unsaturated beach surface segments which separate 4 seepage‐faces during low and rising tides. The locations of these seepage‐faces and adjacent unsaturated zones are quite dynamic due to the complex, dynamic density‐dependent groundwater flow in the aquifer beneath the tidal flat. Seepage‐faces are kind of complex and important boundary for coastal groundwater flow in order to accurately model the groundwater flow and solute transport in intertidal aquifers, the complex nonlinear boundary conditions on the seepage‐faces should be strictly and accurately implemented. This paper made such an attempt. Key Points: Tidal groundwater flow with strict and explicit seepage‐face boundary conditions were numerically simulatedMultiple seepage‐faces were first found to develop on gentle tidal flat with slope around 1‰Multiple seepage‐faces imply multiple local density‐dependent groundwater‐seawater re‐circulations [ABSTRACT FROM AUTHOR]

Predicting the occurrence of internal solitary‐like waves (ISWs) is important for parameterizing turbulent dissipation. ISWs in the northern South China Sea (SCS) are well understood to mainly originate from the Luzon Strait (LS). They are generally predictable due to their phase‐locked relation with tidal forcing in the LS. However, irregular occurrence of ISWs is noted from our 9‐day moored measurements on the SCS shelf. More energetic ISWs unexpectedly appeared during the neap tide than the spring tide. Their occurrence was related to subtidal shelf flows. The tidal and shelf flows had comparable magnitudes. We thus hypothesize that when the total flow meets critical conditions ISWs are generated locally over irregular seafloor. This hypothesis is supported by numerical experiments and validated by observations. Consequently, local generation of ISWs enhances both the intensity and complexity of internal wave field on the shelf, and thus the unpredictability of ISW occurrence. Plain Language Summary: Internal solitary‐like waves (ISWs) are ubiquitous in the ocean. They can propagate several hundreds of kilometers, and induce strong turbulence and mixing. Revealing generation mechanisms and propagation of ISWs is key to further investigating their influence on the distributions of energy dissipation, heat, nutrients, sediment, and pollutants, as well as the safety of submarine voyages and offshore drilling. In this paper, ISWs in the northern South China Sea (SCS), where the most energetic ISWs in the world's oceans are generated, are studied. It is widely accepted that these ISWs generally originate from the Luzon Strait (LS). They usually have regular occurrence, and are phase‐locked to tidal forcing in the LS. However, we present field measurements showing irregular occurrence of ISWs on the northern shelf of the SCS. This irregular occurrence is in striking contrast to the prominent predictability of ISWs originated from the LS. We reveal that the intermittent nature of the occurrence is due to the local generation of ISWs on the shelf; the coupling of different flow components seems to play a significant role in ISW generation on the shelf. The results reported here are expected to be applicable to other shelf regions of the world's oceans. Key Points: Field measurements documented large numbers of internal solitary‐like waves (ISWs) with irregular occurrence on the northeastern shelf of the South China SeaMore energetic and larger numbers of ISWs are observed during the neap tide than during the spring tide as modulated by shelf flowsISWs are locally generated due to the joint effects of tidal and subtidal shelf flows [ABSTRACT FROM AUTHOR]

The PM2.5 (fine particulate matter with diameter ≤2.5 μm) concentration anomalies over the North China Plain (NCP) in early and late winter sometimes show a subseasonal reversal, which brings a great challenge for precise control of air pollution, and mechanisms are not well understood. This paper reveals the key role of the Barents Sea sea‐ice in this reversal. In early winter, a negative Scandinavian‐like pattern causes an anticyclonic anomaly over Northeast Asia and thus leads to the positive PM2.5 concentration anomaly over the NCP. In addition, anomalous warm advection associated with the positive North Atlantic Oscillation‐like pattern accelerates winter sea‐ice loss in the Barents Sea, especially in late winter, which increases the surface turbulent heating flux. These heating causes a negative Polar/Eurasian‐like pattern that induces a cyclonic anomaly over Northeast Asia and eventually leads to a negative PM2.5 concentration anomaly over the NCP in late winter. Vice versa. Plain Language Summary: In this study, we find that there is a significant seesaw pattern of PM2.5 concentration anomalies over the North China Plain (NCP) in early and late winter. Further results reveal that variations in Barents Sea sea‐ice play a crucial role. More specifically, a negative (positive) Scandinavian‐like pattern in early winter tends to induce an anticyclonic (a cyclonic) anomaly over Northeast Asia. This anticyclonic (cyclonic) anomaly leads to a higher (lower) PM2.5 concentration anomaly over the NCP in early winter. In addition, a North Atlantic Oscillation‐related warm (cold) temperature advection promotes (inhibits) the sea‐ice loss in the Barents Sea, especially in late winter. The Barents Sea‐ice loss (increment) in late winter tends to heat (cool) the lower‐level atmosphere and thus triggers an anticyclonic (a cyclonic) response in the upper troposphere there. The anticyclonic (cyclonic) response in the Barents Sea region as a part of the negative (positive) Polar/Eurasian‐like pattern induces a cyclonic (an anticyclonic) anomaly over Northeast Asia, which leads to a lower (higher) PM2.5 concentration anomaly over the NCP in late winter. These findings could be of great value for the subseasonal predictions of PM2.5 concentrations over the NCP in winter. Key Points: PM2.5 concentration anomaly over the North China Plain shows an visibly subseasonal reversal in early and late winter on interannual scaleThe Scandinavia‐like pattern and Polar/Eurasian‐like pattern are responsible for the seesaw pattern of PM2.5 concentration anomaliesThe sea‐ice variations in the Barents Sea play a key role in modulating such atmospheric circulations [ABSTRACT FROM AUTHOR]

The urban barrier effect is one possible physical mechanism by which the urbanization alters the spatial distribution of cloud‐to‐ground (CG) lightning activity in a city. There is a gap in the preceding research on an urban barrier effect to alter patterns of CG lightning in Beijing, and the urban morphology does not receive enough attention as an important influencing factor. By combining lightning location data, ground meteorological data, and urban morphology datasets, we identify a possible urban barrier effect from the perspective of CG lightning in Beijing during 2010–2017. Analysis of one typical thunderstorm on 13 July 2017 revealed that the barrier effect might result in a bifurcation of low‐level airflow and splitting of the convergence line. A more important finding was the modulation of the barrier effect by the scale and density of the built‐up area, and the numerical simulation experiments further confirmed this potential association. Plain Language Summary: Lightning is a common hazard associated with thunderstorms, which can result in $332 million of property damage annually in the United States and cause more than 1,000 casualties per year in China. Beijing, as one of the first‐tier cities in China, is undergoing a rapid urbanization in both city size and building density. Statistical results of cloud‐to‐ground lightning activity over 8 years showed that there were lots of lightning at the edge of the city, and most lightning seemed to be diverted around the city center. Due to a possible barrier effect, the path of a thunderstorm on 13 July 2017 was obviously altered by the city, also resulting in the change of near‐surface wind field. Except for synoptic forcing, wind speed, and urban heat island, this paper pointed out that the size of a city and its building density could influence whether the barrier effect occurred and how strong it was. Key Points: The strong synoptic forcing, strong wind speed, and weak urban heat island were in favor of the formation of a barrier effect in the built‐up area of BeijingWhen thunderstorms passed over city, the bifurcation of low‐level airflow and splitting of convergence line was a result of the urban barrier effectThe size of a city and its building density might influence the occurrence and strength of a barrier effect [ABSTRACT FROM AUTHOR]

This paper reports that there has been a significant relationship between the quasi‐biennial oscillation (QBO) in zonal wind at 50 hPa in summer and precipitation in August in northern China since the late 1970s. The correlation coefficient between them reaches up to −0.61 during 1979–2018. Associated with the easterly (westerly) phase of the QBO, there is a meridional dipole pattern of zonal wind from the stratosphere to troposphere over the Northwest Pacific in August with the easterly (westerly) anomalies in 25–50°N and westerly (easterly) anomalies in 50–65°N. This meridional zonal‐wind dipole favors the strengthening (weakening) of the tropospheric Japan Sea high in the QBO easterly (westerly) phase, causing an increase (decrease) of the water vapor transportation from the Northwest Pacific to northern China and thus more (less) precipitation in these regions. The finding of this study has import implications for predicting the summer precipitation in northern China in the future. Plain Language Summary: The quasi‐biennial oscillation (QBO) is a periodic phenomenon in the tropical stratosphere. It manifested as an alternation of easterly and westerly winds, with an average period of roughly 28 months. It has great influences on the Earth's surface via different pathways. But the previous studies mainly focused on winter. In this study, we are interested in the QBO's impact on the summer precipitation in Asia‐Pacific region. We find that the QBO does have a remarkable relationship with the August precipitation in northern China since the late 1970s. When the QBO is in its easterly phase, it can induce an easterly wind in 25–50°N and a westerly wind in 50–65°N over the Northwest Pacific from the lower stratosphere to lower troposphere, and causes a higher‐than‐normal high pressure over the Sea of Japan, which favors excessive rainfall to its west. Our study provides a direct evidence for the QBO's impact on the Earth's surface in summer. Meanwhile, it also provides potential ideas and methods for predicting the summer precipitation in northern China in the future. Key Points: The quasi‐biennial oscillation (QBO) at 50 hPa has a significantly negative correlation with summer precipitation in northern China since the late 1970sThe QBO can induce a meridional zonal‐wind dipole over the Northwest Pacific that extends from surface into the stratosphereThe enhanced Japan Sea high excited by the meridional zonal‐wind dipole in easterly QBO phase causes more precipitation in northern China [ABSTRACT FROM AUTHOR]

Extremely persistent heavy rainfall (EPHR) events frequently occur in southern China. The mechanisms underlying their generation and development are not well understood, and this leads to difficulties in accurately forecasting these events. In this paper, hourly precipitation grid data and ERA‐5 reanalysis data are used to examine the relationships between precipitation, ageostrophic winds, and low‐level jets (LLJ). The results suggest that ageostrophic winds in the boundary layer mainly exhibit confrontational and asymptotic convergence, with the former being accompanied by double LLJ, which transport more vapor and provide favorable uplift conditions, thus producing stronger precipitation and wider rainbands. The different cooperated relationships between LLJ and ageostrophic winds play an important role in the development of EPHR events, and greater account needs to be taken of these relationships when forecasting. Plain Language Summary: Coastal extremely persistent heavy rainfall (EPHR), long‐duration heavy rainfall in local coastal area, occurs frequently in Southern China, and this kind of rainfall is usually accompanied by southwesterly in the lower atmosphere, which we call as the southwest low‐level jet. Generally, the occurrence and development of rainfall is accompanied by the convergence of the lower air flow. Since the divergence of the geostrophic wind component of the wind field is zero, the convergence situation is characterized by the ageostrophic wind component, which doesn't satisfy the geostrophic balance. In this type of coastal EPHR accompanied by the Southwest Jet, the ageostrophic wind is mainly manifested in two different convergence situations. One is that the ageostrophic wind presents several opposite branches of directions presenting a confrontational pattern around the rainfall area, and the other is an asymptotic convergence in which two similar wind directions, such as northeasterly and southeasterly, gradually converge to the rain area. The former has stronger lower‐level uplift, greater precipitation and wider rain bands, while the latter is relatively weak. This is an interesting phenomenon, of which the specific mechanisms need to be further studied. Key Points: About 74% of extremely persistent heavy rainfall cases along the coast of southern China are accompanied by southwest jets in the marine boundary layerAgeostrophic winds mainly exhibit confrontational and asymptotic convergence, and the former is accompanied by double low‐level jetsCases of ageostrophic winds with confrontational convergence have stronger precipitation and wider rainband [ABSTRACT FROM AUTHOR]

Coastal offshore currents transport terrestrial materials from coastal areas to continental shelf areas in the upper layer; these currents are regarded as unimportant in winter when wind‐driven onshore currents are dominant. In this paper, both long‐term observations and high‐resolution simulations in the East China Sea reveal unique wintertime offshore currents in the upper layer that are widespread in the 30–70‐m isobath area and even overcome the wind‐driven onshore currents at the 50–70‐m isobaths. The mixed layer instabilities, frontogenesis and turbulent thermal wind, which constitute a mechanism in the generation of submesoscale offshore currents, have a growth rate that is fast enough to compete with the wind‐driven dynamics when the winter monsoon is relatively weak (<10 m/s). The alternation of the weak‐wind and strong‐wind modes during the winter monsoons provides a general condition for the repeated occurrence of remarkable submesoscale cross‐shore transport and carbon sequestration in coastal seas. Plain Language Summary: The mechanism by which wintertime cross‐shore currents occur in coastal seas is re‐examined together with wind‐driven mesoscale currents and submesoscale processes. Submesoscale currents, which is shaped like filaments or eddies with a horizontal scale of approximately O(1) km, have frequently been reported in the upper ocean. These submesoscale currents are dynamically defined by conditions where the relative vertical vorticity equals or exceeds the planetary vorticity, where the Rossby number is O(1). Both long‐term observations and high‐resolution simulations in the East China Sea reveal unique wintertime offshore currents in the upper layer that can prevail over wind‐driven onshore currents. The synoptic winter monsoons provide the general conditions for the repeated occurrence of remarkable submesoscale cross‐shore transport and carbon sequestration in coastal seas. Key Points: Significant wintertime offshore currents that overcome wind‐driven onshore currents are investigated in the East China SeaThe submesoscale dynamics are fast enough to compete with the wind‐driven dynamicsThe synoptic winter monsoon provides a general condition for submesoscale cross‐shore events in coastal seas [ABSTRACT FROM AUTHOR]

Extreme summer precipitation often associated with flash floods has devastating impact on the local economies and livelihood of millions of people over East China. Tracking down the drivers of those extreme events will help to understand their formation mechanisms and to improve forecasts. Here the synoptic patterns associated with summer extreme precipitation events over East China during 1961-2018 have been identified systematically and quantitatively using a circulation clustering method. The results show that regional events over East China are dominated by the Eastern Asian summer monsoon associated Meiyu front, landfalling tropical cyclones and low-pressure vortices. Most sub-regions have seen increasing trends of extreme rainfall events during the past 6 decades with comparable contributions from the two main drivers. There was a decreasing trend over the North China Plain driven by the lowlevel southeasterly winds. Plain Language Summary Closely linked to flash floods, extreme precipitation is one of the high impacts but low probability weather phenomena that challenge modern numerical predictions. However, models are generally more skillful in predicting synoptic weather systems than precipitation itself. Taking East China as an example, this paper demonstrates the links between extreme precipitation events and synoptic weather systems using circulation clustering. Two major weather systems, the Eastern Asian Summer Monsoon and landfalling tropical cyclones are responsible for most extreme precipitation events (about 70%) over East China during the summer season. If models can accurately predict the strength and position of these weather systems, there may be enhanced potential of predicting regional precipitation extremes. Such implications are not regionally limited. [ABSTRACT FROM AUTHOR]

Several experiments and field observations have demonstrated seismic waves‐induced permeability increases. However, a few experiments have revealed significant decreases in the permeability of fractured sandstone after shaking produced by dynamic stresses. In this paper, we analyze the tidal responses of the water level in two wells (Junan and Dingyuan) situated on and near, respectively, the Tan‐Lu fault in China. The results sampled by a single well indicate that seismic waves could both increase and decrease the permeability. In addition, our results indicate that the earthquakes that decreased the permeability in a single well were distributed in certain azimuthal zones. Given the azimuthal distribution of distant earthquakes, the observed permeability decrease could be attributed to the seismic wave‐induced clogging of fractures that compose the flow paths in the shallow crust. Our research contributes to an enhanced overall understanding of the effects of earthquakes on the permeability of the shallow crust. Plain Language Summary: Most hydrological phenomena caused by earthquakes can be explained by permeability changes. Many experiments and field observations have demonstrated coseismic increases in permeability. However, coseismic decreases in permeability have seldom been reported. In this paper, we analyze the tidal responses of the water level in two wells. The results indicate that seismic waves could both increase and decrease the permeability. In addition, we find that the earthquakes that decreased the permeability in a single well were distributed in certain azimuthal zones. Given the azimuthal distribution of distant earthquakes, the observed permeability decrease could be attributed to the seismic wave‐induced clogging of fractures that compose the flow paths in the shallow crust. Furthermore, we discuss the possible process of particle remobilization in a natural fracture network caused by seismic waves, as this process may be used to explain the azimuthal distribution of earthquakes that decreased the permeability. Our research contributes to an enhanced overall understanding of the effects of earthquakes on the crustal permeability. In the future, earthquake‐induced increases and decreases in permeability should be considered when discussing controls on permeability for exploring underground resources of groundwater, geothermal energy, oil and gas, and for safely storing nuclear and toxic waste underground. Key Points: Seismic waves could decrease besides increase the crustal permeabilityDecreases in permeability may depend on the earthquake azimuthThe decrease in the permeability could be attributed to the clogging of fractures [ABSTRACT FROM AUTHOR]

Microseisms generated by storms primarily propagate as surface and P waves. However, there has been only one report of S wave microseisms excited by strong storms in the North Atlantic, not from typhoons in the Pacific. In this paper, we present the first observations for S wave microseisms caused by typhoons in Western Pacific, via analyzing ambient seismic waveform recorded by a large‐aperture seismic array in China. The results show that three super typhoons clearly generated S wave microseisms at periods of 5–10 s. The strength of S wave microseism is found to be correlated with P wave. The excitation mechanism for S wave microseism observed in regions with flat seafloor may be related to the interaction between P waves and sub‐ocean sedimentary structure. Plain Language Summary: Besides causing property damages and life losses, ocean storms can also generate seismic waves named as microseisms, which could also be used as signals for studying interior of the Earth. Microseisms mostly propagate as surface waves and P waves. Yet S waves from storms are only detected in the North Atlantic Ocean shown by previous studies. However, there are no reports of S wave microseism by storms in the Pacific Ocean, though the typhoons there are strong and frequent. In this study, we observe and locate S wave microseisms generated by three super typhoons in Western Pacific by processing continuous ambient seismic waveforms recorded in China. The source regions of P and S waves are located very close to each other in the deep ocean near typhoons. Our results show that observability of S wave microseisms depends on P wave energy, which is controlled by strength of typhoons. It is demonstrated that S wave microseisms not only can be generated in the regions with steep topography but also can be excited in the flat seafloor regions. This work is helpful for understanding the excitation mechanism of the S waves. Key Points: We observed body‐wave (P, SV, and probable SH) microseisms from typhoons in the Western PacificWe found that S wave strength is related to P wave power, which in turn relies on typhoon intensityS wave microseisms could be generated in the regions with flat seafloor or bathymetric gradients [ABSTRACT FROM AUTHOR]