Frontiers of Earth Science最新文献

Abstract

Crop type mapping using remote sensing is critical for global agricultural monitoring and food security. However, the complexity of crop planting patterns and spatial heterogeneity pose significant challenges to field data collection, thereby limiting the accuracy of remotely sensed crop mapping. This study proposed a new approach for rapidly collecting field crop data by integrating unmanned aerial vehicle (UAV) images with the YOLOv3 (You Only Look Once version 3) algorithm. The impacts of UAV flight altitude and the number of training samples on the accuracy of crop identification models were investigated using peanut, soybean, and maize as examples. The results showed that the average F1-score for crop type detection accuracy reached 0.91 when utilizing UAV images captured at an altitude of 20 m. In addition, a positive correlation was observed between identification accuracy and the number of training samples. The model developed in this study can rapidly and automatically identify crop types from UAV images, which significantly improves the survey efficiency and provides an innovative solution for acquiring field crop data in large areas.

This study employs the regional Climate-Weather Research and Forecasting model (CWRF) to first investigate the primary physical mechanisms causing biases in simulating summer precipitation over the Yangtze River Basin (YRB), and then enhance its predictive ability through an optimal multi-physics ensemble approach. The CWRF 30-km simulations in China are compared among 28 combinations of varying physics parameterizations during 1980–2015. Long-term average summer biases in YRB precipitation are remotely correlated with those of large-scale circulations. These teleconnections of biases are highly consistent with the observed correlation patterns between interannual variations of precipitation and circulations, despite minor shifts in their primary action centers. Increased YRB precipitation aligns with a southward shifted East Asian westerly jet, an intensified low-level southerly flow south of YRB, and a south-eastward shifted South Asian high, alongside higher moisture availability over YRB. Conversely, decreased YRB precipitation corresponds to an opposite circulation pattern. The CWRF control configuration using the ensemble cumulus parameterization (ECP), compared to other cumulus schemes, best captures the observed YRB precipitation characteristics and associated circulation patterns. Coupling ECP with the Morrison or Morrison-aerosol microphysics and the CCCMA or CAML radiation schemes enhances the overall CWRF skills. Compared to the control CWRF, the ensemble average of these skill-enhanced physics configurations more accurately reproduces YRB summer precipitation’s spatial distributions, interannual anomalies, and associated circulation patterns. The Bayesian Joint Probability calibration to these configurations improves the ensemble’s spatial distributions but compromises its interannual anomalies and teleconnection patterns. Our findings highlight substantial potential for refining the representation of climate system physics to improve YRB precipitation prediction. This is notably achieved by realistically coupling cumulus, microphysics, and radiation processes to accurately capture circulation teleconnections. Further enhancements can be achieved by optimizing the multi-physics ensemble among skill-enhanced configurations.

The south-western Ordos Basin is rich in low-middle rank coalbed methane (CBM) resources; while the geochemical characteristics and genetic mechanism of CBM are not clear. Herein, according to geological and geochemical test data from gas and coal seam water from CBM wells in Bingchang, Jiaoxun, Huangling, Yonglong, and Longdong minging areas, we systematically studied the geochemical characteristics, generation, and evolution mechanism of CBM in Jurassic Yan’an Formation in the south-western Ordos Basin. The results show that the CH₄ content of whole gas is in the range of 42.01 %–94.72%. The distribution ranges of the δ¹³C-CH₄ value is −87.2‰ to −32.5‰, indicating diverse sources of thermogenic gas and biogenic gas. The microbial methane is mainly generated by a CO₂ reduction pathway, with certain methyl-type fermentation spots. The δ¹³C-CH₄ has a positive correlation with burial depth, indicating the obvious fractionation of CBM. The relationship between the genetic types and burial depth of the CBM reservoir indicates that the favorable depth of secondary biogenic gas is less than 660 m. The Late Cretaceous Yanshanian Movement led to the uplift of the Ordos Basin, and a large amount of thermogenic gas escaped from the edge of the basin. Since the Paleogene Period, the coal reservoir in the basin margin has received recharge from atmospheric precipitation, which is favorable for the formation of secondary biogenic methane. The deep area, generally under 1000 m, mainly contains residual thermogenic gas. The intermediate transition zone is mixed gas. Constrained by the tectonic background, the genetic types of CBM in different mining areas are controlled by the coupling of burial depth, coal rank, and hydrogeological conditions. The Binchang mining area contains biogenic gas, and the development of CBM has achieved initial success, indicating that similar blocks with biogenic gas formation conditions is key to the efficient development of CBM. The research results provide a scientific basis for searching for favorable exploration areas of CBM in the southwestern Ordos Basin.

The variations in precipitation have displayed a complex pattern in different regions since the mid-to-late-Holocene. Cloud formation processes may have a significant impact on precipitation, especially during the tropical marine processes and summer monsoon which convey abundant water vapor to coastal southern China and inland areas. Here, we use two 7500 year sedimentary records from the Pearl River Delta and the closed Maar Lake, respectively, in coastal southern China to reconstruct the mid-to-late-Holocene humidity variability and explore its possible relationship with cloud cover modulated by the Earth’s magnetic fields (EMF). Our proxy records document an apparent increase in wetness in coastal southern China between 3.0 and 1.8 kyr BP. This apparent increase in humidity appears to be consistent with the lower virtual axial dipole moments and, in turn, with a lower EMF. This correlation suggests that the EMF might have been superimposed on the weakened monsoon to regulate the mid-to-late-Holocene hydroclimate in coastal southern China through the medium of galactic cosmic rays, aerosols, and cloud cover. However, further investigations are needed to verify this interaction.

In this study the changes of tropical cyclone (TC) size from 2001 to 2021 are analyzed based on linear and quadratic curve fittings of the National Hurricane Center (NHC) / Joint Typhoon Warning Center (JTWC) best track data, based on the radius of maximum wind (RMW) and the average radius of 34-kt wind (AR34), in three oceanic basins of the North Atlantic (NATL), the Western North Pacific (WPAC) and the Eastern North Pacific (EPAC). The computations are done separately for two categories of tropical cyclones: tropical storms (TS) and hurricanes (HT). Size changes of landfalling and non-landfalling TCs are also discussed. Results show that there is a great inter-basin variability among the changes in TC sizes. Major conclusions include: 1) overall, the inner cores of TSs have become larger in all three basins, with the increasing tendencies being significant in the NATL and WAPC, while those of HTs mostly get smaller or remain similar; 2) meanwhile, comparatively large inter-basin differences are observed for the TC outer core sizes, and the sizes of landfalling TCs; 3) particularly, a significant decrease in landfalling HT outer core size is observed over the EPAC; 4) in contrast, significant increases in landfalling TS inner core size are found over the NATL and WPAC. The presented analysis results could benefit future research about TC forecasts, storm surge studies, and the cyclone climate and its changes.

Deep shale gas reservoirs commonly contain connate water, which affects the enrichment and migration of shale gas and has attracted the attention of many scholars. It is significant to quantitatively estimate the amounts of adsorbed and free water in shale matrix pores, considering the different impacts of pore water (adsorbed water and free water) on shale gas. In this paper, pore water in six deep shale samples from the Wufeng-Longmaxi Formations in the Luzhou area, southern Sichuan Basin, China, was quantitatively evaluated by saturation-centrifugation experiments. Further, the impact of shale material composition and microstructure on the pore water occurrence was analyzed. The results show that amounts of adsorbed and free water are respectively 1.7967–9.8218 mg/g (mean 6.4501 mg/g) and 9.5511–19.802 mg/g (mean 13.9541 mg/g) under the experimental conditions (30°C, distilled water). The ratio of adsorbed water to total water is 15.83%–42.61% (mean 30.45%). The amounts of adsorbed and free water are related to the pore microstructure and material compositions of shale. The specific surface area of shale controls the amount of adsorbed water, and the pore volume controls the amount of free water; organic pores developed in shale solid asphalt contribute specific surface area and pore volume, and inorganic pores developed in clay mineral contribute pore volume. Therefore, the pores of shale solid asphalt accumulate the adsorbed water and free water, and the pores of clay minerals mainly accumulate the free water.

Eleven tropical cyclones (TCs) affected Shanghai and crossed the same latitude as Shanghai from 2007 to 2018. According to similar tracks from best-track data, TCs that cause significant precipitation in Shanghai can be divided into three types: landfall TCs, nearshore northward TCs, and western TCs. Based on ERA5 reanalysis data, the dynamic synthesis method was used to synthesize TC circulation situations to compare thermal, dynamic, water vapor, and stability conditions within TC circulations during the period when they affected Shanghai. The conclusions are as follows. 1) When the three TC types are at the same latitude as Shanghai, they are all in the divergent field in the upper troposphere. For the landfall type, the subtropical high at 500 hPa is stronger and farther north than usual, and there is a high-pressure dam on the north side of the TCs. 2) The warm advection of the three TC types at 925 hPa is located in the northern quadrant of the TCs. The dynamic and water vapor conditions are good in the north-western quadrant of landfall and western TCs, and more favorable in the eastern quadrant of nearshore northward TCs. 3) The favorable effects of all three types on precipitation in Shanghai come from the boundary layer. Water vapor, upward motion, and instability conditions of landfall TCs are superior to the other two TC types. The best water vapor, dynamic, and convective instability conditions are at the northern boundary in Shanghai during landfall TCs, and the main sources of water vapor in Shanghai come from the eastern and northern boundaries. During nearshore northward TCs, the main contribution to precipitation is from the eastern boundary, while better dynamic and water vapor conditions come from the western and northern boundaries during western TCs. The above findings provide scientific and technical support for operational forecasting precipitation from TCs affecting mega-cities.

In the north-eastern Qinghai-Tibet Plateau (QTP), the source area of the Yellow River (SAYR) has been experiencing significant changes in climatic and environmental conditions in recent decades. To date, few studies have combined modern hydrological conditions with paleoclimate records to explore the mechanism(s) of these changes. This study seeks to improve understanding of hydrological variability on decadal and centennial timescales in the SAYR and to identify its general cause. We first determined annual fluctuations in the surface area of Lake Ngoring from 1985 to 2020 using multi-temporal Landsat images. The results show that lake surface area changes were generally consistent with variations in precipitation, streamflow and the regional dry-wet index in the SAYR, suggesting that the water balance of the Lake Ngoring area is closely associated with regional hydroclimate changes. These records are also comparable to the stalagmite δ¹⁸O monsoon record, as well fluctuations in the Southern Oscillation Index (SOI). Moreover, an association of high TSI (total solar insolation) anomalies and sunspot numbers with the expansion of Lake Ngoring surface area is observed, implying that solar activity is the key driving factor for hydrologic variability in the SAYR on a decadal timescale. Following this line of reasoning, we compared the δ¹³C_org-based lake level fluctuations of Lake Ngoring for the last millennium, as previously reported, with the hydroclimatic history and the reconstructed TSI record. We conclude that the hydrological regime of Lake Ngoring has been mainly controlled by centennial fluctuations in precipitation for the last millennium, which is also dominated by solar activity. In general, it appears that solar activity has exerted a dominant influence on the hydrological regime of the SAYR on both decadal and centennial timescales, which is clearly manifested in the variations of lake area and water level of Lake Ngoring.

The loess accumulation process has great potential to record patterns of atmospheric circulation change, paleoclimate, and paleoenvironmental evolution. South-eastern Xizang is a climatically sensitive region and here, we analyze a loess profile at Ranwu in order to explore the processes and interactions of dust transport and paleoclimate evolution in the region. Based on parametric grain size end-member analysis, optically stimulated luminescence (OSL) dating, and environmental proxies we show that the Ranwu loess profile comprises five end members (EMs). EM1 represents the fine silt fraction transported by high-altitude westerly winds over long distances; EM2 represents the medium silt fraction accumulated by glacier winds; EM3 is the coarse silt fraction transported by local dust storms under the action of strong glacier winds; EM4 represents the very fine sand fraction transported by strong local dust storms, different wind strengths controls the relative proportion of EM3 and EM4 over time. EM5 is the coarse sand fraction formed from the product of strong weathering of gravels. OSL dating shows loess sedimentation at Ranwu started around 11.16 ka. The prevailing climate was generally warm and wet between 11.6 and 4.2 ka, with four cooling events at 10.50, 9.18, 7.85, and 6.37 ka. Extensive paleosol development between 8.2 and 4.2 ka, a change to dry and cold climate conditions was favorable for loess formation after 4.2 ka. The palaeoenvironmental changes and abrupt climate events recorded in the Ranwu loess sequence are consistent with Holocene global environmental changes.

The layers of Tamarix cones within sedimentary deposits in arid regions have significant chronological and paleoenvironmental implications. Here, we compare the δ¹⁸O values of Tamarix cones in the Hongliujing area of Lop Nur with meteorological data for the Ruoqiang meteorological station for 1960–2019 AD. Linear regression analysis was used to reconstruct the average temperature for April and the precipitation for November in the Hongliujing area over the past 200 years. The results showed that the δ¹⁸O values were significantly negatively correlated with the temperature for February, April, May, August, December, and with the annual mean temperature; significantly negatively correlated with the precipitation for February and April; significantly negatively correlated with the sunshine hours for March and May; significantly positively correlated with the sunshine hours for February, July, August, October, and December, and with the annual mean values; and significantly correlated with the relative humidity for April, July, August, September, October, and November, and with the annual mean values. Based on the δ¹⁸O record of the past 200 years, the Hongliujing area experienced two warm-wet periods (1874–1932 and 2004–2019 AD) and two cold-dry periods (1832–1873 and 1933–2003 AD). Thus, the climate was characterized by alternating warm-wet and cold-dry conditions. Wavelet analysis revealed three main cycles: 45 years, 29 years, and 14 years.