Systematically investigating drought evolution characteristics and vegetation responses to drought is crucial for drought prevention, disaster mitigation, water resource management, and policy formulation. Our study focuses on Xinjiang, Qinghai, and Gansu provinces in mid-western Northwest China. Using the CN05.1 dataset from 1963 to 2022 to calculate the standardized precipitation evapotranspiration index (SPEI), combined with the normalized difference vegetation index (NDVI) from 2003 to 2022, we applied run theory, Theil-Sen slope estimator, Mann-Kendall trend test, and regression analysis to analyze drought evolution across multiple temporal scales and explore drought impacts on vegetation. The results indicate that over the past 60 years, the study area exhibited a drying trend on decadal and interannual scales, with 1998 identified as a significant drought mutation year. Pronounced drying trends were observed in eastern and southern Xinjiang, the Qaidam Basin, and the Hexi Corridor, where light and moderate droughts were predominant. The Hexi Corridor experienced a significantly higher frequency of moderate droughts (greater than 15.00%) than other regions. Drought variability exhibited periodic characteristics of 12-year, 30-year, and 46-year cycles, with notable seasonal differences. In spring, summer, and autumn, the mid-western Northwest China showed significant drying trends, with the most rapid decline in autumn (SPEI rate: -0.034 yr?1). The Hexi Corridor exhibited SPEI decline rates below -0.030 yr?1 across all three seasons, indicating high risks of consecutive droughts, alongside prolonged duration, higher peak intensity, and mean drought intensity. The Tarim Basin also faced severe drought conditions in spring and autumn. Drought exerted measurable impacts on vegetation dynamics. Compared to 2003-2012, NDVI degradation areas increased by approximately 3.52% during 2013-2022. Over the past two decades, vegetation degradation was concentrated in western Xinjiang, eastern and southern Qinghai, where NDVI and SPEI were positively correlated.
Drought stress is a major limiting factor for crop growth. Investigating the photosynthetic characteristics and physiological drought resistance mechanisms of spring maize (Zea mays L.) at the seedling stage is crucial for enhancing drought resilience and promoting crop yield. In this study, taking spring maize as the research object and using pot experiments, four gradients of control treatment (referred to as “CK” treatment), mild drought (T1), moderate drought (T2), and severe drought (T3) were set up to study the photosynthetic physiological characteristics and drought resistance mechanism at the seven-leaf stage of spring maize under different drought degrees. The results show that drought stress significantly reduced both the net photosynthetic rate (Pn) and the maximum net photosynthetic rate (Pnmax) of spring maize leaves. Under moderate and severe drought conditions, the initial slope (α0), the dark respiration rate (Rd), and the apparent quantum efficiency (AQE) of the light response curve decreased significantly, while the light saturation coefficient (γ0) increased markedly, indicating the light energy utilization rate of the leaves decreased significantly. The transpiration rate (Tr) gradually decreased with increasing of drought intensity, the water use efficiency (WUE) significantly enhanced under moderate drought but sharply reduced under severe drought. Stomatal conductance (Gs) progressively declined with drought stress intensification. Stomatal limitation was identified as the primary factor reducing photosynthetic rate under mild and moderate drought conditions, with non-uniform stomatal closure phenomenon under mild drought. Under severe drought condition, intercellular CO2 concentration (Ci) significantly increased, and the stomatal limitation value (Ls) decreased dramatically, demonstrating the dominance of non-stomatal limitation.
It is crucial for disaster prevention, mitigation, and food security to elucidate the temporal and spatial characteristics of multi-scale drought in Heilongjiang Province under the background of climate change. Based on NCEP (National Centers for Environmental Prediction) and GPCC (Global Precipitation Climatology Center) monthly grid meteorological data with a horizontal resolution of 1°×1° in Heilongjiang Province from 1961 to 2023, the standardized precipitation evapotranspiration index (SPEI) was calculated at various time scales. Additionally, the K-means clustering method was employed to divide Heilongjiang Province into three distinct zones, and the spatio-temporal characteristics of meteorological drought for multiple time scales in different zones in Heilongjiang Province from 1961 to 2023. The results show that the SPEI in Heilongjiang Province fluctuated significantly from 1961 to 2023. Notably, the SPEI in the northern mountainous region decreased markedly in April, while the SPEI in the central and western plain areas as well as the southeastern region increased at varying time scales, reflecting obvious spatial disparities in dry and wet conditions across different regions of Heilongjiang Province. Meteorological drought in Heilongjiang Province predominantly manifested as large-scale widespread occurrences, with two prominent high-frequency periods: 1967-1989 and 1999-2011. Furthermore, the spatial extent of drought in summer generally exceeded that in spring. In the recent climatic period (1991-2020), the frequency of drought in Heilongjiang Province increased and the intensity of drought intensified. And this trend poses a challenge to regional ecological security and sustainable development.
In the extreme arid region of Northwest China, the Gobi desert is widely distributed, with strong winds and abundant sand, leading to frequent disasters. A deep understanding of the basic laws of the aeolian sand-dust movement over Gobi surfaces is an important prerequisite for disaster warning and scientific prevention. Given the current difficulty in accurately predicting the instantaneous aeolian sediment transport rate, exploring the statistical laws of airflow and sand-dust characteristic physical quantities in wind-sand events, and then conducting statistical forecasting, may be a feasible way to establish a quantitative relationship between wind and sand on time scales of seconds or less. This study borrows the ideas and methods from the statistical theory of turbulence, and analyzes four field observation datasets of Gobi sand-dust movement using the Hilbert-Huang transform. The results indicate that the Hilbert marginal spectra of the time series of wind speed, kinetic energy and number of saltation sand particle, and dust concentration in the aeolian events all follow the power scaling law. The scaling exponents of variables, characterizing aeolian sand and dust motions, and wind speed range from 0.78 to 1.51 and 0.59 to 1.47, respectively.
As a hot spot of “land-atmosphere coupling”, the current study focuses on the spatial and temporal distribution of land-atmosphere coupling and the influence of hydrothermal conditions in the climate transition regions, but lacks the study on the synergistic effect of multi-factor and the difference in coupling degree among different ecosystems. Based on the station observation data, the paper focused on the coupling relationship between land surface water, thermal and ecological factors and surface fluxes, comparatively analyzed the differences in single-factor and multi-factor synergistic land-atmosphere coupling among different ecosystems, and assessed the contribution of each land surface factor to the coupling degree. The results show that in single-factor coupling, the coupling between latent heat flux and leaf area index is the strongest for sparse vegetation, and the coupling between latent heat flux and soil temperature is the strongest for farmland; the coupling between sensible heat flux and soil temperature is the strongest for grassland, forest and farmland. Multi-factor synergistic coupling is significantly better than that of single factor, and multi-factor synergistic coupling with latent heat flux is significantly enhanced in forest and sparse vegetation, and synergistic coupling with sensible heat flux is obviously enhanced in sparse vegetation. Among the contributions of each factor to the surface fluxes, in grassland system sensible and latent heat fluxes are dominated by thermal and ecological factors, respectively; in sparse vegetation system latent heat flux is co-dominated by the soil moisture factor and leaf area index, and sensible heat flux is dominated by the thermal factor; and latent heat and sensible heat fluxes are dominated by the thermal factor in farmland and forest system. Under drought conditions, the contributions of soil moisture and ecological factors to latent and sensible heat fluxes increase in most ecosystems.
It is of great significance to study the relationship between summer atmospheric heat source over the Qinghai-Xizang Plateau and its surrounding area and the number of high temperature days in the Sichuan-Chongqing Basin for summer high temperature prediction and high temperature and drought disaster prevention. Based on the daily maximum temperature data of 125 meteorological stations in the Sichuan Chongqing Basin and monthly NCEP/NCAR (National Center for Environmental Prediction/National Center for Atmospheric Research) reanalysis dataset, the characteristics of summer high temperature days in the Sichuan-Chongqing Basin and relationship between the inter-annual variation of high temperature days there and atmospheric heat source over the Qinghai-Xizang Plateau and its surrounding area are discussed. The results show that the first mode of EOF decomposition of high temperature days presented a consistent type in the whole region, which can reflect the main distribution characteristic of summer high temperature days. The summer high temperature days in the basin significantly increased from 1979 to 2022, showing obvious inter-annual and inter-decadal variation characteristics. There is a close relationship between the atmospheric heat source over the eastern part and its eastern side of Qinghai-Xizang Plateau and the inter-annual variation of summer high temperature days in the basin. When the heat source over the key region of the plateau is relatively weak (strong), the corresponding high temperature days in the basin are significantly more (less). On the inter-annual scale, when the atmospheric heat source over the eastern part and its eastern side of Qinghai-Xizang Plateau is weaker than normal, the position of South Asian high shifts northeastward, and the western Pacific subtropical high shifts northwestward, The water vapor transport from the South China Sea, western Pacific, and Bay of Bengal to the basin is significantly weakened, combined with significant subsidence anomalies over the basin, resulting in less precipitation and high temperature days there. At the same time, the total cloud cover in the basin is less than normal, and the short-wave solar radiation flux reaching the ground increases significantly, resulting in a rise in ground temperature and an increase in the number of high-temperature days. When the atmospheric heat source over the eastern part of the plateau and its eastern side is relatively strong, the circulation pattern is beneficial for reducing the number of high temperature days in the basin.
This study employs a distributed rainfall-runoff routing model, using minute-resolution radar quantitative precipitation estimation data and 12.5 m-resolution topographic data. Model parameters were calibrated using observed data from the Shuangxi Hydrological Station. A refined simulation was conducted to reproduce the flash flood and debris flow event that occurred in Baishi Township, Beichuan County, Mianyang City, Sichuan Province, on July 16, 2022. The model effectively retraced the runoff generation and confluence processes and analyzed the underlying causes of the disaster. Results show that the localized model performed well, with a high degree of consistency between simulated and observed water depths. The coefficient of determination (R2) reached 0.86, with a maximum water depth error of 0.25 m, a rise magnitude error of 0.59 m, and a runoff onset timing error of 0.1 h, all within acceptable margins and laying a solid foundation for subsequent analyses. From 00:00 Beijing time on July 16, 2022, Beichuan experienced 8.0 hours of continuous rainfall reaching rainstorm levels. The heaviest precipitation occurred in Baishi Township and the upstream basins of the Qingpian and Tiedong rivers, with rainstorms covering 70% of the total basin area and particularly affecting the middle and lower reaches. The maximum hourly rainfall reached 30 mm, the maximum cumulative rainfall was 120 mm, and the average rainfall in the middle and lower reaches was 88 mm. The long duration, high intensity, and wide spatial coverage of rainfall were the primary triggers of the flash flood. In addition, steep terrain gradients and narrow river channels accelerated runoff and increased water depth. Flow velocities at several locations exceeded 3.3 m·s-1. In the lower Qingpian River, water depth continued to rise for 2.0 hours, reaching a flood peak of 15.4 m that lasted for 2.6 hours, contributing significantly to the runoff at the disaster site. At Baishi Township, the peak flood depth reached 21.0 m and remained at a high level for 3.0 hours. Under complex terrain conditions, prolonged heavy rainfall triggered a large-scale flash flood in the small watershed, inundating multiple villages along the river to varying extents. The study area lies within the central Longmenshan Fault Zone and was severely impacted by the 2008 Wenchuan Earthquake. There are 23 identified hazard sites for debris flows and landslides in the basin. The intense flash flood scoured and mobilized abundant loose material, generating a catastrophic debris flow that caused devastating damage to Baishi Township.
An extreme rainstorm occurred in the northeastern region of the Sichuan Basin from 20:00 on 7 to 20:00 on 8 August 2021, with the daily rainfall and the hourly rainfall during the night rain period breaking historical records. The meteorological observation data, Doppler radar data and ERA5 reanalysis data were utilized to analyze the evolution and causes of this process. The results are as follows: 1) Before the process, the pseudo-equivalent potential temperature (θse) at 700 hPa deviated from the climate average by 2.5 standard deviations, and the specific humidity at 700, 850, and 925 hPa all deviated by more than 2 standard deviations. 2) The stable ground convergence zone and the low vortex shear system in the middle and lower layers continuously triggered new convection and constantly merged with the original convection system to form “train effect”, ensuring the maintenance and development of the original convection system; 3) The convergence of low-level warm-humid air and dry-cold air formed a stable θse front area, the meridional warm and humid updraft, together with the updraft of the zonal secondary closed circulation, provided stable energy and water vapor transportation for the convective system. The convergence wind field near 700 hPa, in combination with the blocking effect of the descending and northerly airflow in the meridional secondary circulation at the middle and lower levels, made the convective system present a quasi-stationary “backward propagation” feature, which was conducive to the formation of this extreme rainstorm.
Studying the basic characteristics of sudden rainfall events in mountainous areas, along with the local circulations induced by dynamic and thermodynamic processes, is of great significance for improving the accuracy and timeliness of forecasting sudden heavy rainstorms in such environments. This paper investigates two sudden rainfall events in the mountainous region of Mianyang, China, based on the china multi-source merged precipitation analysis system (CMPAS), black body temperature (TBB) data from the FY-2G satellite, and ERA5 reanalysis data from the European Centre for Medium-Range Weather Forecasts (ECMWF). Diagnostic analyses and numerical simulations were conducted using equations representing flow around, up, and over mountains.The results indicate that both sudden torrential rainfall events in mountainous areas occurred under the influence of the western Pacific subtropical high within a weak synoptic-scale environment. The convergence of cold and warm air masses, combined with favorable moisture supply and high-temperature conditions, served as the primary triggers for the rainstorms. Cross-mountain airflow induced by terrain obstruction generated both around and over flow motions. Within the rainfall zone, these processes enhanced vertical ascent and localized vortices. When weak cold air intruded, flow over dominated, and the combined effects of ascending and deflected flows created favorable dynamic conditions for the initiation and intensification of the sudden rainstorms. In contrast, under strong cold air influence, the contributions of around and over flows to heavy rainfall were significantly reduced. The around and over flows within the rainstorm area were coupled with the intensity of precipitation. During the rainfall process, the cessation of surface heat input led to a significant weakening of thermal disturbances near the rainfall area, preventing the formation of local circulation in the western basin, resulting in weakened around and over flows, and the disappearance of the convergence zone, which led to a noticeable reduction in simulated precipitation and the disappearance of the heavy precipitation center. Numerical simulations for both events demonstrated that latent heat played a more critical role than sensible heat in driving precipitation.
Based on ground automatic station, multi-band weather radar and sounding data, and the fifth generation atmospheric reanalysis ERA5 from the European Centre for Medium-Range Weather Forecasts (ECMWF), the environmental conditions, mesoscale characteristics and maintenance mechanisms of a rare squall line weather process in Yunnan Province on 7 July 2022 were analyzed. The results show that this squall line occurred within the convergence zone of the continental high and subtropical high, with environmental conditions characterized by strong convective available potential energy, moderate vertical wind shear, and significant intrusion of high-level dry and cold air. Changes in C-band radar reflectivity and radial velocity were closely associated with the occurrence of strong surface winds and hail. The squall line exhibited distinct features, with a pronounced inflow jet near the surface layer, accompanied by velocity ambiguity and gust front characteristics. The hail cells exhibited characteristics such as three-body scattering, midlevel convergence, and storm-top divergence. High spatial and temporal resolution observations from the X-band dual-polarization phased array radar showed that mature hail clouds had a strong reflectivity factor exceeding 55 dBZ. At the same time, a prominent differential reflectivity (ZDR) column was observed near the strong updraft, with vertical extension exceeding the height of the wet-bulb temperature 0 ℃ layer. Dual-polarization parameters further indicated that precipitation occurred during the hailfall process. The analysis found that the long-term maintenance of ground convergence lines, the coexistence of strong updrafts and tilted downdrafts within the storm, and the coupling of low-level convergence with high-level divergence was the main mechanisms sustaining this squall line.
Liupan Mountain is an important component of the Loess Plateau-Sichuan-Yunnan ecological barrier and a key water-retaining forest base in Northwest China. To further enhance the understanding of the macro- and microphysical characteristics of orographic clouds and to support scientific weather modification operations, this study investigates a mixed stratiform orographic cloud process that occurred in the Liupan Mountain area from August 21 to 23, 2020. The analysis is based on comprehensive observations from a high-mountain meteorological station at an elevation of 2 842 m, using instruments such as a fog drop spectrometer, raindrop spectrometer, and millimeter-wave cloud radar. The results show that this precipitation system was significantly influenced by orographic effects. The increases in cloud-top height and vertical thickness were approximately equal to the average mountain elevation of 2 162 m. During the period of peak rainfall, the raindrop number concentration, maximum diameter, mean diameter, and liquid water content reached maximum values of 970 m-3, 4.25 mm, 1.23 mm, and 1.36 g·m-3, respectively. The raindrop size distribution was better represented by the Gamma distribution than by the Marshall-Palmer distribution. Based on cloud microphysical data, the cloud system was divided into three mesoscale cloud regions. The widths of regions 2 and 3 were both approximately 400 km. Compared with region 2, region 3 exhibited higher liquid water content, larger mean volume diameter, and larger mean effective diameter, resulting in approximately twice the amount of precipitation.
The occurrence, development mechanism of complex terrain squall line and the warning index of severe weather have been the focus of the short time approaching forecast of severe convective weather in Shanxi Province. On July 25, 2022, a rare squall line with a maximum instantaneous wind of magnitude 12 occurred in southern Shanxi Province. Based on multi-source observations and reanalysis data, the triggering, evolution and organization of the squall line process were analyzed in this paper. The results are as follows: (1) Radar observations showed that the life history of the squall line could be divided into three stages: Firstly, the upstream discrete convective cells moving eastward to form a linear convective system, and then the development of new thunderstorms in mountainous areas formed a multi-cell storm, thereafter, the linear convective system merged into the multi-cell storm and organized into a squall line. At the former two stages, the vertical wind shear was weak, but the favorable environment such as high energy, high humidity and unstable stratification made the convection mainly dominated by multi-cell storms with a low degree of organization. At the third stage, the forward-tilting structure became more obvious, the stratification instability increased significantly, the local vertical wind shear improved significantly due to near low-level disturbance, and the convection quickly organized into a strong squall line. (2) The development and maintenance of surface mesoscale convergence line, dew point front and mesoscale vortex are the main mechanisms of system triggering. The self-organized structure of forward inflow and rear outflow co-existed, the convergence at the low level and divergence in the upper level co-existed in the vertical direction, and the coexistence of environmental inflow and system outflow in the horizontal direction made the squall line maintain and develop. (3) From the perspective of thermal structure, the strong cold pool formed by merging and strengthening of surface cold pool at the third stage is the key cause of extreme thunderstorm wind. (4) The surface convergence line and mesoscale vortex occur more than 20 min earlier than the convective organization and strengthening, and the high value area of radial velocity in the low layer and the radial convergence in the middle layer appear 10-25 min earlier than the surface gale, which has certain indication significance for monitoring and warning.
Inner Mongolia is one of significant seasonal snow-covered regions in China. Snow identification and snow depth inversion are crucial for agricultural production, ecological assessment, and research on spring floods and snow-related disasters. In order to improve the accuracy of local snow identification, a direct comparison snow identification method based on Normalized Difference Snow Index (NDSI) is proposed in this paper, the method involves applying the NDSI difference operation between the snow map to be identified from the Himawari-8 satellite images and the snow-free base map from the current autumn to identify snow, and it is compared with the snow identification methods used in routine business. The results indicate that the Snow Mapping (SNOMAP) algorithm, based on the Normalized Difference Vegetation Index (NDVI), tends to miss some thin snow pixels, while the Fractional Snow Cover (FSC) algorithm can be affected by water bodies in snow identification and ultimately affect its accuracy. In the non-forest areas of Inner Mongolia, the accuracy of NDSI direct comparison was 3.88% higher than SNOMAP and 0.52% higher than FSC. The difference between the accuracy of NDSI direct comparison and FSC in non-forest areas was small. In forest areas, compared with FSC algorithm, NDSI direct comparison method significantly improved the identification accuracy, while the error rate decreased, indicating that NDSI direct comparison method is more suitable for snow identification in forest areas of Inner Mongolia.
To overcome the limitation of traditional meteorological drought indices, which rely on historical climatic probability data of the same period for calculation, this paper establishes a meteorological drought index that can reflect the degree of soil drought but only requires current precipitation and evaporation data. Based on the soil water balance equation and in conjunction with the standard classification of soil drought levels, this study utilizes the relationship between the evaporation intensity of the dry soil surface layer and the water surface, along with soil evaporation calculation methods, to derive the expressions of the critical lines representing various drought levels within the cumulative precipitation-cumulative evaporation coordinate system. The four derived expressions of critical lines correspond to four drought grades, namely, mild drought, moderate drought, severe drought, and extreme drought, respectively. Through a forward daily rolling calculation, the point where the cumulative precipitation and cumulative evaporation coordinates are farthest from their respective critical line is identified as the maximum distance point for that critical line. The drought level is determined by the coordinate point that is located in the highest drought level region and has the maximum distance from the nearest lower critical line. The drought index is then constructed based on the distances from this coordinate point to each critical line. The drought index established in this study reflects the level of soil drought. The drought index calculated using the soil drought level distance index model presented in this paper is compared with soil moisture measurement data and drought disaster records. The results indicate that the variations in the drought index presented in this paper exhibit a high negative correlation with changes in soil moisture and a strong consistency with actual drought impacts. The drought index proposed in this study possesses advantages such as clear physical meaning, convenient calculation, and short response time scale, making it of great significance for practical drought monitoring operations.
Based on meteorological and hydrological observation and NCEP/NCAR (National Center for Environmental Prediction/National Center for Atmospheric Research) reanalysis data, the variation characteristics of compound dry and hot events, and their causes and impact on runoff in the main confluence area of the upper Yellow River in summer were analyzed. The results show that, from the average spatial distribution, the number of high temperature days in summer increases gradually from southwest to northeast in the main confluence area of the upper Yellow River, and the opposite is true for summer precipitation. From temporal and spatial distribution, the number of summer high temperature days in the main confluence area of the upper Yellow River has been increasing consistently since 1961. The precipitation shows fluctuating consistently changes from a long-term trend perspective, but increased consistently after 2000. The compound dry-hot events have increased significantly since this century. From the perspective of multi-time scale changes, the summer compound dry-hot events in the main confluence area of the upper Yellow River exist mainly inter-annual changes and trend changes, and the significant increase of summer compound dry-hot events since 2000 is mainly caused by trend changes. From the perspective of influencing factors, the changes of summer compound dry-hot events in the main confluence areas of the upper Yellow River are mainly influenced by multiple circulation factors, but the influencing factors differ greatly on different time scales. On the inter-annual scale, the influence of westerly circulation, East Asian summer monsoon, South Asian summer monsoon, plateau summer monsoon, and north wind circulation is relatively weak, on the inter-decadal scale, compound dry-hot evencs are mainly influenced by the Tibetan Plateau summer monsoon and they are also influenced by the westerly circulation, the East Asian summer monsoon, the South Asian summer monsoon, the Tibetan Plateau summer monsoon and the north wind circulation on the multi-decadal scale. From the background field of large-scale circulation, the West Pacific subtropical high is stronger and westward, the lack of abnormal water vapor transport in the southwest and the abnormal downward motion in the vertical field are the main reasons for the increase of summer compound dry-hot events in the main confluence area of the upper Yellow River since this century. The increase of compound dry-hot events in the main confluence area of the upper Yellow River will reduce the runoff of Lanzhou station in the basin, the main reason for the increase in runoff of the Yellow River Lanzhou section since 1998 is the increase in precipitation.
The study of the evolution characteristics of agricultural drought disasters and their relationship with climatic factors can identify the critical periods of the impact of climatic factors on drought disasters, reduce the losses caused by drought disasters effectively, and improve the technology and management level of drought disaster risk assessment. By using agricultural drought disaster data, precipitation, average temperature, and effective irrigation area data, this paper analyzed spatiotemporal distribution characteristics of agricultural drought disasters, studied the relationship between agricultural drought disasters and climatic factors, and constructed a drought disaster assessment model based on multiple climatic factors during the critical periods. The results show that from 1978 to 2022, the agricultural drought disaster-affected rate and disaster-damaged rate in Yunnan both showed a decreasing trend. The decreasing rate was 0.49% and 0.09% per 10 years for the agricultural drought disaster-affected rate and disaster-damaged rate, respectively. The agricultural drought disaster-affected rate and disaster-damaged rate experienced two abrupt changes from 1978 to 2022: an increase from less to more in 2004 and a decrease from more to less in 2013. The drought disaster was relatively severe from 2005 to 2013. The comprehensive drought loss rate in 14 prefectures (cities) was 2%-6% higher than the average from 1996 to 2022. From 2014 to 2022, the severity of agricultural drought disaster was lower than the average level from 1996 to 2022 in most areas in Yunnan. Precipitation in May, average temperature in May, and meteorological drought from January to March and from May to September are crucial climate factors to agricultural drought disaster losses in Yunnan, which impact more significantly than those of similar climatic elements at the annual scale. After 2014, precipitation in May generally decreased, average temperature in May and meteorological drought from January to March and from May to September were intensified. However, the agricultural drought disaster in Yunnan was relatively less severe than the average from 1996 to 2022. The one of important reason was that the number of water conservancy facilities increased, the effective irrigation area increased, and the ability of drought disaster prevention and mitigation was enhanced. Based on multiple climatic factors during the critical period, the fitted model has been built, which has a better estimation of agricultural drought disasters in Yunnan and has a better fitting relationship than that with similar climatic elements at the annual scale.
In order to explore whether the deep plateau vortex and southwest vortex system can trigger the material exchange between the atmospheric stratosphere-troposphere, based on the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis data, the dynamic structure, the cross-tropopause mass flux and the trajectory of the gas block during the deep southwest vortex on June 30, 2013 and the plateau vortex on July 23, 2010 were analyzed in detail. The results show that: The deep southwest vortex and plateau vortex develop significantly in the vertical direction, and their internal updrafts develop exceptionally strongly, reaching up to 100 hPa and crossing the troposphere into the stratosphere. There is a strong mass exchange between the stratosphere and troposphere during the formation and mature stages of the vortexes, and the vertical transport term contributes the most to the total flux. The unusually strong upward motion of the low vortex causes part of the tropospheric air mass to be transported to the stratosphere. These results reveal some new important facts: The deep southwest vortex and plateau vortex can cause the mass exchange of the stratosphere-troposphere, and may be a new transport window for the troposphere-stratosphere atmospheric exchange.
The changes of low-level wind field play an important role in the formation of sudden precipitation, which can change the flow field structure in the lower atmosphere, thereby affect the stability and vertical movement of the lower atmosphere and promote the development of convective clouds. Based on wind profile radar data at Chang’an Station, observation data, the fifth generation atmospheric reanalysis data released by the European Center for Medium Range Weather Forecasting, and Doppler radar data, this study analyzed the evolution characteristics of the low-level wind field during three typical sudden precipitation events under the control of the subtropical high at the northern foothills of the Qinling Mountains in midsummer. These events occurred on August 6, 2023, from 11:00 to 12:00 (referred to as “Process I”), July 13, 2023, from 00:00 to 01:00 (referred to as “Process II”), and August 3, 2022, from 18:00 to 19:00 (referred to as “Process III”). The results show that all three events occurred under the circulation background of the subtropical high combined with the intrusion of cold air at low level, exhibiting strong suddenness. For Process I and Process II, the intrusion of cold air at low level was characterized by westerly winds, while for Process III, it was characterized by easterly winds. Before the precipitation, the atmosphere over the Chang’an region was in a significantly unstable state, with weak vertical wind shear in the middle troposphere, which was the main reason for the highly localized nature of these three precipitation events. In midsummer, the multi-year average low-level wind speed at Chang’an Station generally exhibited a single-peak pattern, the wind speed initially increased with height and then decreased. The average wind speed below an altitude of 1 000 meters did not exceed 3.14 m·s?1, and the hourly wind speed shows distinct diurnal variation characteristics. The low-level wind direction displayed a counterclockwise rotation with increasing height, shifting gradually from southwesterly to southeasterly winds.The 4-6 h before the occurrence of three sudden precipitation processes, there was a cold air intrusion process in the low-level over Chang’an, and the wind speed was significantly bigger than the multi-year average. With the continuous invasion of low-level cold air, the 2 m temperature decreased rapidly, the air pressure rose, convection was triggered, and heavy precipitation occurred. The continuous intrusion of low-level cold air could generate strong mesoscale frontogenesis in the lower atmosphere, providing energy and triggering conditions for sudden precipitation. On the other hand, due to the obstruction of the local terrain at the northern foot of the Qinling Mountains and the Guanzhong Basin, the low-level cold air was forced to rise, promoting an increase in precipitation.
Understanding the variation characteristics and possible causes of lake-land breeze speed in Dongting Lake under large-scale climate background is of reference significance for the development and utilization of wind energy resources，water transportation and disaster prevention and reduction. The hourly wind direction and wind speed data at Yueyang Station from 1954 to 2021 are used to analyze the wind speed characteristics and long-term trends of the shoreline and lake-land wind by using the linear tendency estimation，Mann-Kendall mutation detection and sliding t-test methods. The results show that about the wind direction and wind speed characteristics of the four times，02：00，08：00 and 20：00 （Beijing time，the same as below） were almost the same，and the easterly wind and land breeze were dominant，while the westerly wind and lake breeze prevailed at 14：00，and the wind speed at 14：00 was significantly higher than other times. At the inter-annual scale，the variation trend of lakeshore and lake-land wind speed was consistent at four times，both the wind speeds at four times and the land breezes at 02：00，08：00 and 20：00 all showed an obvious decreasing trend，while the change of the lake breeze at 14：00 was not significant. For monthly scale，the wind speed peaks at 02：00，08：00 and 14：00 were all in July，and at 14：00 in winter months land breezes were dominated，and in other months it was dominated by lake breezes. It was dominated by land breezes at 02：00，08：00 and 20：00 in each month，and lake breeze speed was the highest in flood season throughout the year. On the hourly scale，the variation trend of hourly wind speed from 2004 to 2021 was basically the same as that of the lake-land breezes. The frequency of the land breezes was higher than that of the lake breezes，and the wind speed was larger and lake breeze was dominated from 10：00 to 17：00，and the lake breezes with a duration of 3 to 10 hours occurred from 09：00 to 18：00 in each month. The mutation tests find that the wind speed at four times decreased significantly from the 1970s，but only at 14：00 it changed abruptly around 1970，and the abrupt changes of the lake-land breeze speeds at four times occurred in 2002 or 2003. The increase of lake breeze speed and short-term wind speed caused by the change of detection environment can not change the long-term decrease trend of wind speed，and the change of instruments has little effect on wind speed and lake-land wind speed.
High temperature weather occurs frequently in Shandong Province. Studying the variation law of different high temperature indexes and their circulation background is conducive to better defense against the adverse effects of high temperature weather. Based on the daily meteorological observation data of daily maximum temperature, daily minimum temperature and relative humidity of 122 national meteorological stations in Shandong Province from 1981 to 2022, the variation characteristics of high temperature weather in Shandong Province were studied by analyzing the annual maximum temperature, high temperature days, warm night days, and high temperature heat wave days, and the circulation situations of typical high temperature heat wave processes were analyzed. The results show that high temperature weather mainly occurs in the central and western regions of Shandong, and less in the peninsula area. The annual maximum temperature is higher in the west and lower in the east, and high temperature days, warm night days and high temperature heat wave days are characterize by the distribution of more in the west and less in the east. The annual maximum temperature, high temperature days, warm night days and high temperature heat wave days with different grades in Shandong Province show an increasing trend, and the increase range is larger in the west and smaller in the east. The increase range of annual high temperature heat wave days with different grades is mild, moderate and severe in descending order. The wet type high temperature heat wave weather is dominant and shows a significant increasing trend. When there is an obvious circulation situation of “two ridges and two troughs” or “two ridges and one trough” in the middle and high latitudes, it is prone to high temperature and heat wave weathers in Shandong Province, and the subtropical high pressure over the northwest Pacific Ocean is abnormally strong and northerly, and the downdraft under continuous fine weathers causes the temperature to rise and high temperature persist.
The study on the causes of extreme hot weather in Zhejiang Province can provide scientific basis for the prediction and prevention of extreme hot weather. Based on conventional meteorological observations, the ERA5 reanalysis data from European Centre for Medium-Range Weather Forecasts, the outgoing long-wave radiation (OLR) data from National Oceanic and Atmospheric Administration, and daily climate system monitoring indices from the National Climate Center, this study compares and analyzes the spatial and temporal distribution and characteristics of two extreme high temperature events in Zhejiang Province in the summers of 2013 and 2022, and constructs a conceptual model of extreme hot weather. The results indicate that both hot events were characterized by wide coverage, long duration, strong extremity, and severe impacts. The high temperature event in 2022 had a longer duration and wider coverage area, while the single station extreme high temperature value in 2013 was higher. The western Pacific subtropical high (WPSH) in 2022 had a larger extent with a more westward ridge point, whereas in 2013, the duration of the 850 hPa warm ridge temperature exceeding 24 ℃ was longer. The center of extreme high temperature in 2022 was located farther south, with temperatures above 42 ℃ mainly occurring in central and southern Zhejiang, whereas in 2013, extreme high temperatures were concentrated in central and northern Zhejiang. The intensification and westward shift of the WPSH were the direct causes of both extreme high temperature events, corresponding well to a stronger and eastward-displaced South Asian high. When the 1 252 dagpm contour at 200 hPa extends eastward beyond 122°E, the 588 dagpm contour at 500 hPa extends westward beyond 115°E, the 850 hPa warm ridge temperature exceeds 22 ℃, and weak southerly winds prevail in the lower troposphere, it is more likely to experience extreme high temperature exceeding 40 ℃ in Zhejiang. Additionally, negative OLR anomalies over the Maritime Continent (MC) enhance the WPSH by modulating meridional vertical circulation and low-frequency wave propagation. Meanwhile, anomalous zonal vertical circulation between the Pacific at the same latitude as Zhejiang and eastern China further strengthens the WPSH. These findings provide scientific insights for predicting and mitigating extreme heat events in Zhejiang Province.
The National Stadium (Bird’s Nest), Beijing Olympic Park (Osen) and the National Olympic Sports Center (Oti) within a 3 km radius represent three distinct near-ground meteorological detection environments: the dense building sports venue area, the natural underlying surface, and the vicinity of large sports venues, respectively. Using hourly observation data of temperature, rainfall, wind speed and wind direction from meteorological stations in the Bird’s Nest, Osen, and Oti areas from 2020 to 2021, this study conducted a detailed analysis of temperature characteristics in these different environments. Results show that the annual average temperature at the Oti and the Bird’s Nest seat areas was 1.3 and 2.1 ℃ higher than that at the Osen. The temperature differences were more pronounced during the winter half-year than in the summer half-year. Within the Bird’s Nest, the monthly average temperature of the seat area was generally higher than that of the canopy, with the east seats being 0.2 ℃ warmer than the west seats. This difference was closely related to the shading effects of the semi-closed canopy structure, which showed a strong correspondence with changes in the solar elevation angle. The hourly average temperature trends at Oti and Osen were largely consistent, but the temperature changes more rapidly at Osen. The study revealed that temperature differences between stations were associated with variations in wind speed and precipitation conditions.
With the acceleration of climate warming and urbanization, the heat island effect is intensifying. How to take effective measures to mitigate the heat island effect on the basis of quantitative evaluation had become an urgent problem to be solved. Based on annual average temperature of national meteorological stations in Yinchuan and its surroundings from 1961 to 2022 and the data of Landsat8 in summer from 2017 to 2021, the relationship between heat island intensity and urbanization process in Yinchuan urban area was analyzed. The typical high temperature processes were selected, and the cooling effect of different buffer distances of lake wetland was evaluated quantitatively aiming at the lake wetland which could effectively alleviate the urban thermal environment. The results show that the effect of urban heat island was not obvious during 1961-2000 in Yinchuan, but after 2000, the average temperature and warming rate were significantly higher than those of reference stations, indicating that the effect of urban heat island was obvious. The correlation coefficient between heat island intensity and urbanization level reached 0.78, which passed the 0.01 significance test and showed a positive correlation. The typical lake wetland in Yinchuan urban area had obvious cooling effect during the summer high temperature processes, especially the surface temperature cooling effect within 500 m, in which the surface temperature dropped the most (up to 2.5 ℃) within 100 m, and the cooling range gradually decreased with the increase of distance from the water body. The average land surface temperature drops by 0.5 ℃ in the distance of 400-500 m.
An in-depth understanding of the daily variation characteristics of precipitation at classified stations is essential for optimizing and improving accurate forecasting methods. Based on the hourly precipitation observation data from 105 national meteorological stations in Xinjiang during the warm season (May to September) from 2010 to 2019, the stations were classified using the K-means clustering method, and the precipitation characteristics of each category of stations were analyzed according to hourly average precipitation amount, precipitation frequency, and precipitation intensity. The results show that the stations in Xinjiang can be classified into four categories: southern Xinjiang and desert areas (Class I), Tianshan Mountains (Class II), northern Xinjiang and the southern slope of the West Tianshan Mountains (Class III), and valley areas (Class IV). The clustering result is similar to that of classifications based on geographic location and topographic height but it is more detailed and scientific. The distribution of cumulative precipitation and precipitation hours of four types of stations is relatively concentrated, with annual average cumulative precipitation of 54, 354, 110, and 217 mm, corresponding to 67, 311, 118, and 213 hours, respectively. The diurnal variation of precipitation frequency in the warm season at most stations in Xinjiang follows a ‘single-peak’ pattern, but the times of the peaks and valleys vary depending on altitude differences. Precipitation and precipitation intensity generally follow a ‘multi-peak’ structure, and hourly precipitation intensity greater than 1 mm mainly concentrated at Class II stations, with the peaks occurring at 16:00—17:00. Precipitation and precipitation frequency are highest in June and lowest in September. The monthly distribution and the month-to-month diurnal variation characteristics of precipitation intensity differ significantly, but the peak intensity does not show a notable difference. In 2016, all three precipitation indicators characteristics during the warm season were significantly higher than those of other years. In 2010, Class I stations had the highest precipitation intensity, and the precipitation amount and frequency reached a secondary peak, while the changes at the other categories of stations were relatively small.
Studying the evolution characteristics of a gust front and the physical mechanisms of extreme winds behind it using multiple types of radar products is of great reference significance for improving the forecasting and early warning capabilities of catastrophic gale weather. Using conventional upper-air and surface observational data, ERA5 reanalysis data of the European Center for Medium-Range Weather Forecasts, S-band dual-polarization radar data and X-band phased-array radar data, the characteristics of radar products of a gust front and the extreme wind process behind the gust front in Shaoxing of Zhejiang Province on July 10, 2023 were analyzed. The results show that this process occurred under the background of southwest airflow at both high and low altitudes. The upper air was at the edge of the subtropical high, and at 925 hPa, it was in the convergence area of southwest wind speeds. The atmospheric thermal instability and uplift conditions were better. After multiple convective cells merged into a multi-cell storm, the gust front was formed at the outflow boundary of it. The gust front underwent three stages: development, rupture, and weakening. At the weakening stage, a new mesoscale convective zone was triggered behind it, and the backward propagation characteristics were obvious. The maximum wind speed induced by the gust front occurred during its weakening stage, while the extreme wind of the process occurred during the eastward movement and northward lifting of the mesoscale convective band triggered by the gust front. The internal vortex structure of the convective cells which generated the extreme winds only existed at an altitude of 800 m, and the convergence of wind direction and speed was mainly at the middle and upper levels. The gusts of 6-7 levels were generated when the vortex circulation weakened and disappeared, and the core of the reflectivity factor decreased, and the lower levels of the storm turned into downdraft. The extreme wind was generated later when the inflow behind the storm turned back into updraft and converged with the downdraft at middle levels. It was also accompanied by radial convergence in the middle layer horizontally, which indicated an increase in sinking airflow. Due to the relatively small contribution of downward momentum transfer, the extreme wind was mainly caused by strong sinking airflow.
Real-time identification of cloud microphysical characteristics and seeding ability of stratiform cloud precipitation system is beneficial for improving the understanding of the catalytic potential of stratiform cloud precipitation system, and providing technical support for real-time identification of artificial rainfall enhancement. The microphysical characteristics and the seeding ability of stratiform cloud in the middle of Inner Mongolia were analyzed by using airborne detection data of 8 stratiform cloud aircraft operations from 2018 to 2019. The results show that the occurring frequencies of cloud water, liquid water, and supercooled water in stratiform clouds are 59.97%, 82.99% and 70.84%, respectively. The liquid water content is mainly concentrated between 0.001 and 0.100 g·m-3, while the supercooled water content is mainly distributed between 0.010 and 0.100 g·m-3, which indicates good potential for crystal seeding catalysis. The average number concentration of large cloud particles is 8 cm-3, and the number concentration more than 20 cm-3 accounted for 14.10%. The small cloud particle number concentration is 20 cm-3 on average, and the number concentration greater than 20 cm-3 accounted for 28.54%. More than 70% cloud particles are located in the negative temperature region, and the particle number concentration is generally small. When the number concentration of small cloud particle reaches 15 cm-3, the cloud region has certain seeding ability, while when the number concentration of large cloud particle is less than 10 cm-3, the cloud region has highly seeding ability.
In the context of climate change, it is of great significance to explore how winter wheat responds to temperature increase in terms of main agronomic traits and dry matter distribution during its growth stage in the tablelands of Guanzhong, Shaanxi, which can provide references for assessing its sensitivity and adaptability under climate change. In this paper, Zhengmai No. 1860 was selected as the research object to analyze the influence of the warming effect simulated by Open Top Chamber (OTC) on the growth and development of winter wheat. The results show that there were significant differences in temperature inside and outside OTC during growth stage (p<0.05) of winter wheat. The average temperature in OTC was 0.8 ℃ higher than that of the outside. In OTC, all the phenological stages of winter wheat came in advance of those in the control group, with an average of 6 days earlier. The plant height inside OTC was higher than that of the outside. OTC promoted leaf area of wheat at the stage of greening and heading, while inhibited leaf area at the stage of flowering and milk ripening.The warming effect promoted root growth, increasing root length by 37.48% and surface area of fine roots by 35.28%, but inhibited root biomass, decreasing it by 7.60%. The warming effect of OTC can promote the dry matter weight of stem, leaf and ear, except the dry matter weight of leaf at milk-ripening stage. In conclusion, OTC can significantly promote the functional traits of winter wheat at the vegetative growth stage, but at the reproductive growth stage, it can not promote the functional traits and even show a certain inhibitory effect.
In order to assess the potential impact of climate change on agriculture and develop scientific adaptation strategies, the variability characteristics and risk on agriculture of regional high temperature, drought and their compound events in Hubei Province were identified and analyzed based on daily temperature, precipitation and other observations from 76 national meteorological stations during 1994-2023. The analysis employed classification standards for regional high temperature process and monitoring and assessment methods for drought process. The results show that regional high temperature events occurred an average of 4.3 times per year, with an overall increasing trend and 61.2% of severe and strong events occurred in July and August. Regional drought events occurred an average of 1.5 times per year, showing a deceasing trend before 2010 and increasing trend thereafter, with slightly higher frequencies in winter and spring than in summer and autumn. Regional compound high temperature and drought events mainly occurred from June to August, with an significant increase in frequency after 2010. The spatial distribution of intensity and agricultural risk for regional high temperature and drought events was generally similar. High intensity and high risk areas for high temperature events were mainly located in eastern Hubei, while low intensity areas were in the southwest. For drought events, high intensity and high risk areas were mainly located in central-eastern Hubei, decreasing towards surroundings regions. The agricultural risk of compound high temperature and drought events showed a decreasing trend from east to west. The most widely distributed risk levels for regional high temperature, drought and their compound events were classified as high-risk, moderate-risk and extreme high-risk areas, accounting for 37.6%, 53.8% and 46.6% of Hubei Province’s total area, respectively. In the background of global warming and increasing frequency of extreme weather events, the probability and risk of regional extreme high temperature, drought and their compound events are expected to rise in eastern Hubei Province.
Compound high-temperature and drought events is one of the complex extreme climate events with high incidence in the Shiyang River Basin, which has more serious impact on industrial and agricultural production and ecological environment than a single extreme climate event. Based on the average temperature, maximum temperature and precipitation data of five meteorological stations in the Shiyang River Basin from 1961 to 2023, the compound high-temperature and drought events were identified and determined using percentile threshold method and Ped meteorological drought index, and spatial and temporal evolution characteristics of compound high-temperature and drought events were analyzed with linear trend method. The results show that the spatial difference of annual average frequency of compound high-temperature and drought events was small in the Shiyang River Basin, however, the spatial difference of compound high-temperature and drought events frequency was obvious in each decade and increased decade by decade. Annual frequency of compound high-temperature and drought events decreased first and then increased in the Shiyang River Basin, it decreased before 1996 and then increased in the whole basin. Compound high-temperature and drought events mainly occurred from June to August, and the most occurred in July. The frequency of compound high-temperature and drought events with different grades changed greatly, with the increase of drought grade, the frequency of drought increased first and then decreased, the frequency of medium drought was the highest, and the frequency of extreme drought was the least.
Studying the spatial and temporal distribution characteristics of drought at different scales in Chengdu is of great significance to agriculture, economic development and drought disaster prevention and mitigation in this region. Using the monthly precipitation data of 14 national meteorological stations in Chengdu from 1960 to 2022, combining the standardized precipitation index (SPI), the optimal probability distributions of the series of annual and seasonal precipitation data of 14 national meteorological stations in Chengdu were determined based on the optimization of probability distributions belonging to SciPy package firstly. Secondly, based on the optimal probability distribution function, the annual scale SPI (SPI12) and the seasonal scale SPI (SPI3) were calculated, respectively. Finally, based on SPI12 and SPI3, the spatio-temporal distribution characteristics of drought at the annual and seasonal scales in Chengdu were analyzed. The results show that the optimal probability distributions of precipitation series at different scales past the K-S test at significant level of α=0.05, representing the distribution characteristics of precipitation series at different scales in Chengdu. The annual and seasonal drought station ratio, as well as the drought intensity in Chengdu, show a slight increasing trend. The drought frequency at annual and seasonal scales in Chengdu range from 25.40% to 36.51%. There are significant differences in the spatial distribution of drought frequencies at different scales, with spring and summer droughts occurring slightly more frequently compared to autumn and winter droughts. The spatial distribution of different grades of annual drought, spring drought, summer drought, autumn drought, and winter drought in 14 districts and counties of Chengdu show considerable variability, but light and moderate droughts occur with the higher frequency in all cases.
Typhoon “Rumbia” was the most disastrous tropical cyclone, triggering rare floods in Shandong. Based on the conventional meteorological observation data, the reanalysis data from the National Centers for Environmental Prediction, and the precipitation data from automatic weather stations, the frontogenesis mechanism of an extreme rainstorm in Shandong Province caused by Typhoon“Rumbia” from 17 to 20 August 2018 was studied in this paper. The results indicate that the precipitation affected by Typhoon “Rumbia”can be divided into three stages: the precipitation of the typhoon outer cloud system, the precipitation of the interaction of the middle and low latitude weather systems and the precipitation triggered by the typhoon trough. The frontogenesis area of the typhoon rainstorm is mainly in the lower level, and the location of the frontogenesis area is closely related to the location of the cold air. The locations of the heavy precipitation are consistent with the frontogenesis area. The large-value center of frontogenesis intensity corresponds well to the center of the heavy rainstorm, and the intensity of frontogenesis can well indicate the rainfall in the next 6 hours. Favorable convergence flow field on the south side of the dense area of pseudo-equivalent potential temperature (θse) lines was the key to cause frontogenesis. The location of the elongation deformation frontogenesis is consistent with the convergence center of the divergence, the large value center of θse and the total frontogenesis area in this rainstorm process is consistent. The elongation deformation term, shear deformation term and divergence term all contribute positively to the total frontogenesis. The typhoon rainstorm is caused by frontogenerative dynamics, and the area with the strongest ascending motion of the frontal secondary circulation corresponds to the area of the strongest rainstorm. Under the conditions of strong water vapor transport, convergence and strong convective instability, the convergence of typhoon trough and strong frontogenic secondary circulation together produce strong upward movement, and the dynamic uplift effect is rapidly enhanced, resulting in strong convergence of water vapor and transport to the upper level and causing extremely heavy rain in Shandong Province. The rainstorm area is located at the 700 hPa positive helicity center and its right side, and the period of rapid enhancement of positive helicity corresponds to the period of heavy precipitation, and the maximum value center of positive helicity moves down to the vicinity of 900 hPa, which indicates the weakening of typhoon heavy precipitation.
Studying the distribution of convection during the warm season over Fujian Province has important significance for forecasting and warning of convective weather. Based on four weather radars data of Jianyan, Longyan, Changle and Xiamen in Fujian from 2008 to 2017, convection in warm season (from April to August) was identified, and the spatial-temporal and vertical structure of convection, as well as spatial and diurnal variation characteristics of convective systems with different areas and extension heights were analyzed. The results show that the peak period of convective activity is from May to June and August during the warm season in Fujian. The convection in warm season has obvious regional distribution characteristics, the high incidence area is located in the inland from April to June, and from July to August it is in the coastal mountains. From June to August, it is dominated by medium-deep convection, and it is dominated by large area convection in April, and from May to June and from July to August it is dominated by medium and large area convection, medium and small area convection, respectively. The monthly distribution of vertical structure of convection is different. The echo intensity of moderate convection is the maximum from July to August. The echo extension height in northwest Fujian is the highest. There is obvious diurnal variation of convective frequency. Convection often occurs from 15:00 to16:00 (BST), and it is the mainly single peak in the afternoon from July to August, while from April to May there are bimodal or multi-peak. The frequency peaks of convective systems with larger area appear later, and the mid-deep and deep convective peaks are obvious in the afternoon. From May to June, the high incidence of inland convection from night to morning was caused by the diurnal variation of the wind field formed by the inertia oscillation of the boundary layer and the interaction between the disturbing wind and terrain. In August, the high incidence of convection in coastal mountains was caused by the fact that the coastal mountains are located in the near ground heating center, the convergence of wind field and the high energy value area in the afternoon.
As one of the main driving factors of evapotranspiration, the VPD (Vapor Pressure Deficit) reflects the atmospheric capacity to extract water from the surface. Understanding the spatio-temporal variation of VPD is crucial for exploring the response of regional atmospheric dryness and wetness to climate change. Based on data from 11 meteorological stations in the Mt. Qomolangma region of China during 1981-2023, including monthly sunshine duration, average air temperature, maximum and minimum air temperature, precipitation, relative humidity, vapor pressure, and wind speed, this study analyzed the spatio-temporal characteristics and influencing factors of VPD using climate tendency rate, stepwise regression, and the Mann-Kendall test. Results show that the annual and seasonal averages of VPD in the Mt. Qomolangma region generally exhibited lower values in the southwest and higher values in the northeast. Monthly VPD showed a bimodal pattern, with peaks in June and September and a minimum in January. Seasonally, VPD was characterized by higher values in summer, followed by spring, autumn, and the lowest in winter. Over the past 43 years, annual VPD increased at a rate of 0.029 kPa·(10 a)-1, with the most significant growth observed in summer. On a decadal scale, VPD values were relatively low in the 1980s and 1990s, particularly during the 1990s. In the 2000s, VPD was lower in spring and autumn and higher in summer and winter. The 2010s saw elevated VPD values across all seasons, especially in summer and autumn. Spring and flood season VPD mutations occurred in the late 2000s, while mutations in the other three seasons and annual averages appeared in the early 2010s. Changes in VPD were primarily driven by saturated water vapor pressure, particularly in spring and autumn. The significant increase in VPD was dominated by rising air temperatures across all seasons and annually. Additionally, the decrease in water vapor pressure during the flood season contributed to the VPD increase.
The use of different climatological normal periods means the change of the evaluation results of meteorological elements, the abnormal state of climate events and their change characteristics, which have a substantial impact on the climate monitoring and prediction operations. Using temperature and precipitation observation data from national meteorological stations in Ningxia from 1981 to 2021, this study conducted a comparative analysis of the temperature and precipitation characteristics between the old climatological normal period (1981-2010) and the new climatological normal period (1991-2020). Additionally, it explored the changes in extreme characteristics of these factors. The results are as follows: Compared to the old climatological normal period, the annual and seasonal average temperatures in Ningxia are generally higher in the new climatological normal period, which is particularly evident in spring, summer and winter, and the frequency of abnormally high (low) temperatures increases (decreases) accordingly. Yinchuan, the western part of Wuzhong, and the northern part of Zhongwei are areas experiencing significant temperature increase. The overall intensity of extreme high (low) temperature has intensified (weakened), and their frequency has increased (decreased). In summer, the threshold and intensity of extreme high-temperature rise across various regions, especially in the central and northern areas, while in winter, the intensity of extreme low-temperature weakens in most regions, the amplitude of extreme low-temperature varies significantly. The average annual precipitation, as well as the summer, autumn and winter average precipitation, are greater in the new climatological normal period compared to the old. There’s an increased frequency of abnormally more precipitation in summer and autumn, whereas the opposite trend is observed in spring and winter. Meanwhile, the frequency of abnormally less precipitation in all seasons has decreased to some extents. There are significant spatial differences in seasonal precipitation, with a general increase in precipitation in summer and autumn, and a pattern of “decreasing in the north and increasing in the south” in spring and winter. The overall trend of extreme precipitation in spring and autumn (summer and winter) is intensifying (weakening), albeit with fewer (more) extreme precipitation events. In summer, the threshold and intensity variations of extreme precipitation are greater in the north and south, and smaller in the central region, with a notable increase in extreme precipitation in Shizuishan.
In order to deepen the understanding of extreme snowfall and reveal the possibility of anomalous influencing factors leading to extreme snowfall events, the extremes of two major snowfall weather processes in Shanxi Province on February 24, 2021 and from February 27 to March 1 (referred to as “Process I” and “Process II”, respectively) were analyzed by using the meteorological observations and reanalysis data. The results show that Process Ⅰ was a convective snowfall process, caused by the combined influence of a plateau trough, a surface trough and a return flow. The rapid climb of the strong southwest warm and wet jet on the “cold pad” and the symmetric instability together led to the rapid release of potential unstable energy, resulting in a concentrated precipitation range, a large snowfall intensity and a short duration. During this process, cold air quickly invaded and the precipitation phase changed from rain to snow quickly. Process Ⅱ, on the other hand, was characterized primarily by stability, which was influenced by an upper-level trough, a surface cyclone and an inverted trough. During the systematic invasion of cold air, an extreme snowfall event was formed, with a large area of precipitation and a prolonged duration, and the phase changes during this snowfall process were complex. Significant differences were found in the circulation patterns, moisture transport mechanisms, instability mechanisms, and vertical motion characteristics before the precipitation of two snowfall processes. However, compared to the climatological averages, both processes exhibited anomalously high local relative humidity, 700 hPa energy, and vertical upward motion, which was identified as one of the key reasons for the occurrence of extreme weather. The precipitation centers of both events were located in the downstream of the anomalous physical quantity centers 6 to 12 hours before precipitation, and the moisture transport and the thickening of the moist layer were also provided some indication for the precipitation starting time. Additionally, the transition of precipitation phase was closely related to the vertical distribution of temperature and frontal structure.
Moving out of the Qinghai-Xizang Plateau vortex (QXPV) often causes a wide range of disastrous weather such as heavy rain in the lower reaches of the Qinghai-Xizang Plateau. Aiming at the question of why the moving-out QXPV (MQXPV) develops or weakens after moving out of the Qinghai-Xizang Plateau, based on the QXPV database in warm seasons (from May to September) during 1990-2019, the ERA5 reanalysis data from the European Centre for Medium-Range Weather Forecasts (ECMWF) and the 3B42 precipitation data retrieved from the Tropical Rainfall Measuring Mission (TRMM) satellite, this paper discusses the occurrence rules, circulation characteristics, the differences and similarities of the development mechanisms of the two typical cases of the moving out type and the weakening type by using meteorological statistical analysis, synoptic analysis and diagnostic analysis methods. The results show that in the past 30 years, the frequency of the MQXPV showed a weak increasing trend, with an average of 6.5 times per year, and in May it was the largest. It can be divided into the western vortex, the central vortex and the eastern vortex according to the birth location, the western vortex was mainly concentrated in Shiquan River and the north of Gerze, the central vortex was mainly concentrated in the north of Shenzha and the Tuotuo River, and the eastern vortex was mainly located in Zadoi and Qumalai, among them the eastern vortex occurred most and the moving path was mainly eastward. The 200 hPa south Asian high had a large range and strong intensity, which strengthened the horizontal divergence in the upper layer and vertical ascending motion. The 500 hPa western Pacific subtropical high was weaker, and its blocking effect on the movement of the QXPV was weakened, which resulted in the eastward movement of the QXPV. The MQXPV includes two types: developing and weakening. The comparative analysis shows that during the eastward movement of the QXPV, for the developing vortex, the convergence of cold and warm advection was clear and the frontogenesis enhanced gradually, and the potential vorticity presented a zonal distribution from southwest to northeast in the horizontal direction, with an eastward transmission phenomenon, and in the vertical direction, the upper positive potential vorticity passed down obviously. For the weakening vortex, it maintained a warm heart structure, after moving out of the Qinghai-Xizang Plateau, the weakening vortex gradually separated from the large value region of the potential vorticity, and the upper positive potential vorticity passing downward tended to be less obvious. The precipitation caused by the developing vortex appeared in the center and south side of the vortex, the precipitation intensity was stronger and the range was larger, while the intensity was weaker and the range was smaller of the precipitation caused by the weakening vortex.
It is of great significance to study the near-cloud turbulence for improving the prediction ability of aircraft turbulence and ensuring the safety of air transportation. The WRF (Weather Research and Forecasting) model v4.3.1 is used to conduct a high-resolution numerical simulation of a moderate intensity near-cloud turbulence event over Fujian Province, China. The synoptic-scale background and turbulence indices are examined, and the causes of the turbulence event are analyzed. A sensitivity experiment excluding the moist process is conducted to investigate the impact of cloud system evolution on the turbulence generation. The results indicate that the turbulence event was mainly influenced by the low-level clouds area on the periphery of the cold high-pressure system over the southeast coastal region of China. The upper level southern jet stream gradually moved eastward above the turbulence region, and vertical wind shear enhanced and tropopause folding phenomenon was observed. The high-resolution simulation reasonably reproduced the large-scale circulation during the turbulence event. The turbulence indices, including Ri (Richardson number) and NCSU1 index (Version 1 of North Carolina State University Index), effectively indicated the intensity and location of this turbulence event. Inertial instability and high turbulence kinetic energy (TKE) values were distributed around the cloud region near the turbulence area. Specifically, influenced by stratiform clouds, the increment of zonal wind in the turbulence area gradually increased from south to north and the increment of the meridional wind gradually decreased from west to east, both contributing to negative absolute vorticity. Updrafts near the cloud top affected the local wind field in the turbulence region. Downdrafts mixed with saturated moist air through the cloud top, leading to inertial instability and ultimately causing the turbulence event. In contrast, the TKE in the turbulence region disappeared in the absence of clouds, and the vertical wind shear weakened, both two turbulence indices failed to diagnose the turbulence event.
The study of the impacts of climate change and human activities on the vegetation of the Gansu section of the Yellow River main stream is of significance for the construction of ecological security in the Yellow River Basin. Based on the NDVI (Normalized Difference Vegetation Index) of MODIS (Moderate-resolution Imaging Spectroradiometer) and the precipitation and air temperature data of 18 ground meteorological observation stations in the Gansu section of the Yellow River main stream, the spatial and temporal characteristics of NDVI and the contribution of climate change and human activities to the changes of NDVI in the Gansu section of the Yellow River main stream from 2001 to 2020 were quantitatively analyzed by using the methods of linear trend analysis, bias correlation analysis and residual analysis. The results show that the growth rate of NDVI in the Gansu section of the Yellow River main stream was 0.05·(10 a)-1 from 2001 to 2020, of which the 2001-2010 period was a slow increase stage with a growth rate of 0.04·(10 a)-1, and the 2011-2020 period was a rapid increase stage with a growth rate of 0.08·(10 a)-1. The vegetation ecology of the Gansu section of the Yellow River main stream showed a benign development in the past 20 years, and the vegetation improvement area was located in the north-central part of Linxia Hui Autonomous Prefecture, Lanzhou City and the southeastern part of Baiyin City. The climatic factors that dominate the changes of NDVI in the study area are different, the positive correlation between NDVI and air temperature is higher in most parts of Gannan Prefecture, while the correlation between NDVI and precipitation is more significant in the northern part of Linxia Prefecture, Lanzhou City and Baiyin City. The vegetation change in the Gansu section of the Yellow River Basin is the result of the joint action of climate factors and human activities, and the contribution of climate factors to the NDVI change from 2001 to 2020 is 75.27%, while the contribution of human activities is 24.73%. Climate factors are still the dominant factors for the vegetation change in the Gansu section of the Yellow River Basin, but the influence of human activities on the vegetation change is gradually deepening.
It is of great significance to study the change of vegetation cover and the influence of climate factors in Ganzi Prefecture for ecological protection and development. Based on MODIS (Moderate-resolution Imaging Spectroradiometer) -NDVI (Normalized Difference Vegetation Index) data during 2003-2022, trend analysis, Hurst index were used to analyze the changes in vegetation cover in Ganzi Prefecture over the past twenty years. Temperature and precipitation data which are significantly correlated with vegetation index were used to analyze the relationship between vegetation cover change and climate factors. The results indicated that NDVI in Ganzi Prefecture showed an overall upward trend during the study period, and the distribution of vegetation cover was correlated with altitude and watershed. In the past twenty years, the area of vegetation cover increasing accounted for 67.83% of the total area of Ganzi Prefecture, and 25.37% of the area may maintain the same evolution trend as the present in the future. There are regional differences in the spatial distribution of precipitation and temperature, and the precipitation fluctuates significantly in summer, which is related to the influence of Qinghai-Tibet High Pressure in summer. Compared with precipitation, the correlation between temperature and NDVI is more significant. The influence of precipitation and temperature can explain 70% of the regional vegetation cover change during the study period, and the contribution of temperature to vegetation cover change is higher than that of precipitation.
Soil moisture is an important indicator for monitoring soil drought, and studying its relationship with climate change helps to reveal the mechanisms of soil drought under the context of global change. Based on the Web of Science Core Dataset, this study analyzes the literature on the topic of the relationship between climate change and soil moisture. The results show that the number of related publications from 1988 to 2023 follows a trend of “stability, growth, sharp increase”. Chinese scholars and research institutions contributed the most publications, but their overall international influence remains lower compared to developed countries such as the United States. In terms of subject areas, research is primarily concentrated in environmental science and ecology, earth sciences, as well as agricultural and forestry sciences. Key research focuses include soil-climate interactions, soil ecosystem management, hydrometeorology and soil drought monitoring, as well as climate data analysis and ecological modeling. In recent years, research hotspots have gradually shifted to the application of cutting-edge technologies, such as multi-source sensing and artificial intelligence, in soil drought monitoring. Furthermore, the impact of extreme climate events on various ecosystems and their corresponding response strategies have become important research directions.
