Circulation Configuration Research Articles

Differentiated influences of anomalous subtropical high on extreme persistent precipitation and heatwave events in the Yangtze River Valley

AbstractIn the summers of 2020 and 2022, the Western Pacific Subtropical High (WPSH) intensified extremely and extended westward. However, in summer 2020, the Yangtze River Valley (YRV) witnessed record‐breaking floods, while in 2022, an unprecedented and prolonged heatwave occurred. Distinctly, these two extreme events were caused by different effects of the WPSH: one is enhancement of the transportation of water vapor and the other is adiabatic heating caused by the descending airflow. In June–July 2020, the stable extension of the WPSH ridge line to South China directed a southwesterly airflow along its northwest flank, leading to sustained precipitation in the YRV. Additionally, the midlatitude circulation pattern featured two troughs and two ridges. Such a circulation configuration, combined with the strong and westward WPSH, enabled the continuous southward intrusion of cold air and northward transport of warm moist air, converging over the YRV, and thus influenced extreme persistent precipitation. In contrast, the WPSH covered the YRV almost entirely during summer 2022. Under this influence, the clear‐sky condition and descending airflow through adiabatic warming directly resulted in the heatwave. In addition, local land–atmosphere feedback was crucial in its development and persistence. The soil moisture deficit induced by high temperatures increased the sensible heat flux between the soil and atmosphere upward, further enhanced the surface air temperature and strengthened the heat dry condition.

Spatiotemporal Variation and Causes of Typical Extreme Precipitation Events in Shandong Province over the Last 50 Years

In this study, based on hourly ERA5 reanalysis data from July to September, from 1971 to 2020, for Shandong Province, we used mathematical statistical analysis, the Mann–Kendall nonparametric statistical test, cluster analysis, and other methods to extract and analyze the spatiotemporal variation characteristics and causes of typical extreme precipitation events. The results indicated the following: (1) The total number and duration of precipitation events show a nonsignificant upward trend, while the average and extreme rainfall intensities show a nonsignificant downward trend. (2) Extreme precipitation events are primarily concentrated in Qingdao, Jinan, Heze, and Binzhou, with fewer events occurring in central Shandong Province. (3) Extreme precipitation events are classified into four types (namely, patterns I, II, III, and IV). Pattern I exhibits two rain peaks, with the primary rain peak occurring after the secondary rain peak. Similarly, pattern II also displays two rain peaks, with equivalent rainfall amounts for both peaks. In contrast, pattern III has multiple, evenly distributed rain peaks. Finally, pattern IV shows a rain peak during the first half of the precipitation event. Pattern I has the highest occurrence probability (46%), while pattern IV has the lowest (7%). (4) The spatial distributions of the different rain patterns are similar, with most being found in the eastern coastal and western regions. (5) Extreme precipitation events result from interactions between large-scale circulation configurations and mesoscale convective systems. The strong blocking situation and significant circulation transport at middle and low latitudes in East Asia, along with strong convergent uplift, abnormally high specific humidity, and high-water-vapor convergence centers, play crucial roles in supporting large-scale circulation systems and triggering mesoscale convective systems.

Characteristics of spring Mongolian cyclones in the recent 70 years: Background circulations and weather influences

AbstractBased on the fifth‐generation reanalysis (ERA5) data of the European Centre for Medium Range Weather Forecasts (ECMWF), spring Mongolian cyclones were examined from 1950 to 2023. Then, based on the feature parameters of activity path information, machine learning method (k‐means) is used to classify the trajectories of spring Mongolian cyclone. Subsequently, the atmospheric circulation configurations of the three categories of Mongolian cyclones and their influence on the weather in China verify the reasonability of the classification method. Specifically: (1) eastward moving type, affecting Inner Mongolia and Northeast China (Clus‐1). The circulation background of the Clus‐1 cyclone displays a broad negative geopotential height anomaly centre over East Asia; (2) northeastward moving type, affecting northeast part of Northeast China (Clus‐2). The circulation background of the Clus‐2 cyclones shows a “positive–negative–positive” distribution of circulation anomalies in the European Plain, Lake Baikal and the Sea of Japan; (3) southeastward moving type, this type can move to the south of China (Clus‐3). The circulation background of the Clus‐3 cyclones is opposite to that of the Clus‐2 cyclones. The three categories of Mongolian cyclones will cause dust weather and strong winds in different regions of China, mainly affecting Inner Mongolia and Northeast China.

Causes of the Unprecedented Month‐To‐Month Persistent Extreme Heat Event Over South China in Early Summer 2020: Role of Sea Surface Temperature Anomalies in the Tropical Indo‐Pacific Region

AbstractAn unprecedented month‐to‐month persistent extreme heat event attacked South China in early summer (May–July) 2020, with an extremely and persistently stronger‐than‐normal intensity and higher‐than‐normal number of extreme heat days. The underlying mechanisms were investigated from the month‐to‐month perspective using observations, reanalysis data, and numerical experiments. The results showed that a local anomalous anticyclone in the mid‐lower troposphere over South China and the South China Sea directly contributed to this persistent extreme heat event via anomalous warm advection heating and diabatic warming at near‐surface level. Further investigation revealed that the rapid transition from El Niño to La Niña with subsequent development in the central and eastern tropical Pacific, along with persistent warming in the tropical Indian Ocean in the early summer of 2020, were crucial to the maintenance and reinforcement of the local anomalous anticyclone, with greater importance of the latter. On the one hand, rapid cooling in the central and eastern tropical Pacific enhanced the Walker circulation and further triggered an abnormal local Hadley cell, leading to anomalous subsidence over South China and the South China Sea. On the other hand, the persistent warming in the tropical Indian Ocean resulted in an enhanced and westward‐extended western North Pacific subtropical high via eastward‐propagating Kelvin waves and an intensified and eastward‐extended South Asian high by heating the troposphere. These two aspects of atmospheric circulation configuration favored the maintenance and reinforcement of the local anomalous anticyclone in the mid‐lower troposphere over South China and the South China Sea.

Atmospheric Circulation Anomalies and Key Physical Processes behind Two Categories of Anomalous Eurasian Spring Snowmelt

Abstract Eurasian spring snowmelt plays an important role in the subsequent climate and hydrological cycle, however, the understanding of snowmelt itself and its causes remains insufficient. This study explored the basic characteristics of spring snowmelt in the eastern Europe–western Siberia (EEWS) region by classifying snowmelt anomalies into two categories based on the different factors that dominate spring snowmelt, and then investigated the associated atmospheric circulation anomalies and local physical processes. The first category of anomalous snowmelt (category 1) is controlled by both the initial snow mass and the later snowmelt process, while the second category of anomalous snowmelt (category 2) is mainly linked to the later snowmelt process. Specifically, category 1 is characterized by an anomalous trough in EEWS in winter, where water vapor transported and converged, accompanied by anomalous upward motion, which promotes snowfall and snow accumulation, providing initial conditions conducive to snowmelt. In April, this region is controlled by an anomalous ridge, with significant warm advection anomalies and subsidence promoting surface warming, thereby accelerating snow melting. In contrast, the winter circulation anomalies are insignificant in category 2, while the anomalous ridge in April is stronger than in category 1, accompanied by more intense snowmelt processes. In addition, from the surface energy balance perspective, atmospheric downward sensible heat transport is an important factor influencing the anomalous snowmelt in category 1, while shortwave radiation plays a secondary role. Conversely, the snowmelt in category 2 is dominated by shortwave radiation forcing, but the sensible heat effect is slightly weaker. Significance Statement Eurasian spring snowmelt significantly impacts the subsequent climate and hydrological cycle, but the understanding of snowmelt itself and its causes is still inadequate. The purpose of this study is to explore the monthly evolution of atmospheric circulation associated with anomalous snowmelt and its local physical processes associated by categorizing them based on snowmelt characteristics. Category 1 is jointly affected by winter snow accumulation and later warming, while category 2 is dominated by strong snowmelt process in late spring. These two categories are accompanied by different winter and spring circulation configurations. Our results provide a basis for further investigation of snowmelt precursor signals.

Distinctive Features of Monsoon-TC Joint Rainfall over Western North Pacific and its Relationship with the Maritime Continent Thermal Condition

ABSTRACT The East Asian summer monsoon (EASM) and tropical cyclones (TCs) in the Western North Pacific (WNP) are both responsible for the East Asian summer rainfall, yet most studies only examine their rainfall separately. In this study, the East Asian summer rainfall for the past 39-years (1983-2021, May to September) is divided into three categories: monsoon rainfall without TCs’ influence (monsoon-only rainfall), TC rainfall independent of monsoon (TC-only rainfall) and monsoon-TC joint (MS-TC) rainfall. Compared with the other two categories, MS-TC rainfall exhibits distinctive features. During strong MS-TC years, a distinct cyclonic anomaly centre prevails over tropical WNP with anomalous southeasterlies extending from the tropics to the subtropics. Large rainfall centres are located at the west edge of the northern Philippines, the Philippine Basin, and the Korean Peninsula. The WNP Subtropical High (WNPSH) withdrawals eastward, with an eastward extension of the monsoon trough. These circulation configurations provide favourable environmental conditions for more northward movements of TCs, including low-level positive relative vorticity and enhanced vertical motion in WNP. Observational and theoretical analysis results show that anomalous thermal conditions in the southern Maritime Continent (MC) (97.5°−112.5°E, 8°−18°S) in early spring (March to April) can be a precursor for anomalous MS-TC rainfall. Strong MS-TC rainfall is usually preceded by depressed convection and anomalous westerlies near the MC. These anomalies can persist through the following summer and induce the positive feedback of cooling sea surface temperature (SST) in MC and cyclonic anomalies in tropical WNP through the wind-evaporation-SST (WES) effect and local Hadley circulation anomalies. The linear baroclinic model (LBM) experiments demonstrate that enhanced convection in tropical WNP further maintains the anomalous cyclone through Gill's response, which in turn modulates the distribution and amount of MS-TC rainfall.

Influence of boreal summer intraseasonal oscillation on summer precipitation in North China

AbstractTo understand the impact of the boreal summer intraseasonal oscillation (BSISO) propagation on summer precipitation in North China, this study analysed the quantitative relationship between the BSISO and summer precipitation in North China and its influence mechanism using summer precipitation data from 1981 to 2020, the NCEP/NCAR reanalysis data and the NOAA outgoing longwave radiation (OLR). The results show that there is a strong correlation between summer precipitation in North China and the BSISO. Over the past 40 years, the variance in summer precipitation anomalies in North China caused by BSISO1 accounted for 16.8%, whereas the impact of BSISO2 accounted for 13.1%. Their combined impact accounted for 30% of the variance in summer precipitation in North China. The BSISO influences summer precipitation process in North China primarily through upper tropospheric circulation and lower‐level water vapour transport. At 500 and 200 hPa, the BSISO excited some disturbance centres of a deep barotropic structure propagating from west to east along the upper‐tropospheric westerly jet. North China is controlled by the blocking circulation pattern of “high in the east and low in the west” while there is a negative (positive) anomaly near Lake Baikal and a positive (negative) anomaly near the Korean Peninsula. At 850 hPa, the BSISO stimulates an anticyclonic circulation anomaly on the north side of its convective area, and the southerly winds on the west side of the anticyclone enhance water vapour transport to North China. This high‐ and low‐level circulation configuration is favourable for precipitation processes in North China. The results suggest that the influence of both low‐frequency signals in the middle and high latitudes and the BSISO signal in the tropics should be considered to make extended‐range forecasts of precipitation processes in North China.

Contribution of PDO to Decadal Variations of Glaze Dipole Pattern in China

Abstract Based on the in situ observations, reanalysis, and model simulation, the variations in glaze dipole pattern in China and its underlying physical mechanism have been explored. The glaze dipole pattern features an out-of-phase relationship between winter glaze in the south of the Yangtze River valley (YRV) and northern China, accompanied by pronounced interdecadal variation around the late 1970s. The results from synoptic analyses suggest that cold air brought by the northerly winds and warm moist air by the southwesterly winds, as well as the occurrence of inversion layer are vital to the glaze weather in the south of YRV. Further analyses indicate that the interdecadal shift of the Pacific decadal oscillation (PDO) contributes largely to variations in glaze dipole pattern. Specifically, the warm PDO provides a beneficial environment for the occurrence of glaze dipole pattern by stimulating the tropical–extratropical circulation configuration with the deepened East Asian trough, strengthened East Asian westerly jet, anomalous anticyclone over the tropical western Pacific Ocean, and cyclone over the southern Tibetan Plateau at the decadal time scale. Consequently, the enhanced moisture transport brought by southwesterly and cold air intrusion induced by the deepened East Asian trough benefit the glaze weather in the south of YRV, while the decreased precipitation and a much lower temperature in northern China depress the generation of glaze. Moreover, the results from the CAM4 model simulation indicate the atmospheric circulation anomalies forced by PDO-like SST can roughly reproduce the extratropical configuration related to the glaze, but it has difficulties in capturing the tropical circulation anomalies. Significance Statement The purpose of this study is to better understand how the glaze dipole pattern in China responds to climate anomalies, which focuses on the spatial–temporal evolution of glaze days and its underlying physical mechanism. This study highlights the important role of Pacific Ocean sea surface temperature anomalies in the interdecadal variations in glaze dipole pattern, and it gives a clear clarification to demonstrate the impacts of Pacific decadal oscillation on the climatological background for the occurrence of glaze. Our results provide a guide on what drives the interdecadal variation in the glaze dipole pattern and help to understand the climate dynamic mechanism of variations in freezing events in China.

Contribution of the Tibetan Plateau Snow Cover to the Record-breaking Rainfall Over the Yangtze River Valley in June 2020

ABSTRACT In June 2020, the Yangtze River Valley (YRV) experienced the most severe and long-persisting rainfall in past decades, causing hundreds of people’s death and huge property loss. Based on the high-resolution MODIS/Terra snow cover data for the 2000–2021 period and the linear baroclinic model (LBM), this study investigates the dynamic influence and the potentially predictable source from the Tibetan Plateau (TP) snow cover (TPSC) on this record-breaking YRV rainfall. The TPSC anomalies in the critical area (76°−83°E, 28°−35°N) can persist for more than six months and induce a Rossby wave train propagating northeastward. For the excessive TPSC, barotropic negative geopotential height anomalies prevail over northern East Asia and positive geopotential height anomalies over the northwestern Pacific. Such a circulation configuration favours a deepened cold vortex over Northeast China and the westward-developing western Pacific subtropical high, which leads to an intensified Meiyu-Baiu-Changma front and rich Meiyu rainfall. The LBM sensitive experiments can reproduce the above physical processes associated with the TPSC anomalies, realistically. Moreover, the quantitative contributions of the indices representing the TPSC and the three oceans (Pacific, Indian Ocean, and Atlantic) to the record-breaking Meiyu rainfall are examined, and the TPSC index explains most of the total variance of the rainfall anomaly among the indices, reaching 42.75%. This highlights the importance of the TP thermal forcing to the extreme climate conditions in June 2020.

The key atmospheric drivers linking regional Arctic amplification with East Asian cold extremes

Across the winters of 1999–2011, both the frequency of extreme cold events in East Asia and the Arctic Amplification showed an upward trend. Here we constructed regional AA index in the winters of 1979–2019, and examine the atmospheric driver that linked with strong AA scenarios and cold extreme in EA. Results show that the local atmospheric blocking are key drivers for regional AA occurrence on the intraseasonal timescale. Ural-Siberia blocking, which is tightly linked to the warming over Barents-Kara Seas (BK), is the primary atmospheric mode when regional AA corresponds with extreme East Asian cold days. However, when there is no warming over BK during the associated cold days in EA, the warming over western hemisphere become prominent, accompanied by the negative phase of the North Atlantic Oscillation (NAO−) and blocking dipole system located at mid-Siberian and East Asian continent. Under this circulation configuration, not only does EA exhibits extremely cooling, but also northeastern North America experiences significant cold anomalies. Furthermore, precursor signals at 2–10 days are found between NAO− and each regional AA event when the days of BK warming are excluded. Our results highlight the importance of atmospheric circulation on linking the warming of different Arctic sectors and cold extremes in the mid-latitude continent, and point out the independent role of NAO− and BKS warming on regional AA.

Promoting spatial cognition in hospital buildings using space syntax analyses

Spatial cognition is a pivotal consideration in hospital buildings. Potential circulation configuration alternatives in hospitals have been filtered to 59 significant alternatives, to be simulated using space syntax analyses for measuring connectivity, visual integration, and visual step depth. Best alternatives have been determined via comparison and correlation analysis. A survey has been conducted, comparing the preferences of professional architects with simulation outcomes. Results show that best and worst alternatives exist in triangular and circular buildings, respectively. Also, cul-de-sac corridors are argued to promote spatial cognition better than looped corridors. Notably, cognition is inversely proportional to the curvature of the circulation spaces, number of nodes, and the number of branches. The survey showed a weak perception of the best alternatives, while respondents have a better perception of highly visible locations in these alternatives.

The southward movement preference of large-scale persistent extreme precipitation events over the Yangtze River Valley during Mei-yu period

Among all rainfall events, heavy rainfall that affects large areas and persists for days can cause serious flooding, severe casualties, and substantial economic losses. In this study, we identify the extreme precipitation events that affect a large area and have persistence of more than three days (LPEPEs) in China during 1961–2013. The LPEPEs incline to occur in summer and over southern China, coinciding well with the rainy season in China. The movement of LPEPEs is dominated by the tendency to extend southward, especially for the LPEPEs during the Mei-yu period (LPEPEs-M). The dynamical composite analyses on the southward-moving LPEPEs-M show that the southward extension is generated by the combined effect of the large-scale circulation configuration and the diabatic heating caused by large-scale condensation. The latent heating results in a cyclonic anomaly from the middle to the lower levels. Therefore, westerly and easterly anomalies are produced to the south and north of the heating center, respectively. The two anomalies manipulate the vertical distribution of zonal winds which is dominated by the large-scale circulation to both sides of the rainband. This kind of anomaly configuration, together with the quasi east–west zonal distribution of diabatic heating, creates a pattern with positive and negative PV tendency to the south and north of the heating center, leading to LPEPE system development to the south of the precipitation center. Therefore, the LPEPE rainband has a preference to move southward over the Yangtze River Valley during the Mei-yu period. We also test the dynamical mechanism with a numerical sensitivity experiment using the WRF model. In the experiment without latent heating feedback, the Mei-yu rainband moves northward and extends to central and North China. While in the experiment with the latent heating feedback, the rainband intensifies and extends southward. The southward extension preferences of the LPEPE can provide an internal dynamical of the stagnation of East Asian summer evolution.

Comparisons of the Synoptic Characteristics of 14-Day Extreme Precipitation Events in Different Regions of Eastern China

How to predict and study persistent extreme precipitation events (PEPEs) with a prediction period of 1–2 weeks is an important scientific problem faced by the meteorological circles at home and abroad. Based on the accurate description of the flood range caused by 14-day PEPEs, the comprehensive analysis method was used to obtain the weather characteristics related to 14-day PEPEs (including abnormal trough/ridge, westerly jet, atmospheric river (AR) activity, teleconnections, etc.). First, we selected three regions in China, North China (NC), the Yangtze River valley (YRV), and South China (SC), analyzed their 14-day PEPEs in summer (June to August), and composited them into an average circulation (500 hPa geopotential height field) to compare the weather patterns related to PEPEs in these regions. Then, several variables are composited to understand the evolution of the atmospheric state before and during the occurrence of PEPEs. Finally, potential applications of several teleconnection types and composites in advance prediction are studied. The main findings include: the common weather signals during the occurrence of PEPEs are characterized by obvious and continuous a high-low-high saddle field circulation configuration (conducive to the formation of frequent heavy rainfall), active westerly jet (westerly jet is the controlling factor of precipitation), and enhanced water vapor transport (significantly increased atmospheric river activity). In this study, some key characteristics and development of PEPEs were identified, the formation mechanism of China’s 14-day PEPEs was revealed, the role of ARs in PEPEs was recognized, and the PEPEs precursor signal was extracted. Furthermore, PEPEs in different regions were also compared, which played an important role in understanding and predicting similar events.

Sustained mid-Pliocene warmth led to deep water formation in the North Pacific

Geologic intervals of sustained warmth such as the mid-Pliocene Warm Period can inform our understanding of future climate change, including the long-term consequences of oceanic uptake of anthropogenic carbon. Here we generate carbon isotope records and synthesize existing records to reconstruct the position of water masses and determine circulation patterns in the deep Pacific Ocean. We show that the mid-depth carbon isotope gradient in the North Pacific was reversed during the mid-Pliocene compared with today, which implies water flowed from north to south and deep water probably formed in the subarctic North Pacific Deep Water. An isotopically enabled climate model that simulates this North Pacific Deep Water reproduces a similar carbon isotope pattern. Modelled levels of dissolved inorganic carbon content in the North Pacific decrease slightly, although the amount of carbon stored in the ocean actually increases by 1.6% relative to modern due to an increase in dissolved inorganic carbon in the surface ocean. Although the modelled Pliocene ocean maintains a carbon budget similar to the present, the change in deep ocean circulation configuration causes pronounced downstream changes in biogeochemistry. Marine carbon isotope patterns point to substantial deep water formation in the North Pacific during the mid-Pliocene Warm Period, according to a synthesis of carbon isotope records and isotope-enabled climate modelling.

Diagnostic Analysis of Multimodel Rainstorm Forecast for Cases Based on MODE Method

It is very important to analyze the performance of the model rainstorm forecast for understanding the intensity and position deviation of the model precipitation and improving the forecast ability. This paper uses classical scoring and the MODE (Method for Object-Based Diagnostic Evaluation) method to evaluate the forecast performance of different products. The forecast and observation data used in the article mainly include CMA (China Meteorological Administration) multi-source merged precipitation analysis, the precipitation forecast of ECMWF (European Centre for Medium-Range Weather Forecasts), CMA-Meso (Mesoscale model forecast of CMA) and SCMOC (National Meteorological Center grid precipitation forecast guidance product) data. At the same time, the possible correction method of heavy rainfall area is explored by using the high and low-level circulation configuration of the ECMWF model. The main conclusions are as follows: ① MODE spatial verification shows that the number, intensity, area and location of ECMWF rainstorm precipitation objects match the observed precipitation best, which is obviously better than SCMOC and CMA-Meso precipitation forecasts. There are significantly fewer SCMOC rainstorm precipitation objects, and the area of each single precipitation object is significantly larger, which often fails to report the small area objects of convective precipitation. ② The reason for the high TS score of SCMOC is that it reduces the number of small area rainstorm objects and avoids the “double punishment” phenomenon caused by position forecast error, which leads to the failure of SCMOC in local rainstorm forecasts. ③ Analyzing the relationship between the circulation situation of the ECMWF model and the location of rainstorm forecasts by the model, it is found that the location of the rainstorm area is consistent with the upper circulation system, especially with the strong rising area of the vertical velocity of 700 hPa and the high value area of the specific humidity of 850 hPa. When the rainstorm area coincides with the upper air system but is not consistent with the ground convergence area and the high value area of velocity potential, the rainstorm location often has a large deviation. The location of the surface convergence area can be used as a reference to improve the performance of the rainstorm forecast.

Fluidization of the spherocylindrical particles: Comparison of multi-sphere and bond-sphere models

The fluidization of spherocylindrical particles is widely encountered in many engineering applications. In this work, by using the computational fluid dynamic-discrete element method (CFD-DEM), both the multi-sphere and bond-sphere models are developed, validated, and applied to investigate the fluidization characteristics of almost rigid spherocylinders. The differences between the two models are highlighted. Results show that the particle resolution of the multi-sphere model has a great impact on the pressure drop of the stationary bed but a small effect on full fluidization behaviors. More particles tend to be in the vertical direction by using the bond-sphere model, whereas the multi-sphere model predicts a more horizontal particle orientation. A left–right symmetrical particle flow pattern is observed for the multi-sphere model while a particle circulation configuration across the whole bed width appears for the bond-sphere model. All these observations are helpful for selecting particle model and deeply understanding the fluidization of spherocylindrical particles.

Association between regional summer monsoon onset in South Asia and Tibetan Plateau thermal forcing

By using multiple data sources and numerical simulation results from the atmospheric general circulation model (AGCM) of CAM4.0 (Community Atmosphere Model, version 4), we investigated the effect of thermal forcing over the Tibetan Plateau (TP) on the onset of the summer monsoon in the Arabian Sea (AS) and India. Results indicate that the spatial distribution of diabatic heating over the TP in May is a southeastern–northwestern inverse pattern. This diabatic heating shows a robust negative relationship with the onset date of the summer monsoon over the AS and India. A positive diabatic heating seesaw pattern can enhance the ascending (descending) motion over the southeastern (northwestern) TP, corresponding to above (below) normal in- situ precipitation. Temperature budget diagnosis reveals that the adiabatic heating due to anomalous vertical motion and relevant horizontal advection of temperature in the mid-upper troposphere are responsible for the warming over the TP. Consequently, the reversal of the meridional temperature gradient from winter to summer over South Asian regions occurs earlier than the climate-mean state. Furthermore, the diabatic heating over the TP induces an enhanced and westward-extended South Asian High (SAH), which together with the easterly along the southern flank of the SAH superimpose on the low-level westerly flow over the AS and India, results in intensive upper-level divergence-pumping and upward motion. This anomalous circulation configuration in lower and upper levels further facilitates an earlier onset of the summer monsoon in AS and India. These findings are corroborated in the sensitivity runs based on CAM4.0.

Large‐scale circulation features responsible for different types of extreme high temperatures with extreme coverage over South China

AbstractThis study classifies the summer extreme high temperatures (EHT) over South China into Day‐EHT, Night‐EHT and Mixed‐EHT (corresponding to EHT with extreme coverage only in daytime, only at night and both in daytime and at night) and compares their associated circulation patterns. All types of EHT are associated with the intensification and expansion of western Pacific subtropical high (WPSH). Concurrently, anomalous subsidence occurs over South China, which reduces the cloud cover and favours daytime warming. However, different types of EHT feature different circulation configurations. Day‐EHT are influenced by the northwestward‐extending WPSH and tropical cyclones, manifesting an abnormal anticyclonic‐cyclonic pair. Anomalous northeasterly winds dominate over South China, which cause lower humidity and faster heat dissipation and thus the EHT disappear at night. Night‐EHT are related to the westward‐extending WPSH and intensified East Asian westerly trough, presenting a subtropical anticyclonic anomaly and a mid‐latitude cyclonic anomaly. Anomalous southwesterly and northerly flows meet around South China, favouring more water vapour convergence in situ. The above‐normal humidity results in smaller amplitude of cloud cover reduction and weaker daytime warming compared to Day‐EHT, but favours heat maintenance and Night‐EHT. Moreover, the mid‐high‐latitude wave train preceding Day‐EHT (Night‐EHT) lies more northward (southward) with the upstream high‐pressure anomaly occurring around the Ural Mountains (Caspian Sea). Mixed‐EHT combine the anomalies for Day‐EHT and Night‐EHT and weak humidity anomaly occurs over South China, thus the daytime EHT could persist until night albeit cools down slightly. These distinguished features indicate that different types of EHT should be considered separately for better forecast and projection of EHT.

Linear stability analysis of flashing instability based on the homogeneous equilibrium model

Flashing instability is a two-phase flow instability widely reported in low-pressure natural circulation that induces undesirable flow oscillations with large magnitudes challenging the operation of facilities. The current study performs linear stability analysis on the flashing instability using a homogenous equilibrium model developed for a typical natural circulation configuration. Physical simplifications are performed, and the governing equations are analytically solved lumping the dynamics of the flow into the evolution of the inlet flow rate governed by a linear ordinary integro-differential equation. Similarity groups governing the dynamics is extracted from the non-dimensionalization. Static stability of the flow is checked based on the classic criterion of the excursive instability, which confirms negligible potential of the static instability and proves the dynamic nature of the flashing instability. The Laplace transformation is performed providing the characteristic function governing the dynamic stability, and the D-partition method is implemented producing the stability boundaries. Through reviewing the characteristic function, a simplified model is obtained around the stability boundary with the low exit flow quality, revealing three dominant feedback effects. For conditions with a short two-phase region near the top of the chimney, the flashing is found to introduce a strong delayed feedback between the gravitational driving force and the inlet flow rate, which essentially triggers the instability under favorable oscillation frequencies properly shifting the phase differences. The extracted dominant dynamics explains the parametric effect on the stability, and analytically predicts a proportional relation between oscillation period and the fluid residence time in the hot leg. The modeling and analysis in the current study supplements the analytical basis for understanding the flashing instability and provides valuable reference for further analytical studies.

Classification of Northeast China Cold Vortex Activity Paths in Early Summer Based on K-means Clustering and Their Climate Impact

The classification of the Northeast China Cold Vortex (NCCV) activity paths is an important way to analyze its characteristics in detail. Based on the daily precipitation data of the northeastern China (NEC) region, and the atmospheric circulation field and temperature field data of ERA-Interim for every six hours, the NCCV processes during the early summer (June) seasons from 1979 to 2018 were objectively identified. Then, the NCCV processes were classified using a machine learning method (k-means) according to the characteristic parameters of the activity path information. The rationality of the classification results was verified from two aspects, as follows: (1) the atmospheric circulation configuration of the NCCV on various paths; and (2) its influences on the climate conditions in the NEC. The obtained results showed that the activity paths of the NCCV could be divided into four types according to such characteristics as the generation origin, movement direction, and movement velocity of the NCCV. These included the generation-eastward movement type in the east of the Mongolia Plateau (eastward movement type or type A); generation-southeast long-distance movement type in the upstream of the Lena River (southeast long-distance movement type or type B); generation-eastward less-movement type near Lake Baikal (eastward less-movement type or type C); and the generation-southward less-movement type in eastern Siberia (southward less-movement type or type D). There were obvious differences observed in the atmospheric circulation configuration and the climate impact of the NCCV on the four above-mentioned types of paths, which indicated that the classification results were reasonable.