2024 Vol. 30, No. 2
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2024, 30(2): .
Abstract:
2024, 30(2): 97-105.
doi: 10.3724/j.1006-8775.2024.010
Abstract:
Concurrent extreme weather events in geographically distant areas potentially cause high-end risks for societies. By using network analysis, the present study managed to identify significant nearly-simultaneous occurrences of heatwaves between the grid cells in East Asia and Eastern Europe, even though they are geographically far away from each other. By further composite analysis, this study revealed that hot events first occurred in Eastern Europe, typically with a time lag of 3–4 days before the East Asian heatwave events. An eastward propagating atmospheric wave train, known as the circumglobal teleconnection (CGT) pattern, bridged the sequent occurrences of extreme events in these two remote regions. Atmospheric blockings, amplified by surface warming over Eastern Europe, not only enhanced local heat extremes but also excited a CGT-like pattern characterized by alternative anomalies of high and low pressures. Subsequent downstream anticyclones in the middle and upper troposphere reduced local cloud cover and increased downward solar radiation, thereby facilitating the formation of heatwaves over East Asia. Nearly half of East Asian heatwave events were preceded by Eastern European heatwave events in the 10-day time range before East Asian heatwave events. This investigation of heatwave teleconnection in the two distant regions exhibits strong potential to improve the prediction accuracy of East Asian heatwaves.
Concurrent extreme weather events in geographically distant areas potentially cause high-end risks for societies. By using network analysis, the present study managed to identify significant nearly-simultaneous occurrences of heatwaves between the grid cells in East Asia and Eastern Europe, even though they are geographically far away from each other. By further composite analysis, this study revealed that hot events first occurred in Eastern Europe, typically with a time lag of 3–4 days before the East Asian heatwave events. An eastward propagating atmospheric wave train, known as the circumglobal teleconnection (CGT) pattern, bridged the sequent occurrences of extreme events in these two remote regions. Atmospheric blockings, amplified by surface warming over Eastern Europe, not only enhanced local heat extremes but also excited a CGT-like pattern characterized by alternative anomalies of high and low pressures. Subsequent downstream anticyclones in the middle and upper troposphere reduced local cloud cover and increased downward solar radiation, thereby facilitating the formation of heatwaves over East Asia. Nearly half of East Asian heatwave events were preceded by Eastern European heatwave events in the 10-day time range before East Asian heatwave events. This investigation of heatwave teleconnection in the two distant regions exhibits strong potential to improve the prediction accuracy of East Asian heatwaves.
2024, 30(2): 106-117.
doi: 10.3724/j.1006-8775.2024.011
Abstract:
This study investigates the effect of the initial tropical cyclone (TC) vortex structure on the intensity change during the eyewall replacement cycle (ERC) of TCs based on two idealized simulations using the Weather Research and Forecasting (WRF) model. Results show that an initially smaller TC with weaker outer winds experienced a much more drastic intensity change during the ERC than an initially larger TC with stronger outer winds. It is found that an initially larger TC vortex with stronger outer winds favored the development of more active spiral rainbands outside the outer eyewall, which slowed down the contraction and intensification of the outer eyewall and thus prolonged the duration of the concentric eyewall and slow intensity evolution. In contrast, the initially smaller TC with weaker outer winds corresponded to higher inertial stability in the inner core and weaker inertial stability but stronger filamentation outside the outer eyewall. These led to stronger boundary layer inflow, stronger updraft and convection in the outer eyewall, and suppressed convective activity outside the outer eyewall. These resulted in the rapid weakening during the formation of the outer eyewall, followed by a rapid re-intensification of the TC during the ERC. Our study demonstrates that accurate initialization of the TC structure in numerical models is crucial for predicting changes in TC intensity during the ERC. Additionally, monitoring the activity of spiral rainbands outside the outer eyewall can help to improve short-term intensity forecasts for TCs experiencing ERCs.
This study investigates the effect of the initial tropical cyclone (TC) vortex structure on the intensity change during the eyewall replacement cycle (ERC) of TCs based on two idealized simulations using the Weather Research and Forecasting (WRF) model. Results show that an initially smaller TC with weaker outer winds experienced a much more drastic intensity change during the ERC than an initially larger TC with stronger outer winds. It is found that an initially larger TC vortex with stronger outer winds favored the development of more active spiral rainbands outside the outer eyewall, which slowed down the contraction and intensification of the outer eyewall and thus prolonged the duration of the concentric eyewall and slow intensity evolution. In contrast, the initially smaller TC with weaker outer winds corresponded to higher inertial stability in the inner core and weaker inertial stability but stronger filamentation outside the outer eyewall. These led to stronger boundary layer inflow, stronger updraft and convection in the outer eyewall, and suppressed convective activity outside the outer eyewall. These resulted in the rapid weakening during the formation of the outer eyewall, followed by a rapid re-intensification of the TC during the ERC. Our study demonstrates that accurate initialization of the TC structure in numerical models is crucial for predicting changes in TC intensity during the ERC. Additionally, monitoring the activity of spiral rainbands outside the outer eyewall can help to improve short-term intensity forecasts for TCs experiencing ERCs.
2024, 30(2): 118-131.
doi: 10.3724/j.1006-8775.2024.012
Abstract:
This study investigated the growth of forecast errors stemming from initial conditions (ICs), lateral boundary conditions (LBCs), and model (MO) perturbations, as well as their interactions, by conducting seven 36 h convection-allowing ensemble forecast (CAEF) experiments. Two cases, one with strong-forcing (SF) and the other with weak-forcing (WF), occurred over the Yangtze-Huai River basin (YHRB) in East China, were selected to examine the sources of uncertainties associated with perturbation growth under varying forcing backgrounds and the influence of these backgrounds on growth. The perturbations exhibited distinct characteristics in terms of temporal evolution, spatial propagation, and vertical distribution under different forcing backgrounds, indicating a dependence between perturbation growth and forcing background. A comparison of the perturbation growth in different precipitation areas revealed that IC and LBC perturbations were significantly influenced by the location of precipitation in the SF case, while MO perturbations were more responsive to convection triggering and dominated in the WF case. The vertical distribution of perturbations showed that the sources of uncertainties and the performance of perturbations varied between SF and WF cases, with LBC perturbations displaying notable case dependence. Furthermore, the interactions between perturbations were considered by exploring the added values of different source perturbations. For the SF case, the added values of IC, LBC, and MO perturbations were reflected in different forecast periods and different source uncertainties, suggesting that the combination of multi-source perturbations can yield positive interactions. In the WF case, MO perturbations provided a more accurate estimation of uncertainties downstream of the Dabie Mountain and need to be prioritized in the research on perturbation development.
This study investigated the growth of forecast errors stemming from initial conditions (ICs), lateral boundary conditions (LBCs), and model (MO) perturbations, as well as their interactions, by conducting seven 36 h convection-allowing ensemble forecast (CAEF) experiments. Two cases, one with strong-forcing (SF) and the other with weak-forcing (WF), occurred over the Yangtze-Huai River basin (YHRB) in East China, were selected to examine the sources of uncertainties associated with perturbation growth under varying forcing backgrounds and the influence of these backgrounds on growth. The perturbations exhibited distinct characteristics in terms of temporal evolution, spatial propagation, and vertical distribution under different forcing backgrounds, indicating a dependence between perturbation growth and forcing background. A comparison of the perturbation growth in different precipitation areas revealed that IC and LBC perturbations were significantly influenced by the location of precipitation in the SF case, while MO perturbations were more responsive to convection triggering and dominated in the WF case. The vertical distribution of perturbations showed that the sources of uncertainties and the performance of perturbations varied between SF and WF cases, with LBC perturbations displaying notable case dependence. Furthermore, the interactions between perturbations were considered by exploring the added values of different source perturbations. For the SF case, the added values of IC, LBC, and MO perturbations were reflected in different forecast periods and different source uncertainties, suggesting that the combination of multi-source perturbations can yield positive interactions. In the WF case, MO perturbations provided a more accurate estimation of uncertainties downstream of the Dabie Mountain and need to be prioritized in the research on perturbation development.
2024, 30(2): 132-148.
doi: 10.3724/j.1006-8775.2024.013
Abstract:
The increasing temperature in the Yellow River Basin has led to a rapid rise in the melting level height, at a rate of 5.98 m yr–1 during the cold season, which further contributes to the transition from snowfall to rainfall patterns. Between 1979 and 2020, there has been a decrease in snowfall in the Yellow River Basin at a rate of –3.03 mm dec–1, while rainfall has been increasing at a rate of 1.00 mm dec–1. Consequently, the snowfall-to-rainfall ratio (SRR) has decreased. Snowfall directly replenishes terrestrial water storage (TWS) in solid form until it melts, while rainfall is rapidly lost through runoff and evaporation, in addition to infiltrating underground or remaining on the surface. Therefore, the decreasing SRR accelerates the depletion of water resources. According to the surface water balance equation, the reduction in precipitation and runoff, along with an increase in evaporation, results in a decrease in TWS during the cold season within the Yellow River Basin. In addition to climate change, human activities, considering the region's dense population and extensive agricultural land, also accelerate the decline of TWS. Notably, irrigation accounts for the largest proportion of water withdrawals in the Yellow River Basin (71.8%) and primarily occurs during the warm season (especially from June to August). The impact of human activities and climate change on the water cycle requires further in-depth research.
The increasing temperature in the Yellow River Basin has led to a rapid rise in the melting level height, at a rate of 5.98 m yr–1 during the cold season, which further contributes to the transition from snowfall to rainfall patterns. Between 1979 and 2020, there has been a decrease in snowfall in the Yellow River Basin at a rate of –3.03 mm dec–1, while rainfall has been increasing at a rate of 1.00 mm dec–1. Consequently, the snowfall-to-rainfall ratio (SRR) has decreased. Snowfall directly replenishes terrestrial water storage (TWS) in solid form until it melts, while rainfall is rapidly lost through runoff and evaporation, in addition to infiltrating underground or remaining on the surface. Therefore, the decreasing SRR accelerates the depletion of water resources. According to the surface water balance equation, the reduction in precipitation and runoff, along with an increase in evaporation, results in a decrease in TWS during the cold season within the Yellow River Basin. In addition to climate change, human activities, considering the region's dense population and extensive agricultural land, also accelerate the decline of TWS. Notably, irrigation accounts for the largest proportion of water withdrawals in the Yellow River Basin (71.8%) and primarily occurs during the warm season (especially from June to August). The impact of human activities and climate change on the water cycle requires further in-depth research.
2024, 30(2): 149-167.
doi: 10.3724/j.1006-8775.2024.014
Abstract:
This study employed numerical simulations to explore the impact of varying ice nucleation processes on the microphysics and electrification within thunderstorm clouds. A two-dimensional cumulus model, incorporating both non-inductive and inductive charge separation schemes, was utilized. The findings revealed that the freezing nucleation mechanism significantly influenced the microphysical development, electrification, and charge structure of thunderstorms. Homogeneous freezing generated a large quantity of small ice crystals near the cloud tops, which were primarily responsible for the development of positive charge regions through a non-inductive charging process. Conversely, heterogeneous freezing resulted in larger ice crystals, enhancing graupel formation and leading to a more rapid and intense charge separation rate of around?15℃. Ice crystals formed heterogeneously and charged negatively during the development stage, resulting in an inverted dipole charge structure. When both immersion and homogeneous freezing processes were considered, the competition between these two distinct freezing processes resulted in reduced cloud water content and weaker electrification. Under conditions of low cloud water content at lower storm levels, graupel particles were negatively charged through non-inductive charging, causing the charge structure to quickly revert to a normal dipole structure.
This study employed numerical simulations to explore the impact of varying ice nucleation processes on the microphysics and electrification within thunderstorm clouds. A two-dimensional cumulus model, incorporating both non-inductive and inductive charge separation schemes, was utilized. The findings revealed that the freezing nucleation mechanism significantly influenced the microphysical development, electrification, and charge structure of thunderstorms. Homogeneous freezing generated a large quantity of small ice crystals near the cloud tops, which were primarily responsible for the development of positive charge regions through a non-inductive charging process. Conversely, heterogeneous freezing resulted in larger ice crystals, enhancing graupel formation and leading to a more rapid and intense charge separation rate of around?15℃. Ice crystals formed heterogeneously and charged negatively during the development stage, resulting in an inverted dipole charge structure. When both immersion and homogeneous freezing processes were considered, the competition between these two distinct freezing processes resulted in reduced cloud water content and weaker electrification. Under conditions of low cloud water content at lower storm levels, graupel particles were negatively charged through non-inductive charging, causing the charge structure to quickly revert to a normal dipole structure.
2024, 30(2): 168-179.
doi: 10.3724/j.1006-8775.2024.015
Abstract:
Based on ERA5 reanalysis data, the present study analyzed the thermal energy development mechanism and kinetic energy conversion characteristics of two extreme rainstorm processes in relation to the shallow southwest vortex in the warm-sector during a "rain-generated vortex" process and the deep southwest vortex in a "vortex-generated rain" process. The findings were as follows: (1) During the extreme rainstorm on August 11, 2020 (hereinafter referred to as the "8·11" process), intense surface heating and a high-energy unstable environment were observed. The mesoscale convergence system triggered convection to produce heavy rainfall, and the release of latent condensation heat generated by the rainfall promoted the formation of a southwest vortex. The significant increase (decrease) in atmospheric diabatic heating and kinetic energy preceded the increase (decrease) in vorticity. By contrast, the extreme rainstorm on August 16, 2020 (hereinafter referred to as the "8·16" process) involved the generation of southwest vortex in a low-energy and high-humidity environment. The dynamic uplift of the southwest vortex triggered rainfall, and the release of condensation latent heat from rainfall further strengthened the development of the southwest vortex. The significant increase (decrease) in atmospheric diabatic heating and kinetic energy exhibited a delayed progression compared to the increase (decrease) in vorticity. (2) The heating effect around the southwest vortex region was non-uniform, and the heating intensity varied in different stages. In the "8·11" process, the heating effect was the strongest in the initial stage, but weakened during the vortex's development. On the contrary, the heating effect was initially weak in the "8·16" process, and intensified during the development stage. (3) The available potential energy of the "8·11" process significantly increased in kinetic energy converted from rotational and divergent winds through baroclinic action, and the divergent wind energy continued to convert into rotational wind energy. By contrast, the "8·16" process involved the conversion of rotational wind energy into divergent wind energy, which in turn converted kinetic energy back into available potential energy, thereby impeding the further development and maintenance of the southwest vortex.
Based on ERA5 reanalysis data, the present study analyzed the thermal energy development mechanism and kinetic energy conversion characteristics of two extreme rainstorm processes in relation to the shallow southwest vortex in the warm-sector during a "rain-generated vortex" process and the deep southwest vortex in a "vortex-generated rain" process. The findings were as follows: (1) During the extreme rainstorm on August 11, 2020 (hereinafter referred to as the "8·11" process), intense surface heating and a high-energy unstable environment were observed. The mesoscale convergence system triggered convection to produce heavy rainfall, and the release of latent condensation heat generated by the rainfall promoted the formation of a southwest vortex. The significant increase (decrease) in atmospheric diabatic heating and kinetic energy preceded the increase (decrease) in vorticity. By contrast, the extreme rainstorm on August 16, 2020 (hereinafter referred to as the "8·16" process) involved the generation of southwest vortex in a low-energy and high-humidity environment. The dynamic uplift of the southwest vortex triggered rainfall, and the release of condensation latent heat from rainfall further strengthened the development of the southwest vortex. The significant increase (decrease) in atmospheric diabatic heating and kinetic energy exhibited a delayed progression compared to the increase (decrease) in vorticity. (2) The heating effect around the southwest vortex region was non-uniform, and the heating intensity varied in different stages. In the "8·11" process, the heating effect was the strongest in the initial stage, but weakened during the vortex's development. On the contrary, the heating effect was initially weak in the "8·16" process, and intensified during the development stage. (3) The available potential energy of the "8·11" process significantly increased in kinetic energy converted from rotational and divergent winds through baroclinic action, and the divergent wind energy continued to convert into rotational wind energy. By contrast, the "8·16" process involved the conversion of rotational wind energy into divergent wind energy, which in turn converted kinetic energy back into available potential energy, thereby impeding the further development and maintenance of the southwest vortex.
2024, 30(2): 180-188.
doi: 10.3724/j.1006-8775.2024.016
Abstract:
The Northeastern China cold vortex (NCCV) is one type of strong cyclonic vortex that occurs near Northeastern China (NEC), and NCCV activities are typically accompanied by a series of hazardous weather. This paper employed an automatic algorithm to identify the NCCVs from 1979 to 2018 and analyzed their circulation patterns and climatic impacts by using the defined NCCV intensity index (NCCVI). The analysis revealed that the NCCV activities in summer exhibited a strong inter-annual variability, with an obvious periodicity of 3–4 years and 6–7 years, but without significant trends. In years when the NCCVI was high, NEC experienced negative geopotential height anomalies, cyclonic circulation, and cooler temperature anomalies, which were conducive to the maintenance and development of NCCV activities. Furthermore, large amounts of water vapor converged in NEC through two transportation routes as the NCCVs intensified, leading to a significant positive (negative) correlation with the summer precipitation (surface temperature) in NEC. The Atlantic sea surface temperature (SST) anomalies were closely related to summer NCCV activities. As the Atlantic SST rose, large amounts of surface sensible and latent heat flux were transported into the lower troposphere, inducing a positive geopotential height anomaly that occurred on the east side of the heat source. As a result, an eastward diverging flow was formed in the upper troposphere and propagated downstream, i.e., the eastward propagating Rossby wave train, which eventually led to a coupled circulation in the Ural Mountains and NEC, as well as more intensive NCCV activities in summer.
The Northeastern China cold vortex (NCCV) is one type of strong cyclonic vortex that occurs near Northeastern China (NEC), and NCCV activities are typically accompanied by a series of hazardous weather. This paper employed an automatic algorithm to identify the NCCVs from 1979 to 2018 and analyzed their circulation patterns and climatic impacts by using the defined NCCV intensity index (NCCVI). The analysis revealed that the NCCV activities in summer exhibited a strong inter-annual variability, with an obvious periodicity of 3–4 years and 6–7 years, but without significant trends. In years when the NCCVI was high, NEC experienced negative geopotential height anomalies, cyclonic circulation, and cooler temperature anomalies, which were conducive to the maintenance and development of NCCV activities. Furthermore, large amounts of water vapor converged in NEC through two transportation routes as the NCCVs intensified, leading to a significant positive (negative) correlation with the summer precipitation (surface temperature) in NEC. The Atlantic sea surface temperature (SST) anomalies were closely related to summer NCCV activities. As the Atlantic SST rose, large amounts of surface sensible and latent heat flux were transported into the lower troposphere, inducing a positive geopotential height anomaly that occurred on the east side of the heat source. As a result, an eastward diverging flow was formed in the upper troposphere and propagated downstream, i.e., the eastward propagating Rossby wave train, which eventually led to a coupled circulation in the Ural Mountains and NEC, as well as more intensive NCCV activities in summer.
2024, 30(2): 189-199.
doi: 10.3724/j.1006-8775.2024.017
Abstract:
The X-band phased array radar offers faster scanning speed and higher spatial resolution compared to the S-band radar, making it capable of enhancing tornado monitoring and early warning capabilities. This study analyzed the characteristics and nowcasting signals of a tornado case that occurred on June 16, 2022 in the Guangzhou region. Our findings indicate that the violent contraction of rotation radius and the dramatic increase in rotation speed were important signal characteristics associated with tornado formation. The X-band phased array radar, with its high temporal and spatial resolution, provided an opportunity to capture early warning signals from polarimetric characteristics. The X-band phased array radar demonstrated noteworthy ability to identify apparent tornado vortex signature (TVS) features in a 10-minute lead time, surpassing the capabilities of the CINRAD/SA radar. Additionally, due to its higher scanning frequency, the X-band phased-array radar was capable of consistently identifying TVS with shorter intervals, enabling a more precise tracking of the tornado's path. The application of professional radars, in this case, provides valuable insights for the monitoring of evolutions of severe local storms and even tornadoes and the issuance of early warning signals.
The X-band phased array radar offers faster scanning speed and higher spatial resolution compared to the S-band radar, making it capable of enhancing tornado monitoring and early warning capabilities. This study analyzed the characteristics and nowcasting signals of a tornado case that occurred on June 16, 2022 in the Guangzhou region. Our findings indicate that the violent contraction of rotation radius and the dramatic increase in rotation speed were important signal characteristics associated with tornado formation. The X-band phased array radar, with its high temporal and spatial resolution, provided an opportunity to capture early warning signals from polarimetric characteristics. The X-band phased array radar demonstrated noteworthy ability to identify apparent tornado vortex signature (TVS) features in a 10-minute lead time, surpassing the capabilities of the CINRAD/SA radar. Additionally, due to its higher scanning frequency, the X-band phased-array radar was capable of consistently identifying TVS with shorter intervals, enabling a more precise tracking of the tornado's path. The application of professional radars, in this case, provides valuable insights for the monitoring of evolutions of severe local storms and even tornadoes and the issuance of early warning signals.
2024, 30(2): 200-210.
doi: 10.3724/j.1006-8775.2024.018
Abstract:
In the present study, a severe squall line (SL) was analyzed by using intensive observational surface data and radar monitoring products. In this process, mesoscale convergence lines, such as the sea breeze front (SBF), gust front and dry line, served as the main triggering and strengthening factors. The transition from convection triggering to the formation of the initial shape was mainly affected by the convergence line of the SBF, which combined with thermal convection to form the main parts of the SL. In the later stage, the convergence line of the gust front merged with other convergence lines to form a series of strong convective cells. The SBF had good indicative significance in terms of severe convective weather warnings. The suitable conditions of heat, water vapor and vertical wind shear on the Shandong Peninsula were beneficial to the maintenance of the SL. Before SL occurrence, tropopause folding strengthened, which consequently enhanced the baroclinic property in the middle and upper troposphere. The high sensible heat flux at the surface easily produced a positive potential vorticity anomaly in the low layer, resulting in convective instability, which was conducive to the maintenance of these processes. In the system, when precipitation particles passed through the unsaturated air layer, they underwent strong evaporation, melting or sublimation, and the cooling effect formed negative buoyancy, which accelerated the sinking of the air and promoted the sustained development of the surface gale. Together with the development of low-level mesocyclones, the air pressure decreased rapidly, which was conducive to gale initiation.
In the present study, a severe squall line (SL) was analyzed by using intensive observational surface data and radar monitoring products. In this process, mesoscale convergence lines, such as the sea breeze front (SBF), gust front and dry line, served as the main triggering and strengthening factors. The transition from convection triggering to the formation of the initial shape was mainly affected by the convergence line of the SBF, which combined with thermal convection to form the main parts of the SL. In the later stage, the convergence line of the gust front merged with other convergence lines to form a series of strong convective cells. The SBF had good indicative significance in terms of severe convective weather warnings. The suitable conditions of heat, water vapor and vertical wind shear on the Shandong Peninsula were beneficial to the maintenance of the SL. Before SL occurrence, tropopause folding strengthened, which consequently enhanced the baroclinic property in the middle and upper troposphere. The high sensible heat flux at the surface easily produced a positive potential vorticity anomaly in the low layer, resulting in convective instability, which was conducive to the maintenance of these processes. In the system, when precipitation particles passed through the unsaturated air layer, they underwent strong evaporation, melting or sublimation, and the cooling effect formed negative buoyancy, which accelerated the sinking of the air and promoted the sustained development of the surface gale. Together with the development of low-level mesocyclones, the air pressure decreased rapidly, which was conducive to gale initiation.
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