The characteristics of helicity in a hurricane are presented by calculating the MM5 model output in addition to theoretical analysis. It is found that helicity in a hurricane mainly depends on its horizontal component...The characteristics of helicity in a hurricane are presented by calculating the MM5 model output in addition to theoretical analysis. It is found that helicity in a hurricane mainly depends on its horizontal component, whose magnitude is about 100 to 1000 times larger than its vertical component. It is also found that helicity is approximately conserved in the hurricane. Since the fluid has the intention to adjust the wind shear to satisfy the conservation of helicity, the horizontal vorticity is even larger than the vertical vorticity, and the three-dimensional vortices slant to the horizontal plane except in the inner eye. There are significant horizontal vortices and inhomogeneous helical flows in the hurricane. The formation of the spiral rainband is discussed by using the law of horizontal helical flows. It is closely related to the horizontal strong vortices and inhomogeneous helical flows.展开更多
The Advanced Weather Research and Forecasting Model （ARW） is used to simulate the local heavy rainstorm process caused by Typhoon Matsa over the northeastern coast of Zhejiang Province in 2005. The results show that...The Advanced Weather Research and Forecasting Model （ARW） is used to simulate the local heavy rainstorm process caused by Typhoon Matsa over the northeastern coast of Zhejiang Province in 2005. The results show that the rainstorm was caused mainly by the secondary spiral rainband of the Stationary Band Complex （SBC） structure. Within the secondary spiral rainband there was a strong meso-β-scale convergence line generated in the boundary layer, corresponding very well to the Doppler radar echo band. The convergence line comprised several smaller convergence centers, and all of these convergence columns inclined outward. Along the convergence line there was precipitation greater than 20 mm occurring during the following one hour. During the heavy rainstorm process, the Doppler radar echo band, convergence line, and the precipitation amount during the following one hour, moved and evolved synchronously. Further study reveals that the vertical shear of radial wind and the low-level jet of tangential wind contributed to the genesis and development of the convergence columns. The combined effect of the ascending leg of the clockwise secondary circulation of radial wind and the favorable environment of the entrance region of the low-level jet of tangential wind further strengthened the convergence. The warm, moist inflow in the lower levels was brought in by the inflows of the clockwise secondary circulation and uplifted intensely at the effect of convergence. In the convectively instable environment, strong convection was triggered to produce the heavy rainstorm.展开更多
Spiral rainband is a prominent structure of tropical cyclone. Though its forming mechanism, vortex Rossby wave theory, has been widely accepted in recent years, its internal structural features are still not well know...Spiral rainband is a prominent structure of tropical cyclone. Though its forming mechanism, vortex Rossby wave theory, has been widely accepted in recent years, its internal structural features are still not well known. The spiral rainband in the severe tropical storm Kammuri (2002), which caused heavy rainfall in southeast China, is simulated using the mesoscale model MM5 (V3). Results show that the simulated spiral rainband propagates azimuthally at a speed close to that of vortex Rossby wave in theory, and is accom- panied with energy dispersion in the radial direction. The structural features of simulated spiral rainband are analyzed with the high-resolution model output including the full physical process. Positive vorticity, ascending motion, hori- zontal momentum and so on are highly concentrated in the spiral rainband. The convergent moisture of spiral rainband comes mostly from the planetary boundary layer under 1 km. Airflow from the outside of spiral rainband is convective instability, which can provide instability energy for convec- tion development. However, the atmospheric stratification in the inside of spiral rainband is neutral, implying that the instability energy has been released. There is a mesoscale strong wind band just near the spiral rainband in the outer side with a maximum wind speed exceeding 30 m/s, which results from the pressure force acceleration when the air flows into the spiral rainband along the gradient of pressure.展开更多
During the movement of Typhoon Hato(2017)over land,heavy rainfall occurred when the spiral rainband which was about 100 km distance away from the center of the typhoon passed the Dayao Mountain(with an elevation of 1....During the movement of Typhoon Hato(2017)over land,heavy rainfall occurred when the spiral rainband which was about 100 km distance away from the center of the typhoon passed the Dayao Mountain(with an elevation of 1.2 km).In this study,the structures and forming mechanism of the heavy rainband along the mountain range are investigated by using high-resolution model simulations.The results show the importance of topography in causing the heavy rainband.Upslope of the steep terrain lifts the cyclonic flow to produce strong upward motion when the rainband passes across with high wind speed.At the same time,the warm and humid air is lifted to the steep slope,causing unstable energy to accumulate over the windward slope,which is conducive to the occurrence of rainfall.In particular,the convective cells generated upstream of rainband will further strengthen and develop due to the uplift when they move close to the mountain foot.Some precipitation particles in the convective cells fall to the ground while others move downstream with the intense updrafts,forming heavy rainfall near the summit.As a result,the largest accumulative rainfall coincides well with the orientation of the mountain ridge.展开更多
Typhoon Rananim （2004） was one of the severest typhoons landfalling the Chinese mainland from 1996 to 2004. It brought serious damage and induced prodigious economical loss. Using a new generation of mesoscale model...Typhoon Rananim （2004） was one of the severest typhoons landfalling the Chinese mainland from 1996 to 2004. It brought serious damage and induced prodigious economical loss. Using a new generation of mesoscale model, named the Weather Research and Forecasting （WRF） modeling system, with 1.667 km grid horizontal spacing on the finest nested mesh, Rananim was successfully simulated in terms of track, intensity, eye, eyewall, and spiral rainbands. We compared the structures of Rananim to those of hurricanes in previous studies and observations to assess the validity of simulation. The three-dimensional （3D） dynamic and thermal structures of eye and eyewall were studied based on the simulated results. The focus was investigation of the characteristics of the vortex Rossby waves in the inner-core region. We found that the Rossby vortex waves propagate azimuthally upwind against the azimuthal mean tangential flow around the eyewall, and their period was longer than that of an air parcel moving within the azimuthal mean tangential flow. They also propagated outward against the boundary layer inflow of the azimuthal mean vortex. Puthermore, we studied the connection between the spiral potential vorticity （PV） bands and spiral rainbands, and found that the vortex Rossby waves played an important role in the formation process of spiral rainbands.展开更多
在台风的发生、发展过程中,水平湍流混合是重要的物理过程,该过程的参数化方案对台风个例的数值模拟结果有很大影响.研究通过调整WRF(Weather Research and Forecasting Model)模式水平湍流参数化方案中的Smagorinsky系数Cs(Smagorinsky...在台风的发生、发展过程中,水平湍流混合是重要的物理过程,该过程的参数化方案对台风个例的数值模拟结果有很大影响.研究通过调整WRF(Weather Research and Forecasting Model)模式水平湍流参数化方案中的Smagorinsky系数Cs(Smagorinsky Coefficient)控制水平湍流混合的强弱,对比分析了水平湍流混合强度在台风莫拉克(2009)数值模拟中对台风强度和结构的影响.结果表明:水平湍流混合对莫拉克台风路径的模拟没有显著影响,但对台风的最大强度有显著影响,随着Cs增大台风强度减弱,热力场分析表明过大的水平混合不利于台风暖心的维持.从轴对称风场特征来看,Cs变化的影响并非集中在边界层中,台风中上层的风场均发生了改变,但边界层中变化更大,随着Cs增大最大风速半径外扩.从雨带和对流的发展特征来看,Cs越小越有利于模拟出单点发展的对流胞,但这些对流胞不易组合发展为有组织的对流带,而Cs过大时,对流胞出现涡丝化发展形态而发展为平滑的长雨带,减弱了雨带内的对流强度但使对流区的分布更趋于对称化,且过大的水平交换作用不利于雨带精细结构的模拟.展开更多
基金sponsored by the National Natural Science Foundation of China under Grant No.400750112001 PIA 20026 the National Key program for Developing Basic Sciences:CHeRES(G 1998040907).
文摘The characteristics of helicity in a hurricane are presented by calculating the MM5 model output in addition to theoretical analysis. It is found that helicity in a hurricane mainly depends on its horizontal component, whose magnitude is about 100 to 1000 times larger than its vertical component. It is also found that helicity is approximately conserved in the hurricane. Since the fluid has the intention to adjust the wind shear to satisfy the conservation of helicity, the horizontal vorticity is even larger than the vertical vorticity, and the three-dimensional vortices slant to the horizontal plane except in the inner eye. There are significant horizontal vortices and inhomogeneous helical flows in the hurricane. The formation of the spiral rainband is discussed by using the law of horizontal helical flows. It is closely related to the horizontal strong vortices and inhomogeneous helical flows.
基金supported by the State Key Program of the National Natural Science Foundation of China (Grant No 40830958)the Research Project of Serious Oceanic Disasters Alerting and Application Technology (Grant No 2006BAC03B00)+1 种基金the Key Program of the State Key Laboratory of Disaster Weather (Grant No 2008LASW-A03)the National Natural Science Foundation of China(Grant No 40975021)
文摘The Advanced Weather Research and Forecasting Model （ARW） is used to simulate the local heavy rainstorm process caused by Typhoon Matsa over the northeastern coast of Zhejiang Province in 2005. The results show that the rainstorm was caused mainly by the secondary spiral rainband of the Stationary Band Complex （SBC） structure. Within the secondary spiral rainband there was a strong meso-β-scale convergence line generated in the boundary layer, corresponding very well to the Doppler radar echo band. The convergence line comprised several smaller convergence centers, and all of these convergence columns inclined outward. Along the convergence line there was precipitation greater than 20 mm occurring during the following one hour. During the heavy rainstorm process, the Doppler radar echo band, convergence line, and the precipitation amount during the following one hour, moved and evolved synchronously. Further study reveals that the vertical shear of radial wind and the low-level jet of tangential wind contributed to the genesis and development of the convergence columns. The combined effect of the ascending leg of the clockwise secondary circulation of radial wind and the favorable environment of the entrance region of the low-level jet of tangential wind further strengthened the convergence. The warm, moist inflow in the lower levels was brought in by the inflows of the clockwise secondary circulation and uplifted intensely at the effect of convergence. In the convectively instable environment, strong convection was triggered to produce the heavy rainstorm.
基金supported by the National Grand Fundamental Research 973 Prograrm of China(Grant No.2004CB418306)the National Natural Science Foundation of China(Grant Nos.40375017,40233036 and 40305004)the Ministry of Science and Technology Project of China(Grant No.2001CCA02200).
文摘Spiral rainband is a prominent structure of tropical cyclone. Though its forming mechanism, vortex Rossby wave theory, has been widely accepted in recent years, its internal structural features are still not well known. The spiral rainband in the severe tropical storm Kammuri (2002), which caused heavy rainfall in southeast China, is simulated using the mesoscale model MM5 (V3). Results show that the simulated spiral rainband propagates azimuthally at a speed close to that of vortex Rossby wave in theory, and is accom- panied with energy dispersion in the radial direction. The structural features of simulated spiral rainband are analyzed with the high-resolution model output including the full physical process. Positive vorticity, ascending motion, hori- zontal momentum and so on are highly concentrated in the spiral rainband. The convergent moisture of spiral rainband comes mostly from the planetary boundary layer under 1 km. Airflow from the outside of spiral rainband is convective instability, which can provide instability energy for convec- tion development. However, the atmospheric stratification in the inside of spiral rainband is neutral, implying that the instability energy has been released. There is a mesoscale strong wind band just near the spiral rainband in the outer side with a maximum wind speed exceeding 30 m/s, which results from the pressure force acceleration when the air flows into the spiral rainband along the gradient of pressure.
基金Key-Area Research and Development Program of Guangdong Province of China(2020B1111200001)National Natural Science Foundation of China(41875070,41575040)+1 种基金Beijige Open Research Fund for Nanjing Joint Center of Atmospheric Research(NJCAR2018MS02)Science and Technology Program of Yunnan(2018BC007)。
文摘During the movement of Typhoon Hato(2017)over land,heavy rainfall occurred when the spiral rainband which was about 100 km distance away from the center of the typhoon passed the Dayao Mountain(with an elevation of 1.2 km).In this study,the structures and forming mechanism of the heavy rainband along the mountain range are investigated by using high-resolution model simulations.The results show the importance of topography in causing the heavy rainband.Upslope of the steep terrain lifts the cyclonic flow to produce strong upward motion when the rainband passes across with high wind speed.At the same time,the warm and humid air is lifted to the steep slope,causing unstable energy to accumulate over the windward slope,which is conducive to the occurrence of rainfall.In particular,the convective cells generated upstream of rainband will further strengthen and develop due to the uplift when they move close to the mountain foot.Some precipitation particles in the convective cells fall to the ground while others move downstream with the intense updrafts,forming heavy rainfall near the summit.As a result,the largest accumulative rainfall coincides well with the orientation of the mountain ridge.
基金supported by the National Key Basic Research and Development Project of China (Grant Nos. 2004CB418301,2009CB421503)National Natural Science Foundation of China (Grant No. 40775033)the Chinese Special Scientific Research Project for Public Interest (Grant No.GYHY200806009)
文摘Typhoon Rananim （2004） was one of the severest typhoons landfalling the Chinese mainland from 1996 to 2004. It brought serious damage and induced prodigious economical loss. Using a new generation of mesoscale model, named the Weather Research and Forecasting （WRF） modeling system, with 1.667 km grid horizontal spacing on the finest nested mesh, Rananim was successfully simulated in terms of track, intensity, eye, eyewall, and spiral rainbands. We compared the structures of Rananim to those of hurricanes in previous studies and observations to assess the validity of simulation. The three-dimensional （3D） dynamic and thermal structures of eye and eyewall were studied based on the simulated results. The focus was investigation of the characteristics of the vortex Rossby waves in the inner-core region. We found that the Rossby vortex waves propagate azimuthally upwind against the azimuthal mean tangential flow around the eyewall, and their period was longer than that of an air parcel moving within the azimuthal mean tangential flow. They also propagated outward against the boundary layer inflow of the azimuthal mean vortex. Puthermore, we studied the connection between the spiral potential vorticity （PV） bands and spiral rainbands, and found that the vortex Rossby waves played an important role in the formation process of spiral rainbands.
文摘在台风的发生、发展过程中,水平湍流混合是重要的物理过程,该过程的参数化方案对台风个例的数值模拟结果有很大影响.研究通过调整WRF(Weather Research and Forecasting Model)模式水平湍流参数化方案中的Smagorinsky系数Cs(Smagorinsky Coefficient)控制水平湍流混合的强弱,对比分析了水平湍流混合强度在台风莫拉克(2009)数值模拟中对台风强度和结构的影响.结果表明:水平湍流混合对莫拉克台风路径的模拟没有显著影响,但对台风的最大强度有显著影响,随着Cs增大台风强度减弱,热力场分析表明过大的水平混合不利于台风暖心的维持.从轴对称风场特征来看,Cs变化的影响并非集中在边界层中,台风中上层的风场均发生了改变,但边界层中变化更大,随着Cs增大最大风速半径外扩.从雨带和对流的发展特征来看,Cs越小越有利于模拟出单点发展的对流胞,但这些对流胞不易组合发展为有组织的对流带,而Cs过大时,对流胞出现涡丝化发展形态而发展为平滑的长雨带,减弱了雨带内的对流强度但使对流区的分布更趋于对称化,且过大的水平交换作用不利于雨带精细结构的模拟.
