Tropical Cyclone-Induced Strong Wind And Its Engineering Parameter Characteristics In The Coastal Areas Of China

Strong winds induced by tropical cyclones(TCs)must be taken into consideration when performing engineering wind-resistant design and TC disaster mitigation in the coastal areas of China that are subject to the frequent influence of TCs.According to the actual needs of engineering wind-resistant design,characteristics and mechanisms of TC-induced strong wind and its engineering parameters are investigated,by use of statistical analysis,observational analysis,dynamic diagnostics and numerical simulation.Our results are supposed to get better understanding on this issue,and provide some clues for engineering wind-resistant design and risk assessment.At first,climatological characteristics of the radius of maximum wind(RMW),the maximum surface wind speed,the return period wind,the most dangerous TC track and the wind destructiveness index(WDI)are investigated.Results indicate that: 1)the averaged RMW is 40.5km in coastal areas of China.And stronger TCs are mainly located near the eastern side of Taiwan Island(TW)and the northeast side of Hainan Island(HN).2)Methods of searching the most destructive TC track and calculating the return period wind speeds induced by TCs are proposed based on the best track datasets and the reanalysis datasets.3)The WDI we proposed can indicate the most dangerous regions which are subject to the influence of TC-induced strong winds efficiently.And the sea areas on the eastern side of TW and HN are within higher risks.Statistical analysis is performed on the wind structure of TC to investigate the variations and differences on different directions related to the TW and HN islands.Results show that: 1)When TCs locate on different directions of the Island,the low-level external wind fields are in different asymmetric distributions.Vertical vorticity belts are distributed in the positive and negative values around the TC center.And the higher the terrain is,the more obvious these characteristics will be.2)In regions on the east and northeast of TW,the Taiwan Strait,the northeast of HN and the northwest of Beibu Gulf,the stronger external wind of TCs occur more frequently.3)For the inner core winds,the averaged RMW in regions around the TW(HN)is 51.3km(58.3km).The radiuses of 34 kts wind(R34)are deformed,and the largest one appears on the eastern quadrant.The stronger the TC is,the bigger the R34 will be.4)When TC locates on the northwestern(southwestern)side of the TW(HN)island,its wind circle is severely deformed.Stronger TC tends to be deformed more severely than the weaken ones in regions near the TW.But the conclusion is opposite when TCs locate near the HN.Taking Mujigae(2015)as an example,observational analysis is performed to studies the wind characteristics in the near-surface layer.1)When under the effects of eyewall and the spiral rainband,the averaged wind speeds at different heights change as the shape of letter “M”.The wind direction rotates around by 180o,and the vertical velocity increases 3-4 times when under the attack of eyewall.The attack anger also increases and exceeds the design specification recommendation under eyewall's effect.The wind profile does not meet the power law recommended by the specification in the eyewall and the spiral rainband.2)The value of gust factor increases when the eyewall and the spiral rainband pass by.After TC made landfall,its fluctuation amplitude increased significantly.3)The turbulent intensity and turbulent energy spectrum are all significantly increases when the eyewall and the spiral rainband pass through.The assumption of turbulence isotropy and the "-5/3" law are no longer satisfied.Using the fine winds simulated by the WRF model,TC wind engineering parameters could be quantitatively assessed.The assessment results show that: 1)Severe TC winds are distributed along its track.And the wind speed on the right side of the path is greater than the ones on the left side.2)Larger wind attack angles are mainly located near the TC path and terrains.Their absolute values can exceed 3o.In the windward(leeward)area,the attack angles are positive(negative).The higher the terrain is,the larger the angle will be.3)Engineering wind parameters such as the wind attack angle,the maximum wind speed,horizontal wind speed shear,temporal variation of wind speed and direction,and storm relative helicity can qualify the effects of TC-induced strong winds on engineering structures and efficiently indicate higher risk regions of TC,such as the inner core and terrains.4)Both the height and shape of terrain can affect the characteristics of wind engineering parameters.After reducing the height of mountains,the frequency and value of wind speed will decrease(increase)on the windward slope(leeward slope).And the wind attack angle decreases to the recommended range of the code.Finally,using the simulation results,characteristics and mechanisms of strong wind variation are discussed.Results show: 1)During the TC making landfall,the total range of azimuthally averaged gale increases first and then decreases,while variation of the RMW is opposite.These changes of range are closely related to the radial absolute angular momentum flux at low-levels.2)Affected by the joint influences of movement and vertical wind shear,TC winds are asymmetrically distributed.Winds in the northeast and northwest quadrants tend to be stronger.In addition,regions in the eyewall are affected by the Vortex Rossby Wave propagation,making the asymmetric distribution fluctuated.3)The rolls in the TC boundary layer can transmit kinetic energy from higher levels to the ground,creating a strip-like gale distribution in the eyewall.