Typhoon Wind Modeling and Flutter Fragility Analysis of Long-Span Bridges in Coastal Regions of China

Typhoon or hurricane or tropical cyclone, which is a large-scale air rotating system around a low atmospheric pressure center, frequently causing devastating economic loss and human casualties along coastal regions due to violent winds, heavy rainfall, massive storm surges, flash flooding or even landslides in mountainous areas. The coastal region of China, which is characterized by high population densities and well-developed cities, is always exposed to typhoon threats with 7~8 landfall typhoons every year since Western Pacific Basin is the most active typhoon basin on earth, accounting for almost one-third of global annual storms. With more long-span bridges are being constructed along this coastal area, it is of great importance to perform the risk assessments on these flexible or wind-sensitive structures subjected to typhoon winds. To reconstruct the mean typhoon wind speed field, a semi-analytical height-resolving typhoon boundary layer wind field model, including a parametric pressure model and an analytical wind model was first developed in Chapter 2 using a scale analysis technique. Some basic characteristics of the inner structure of typhoon wind field, such as the logarithmic vertical wind profile near the ground and super-gradient winds were reproduced. Then, Chapter 3 develops a dataset of two wind field parameters, i.e. the radius to maximum wind speed, R_(max,s) and the Holland pressure profile parameter, B_s in Western Pacific Ocean using the wind data information from best track dataset archived by the Japan Meteorological Agency (JMA) coupled with the present wind field model. The proposed dataset of R_(max,s) and B_s is able to reproduce the JMA wind observations as closely as possible, which allows performing more accurate typhoon wind hazard estimation. On this basis, the maximum wind hazard footprints for over-water, roughness only and roughness and topography combined conditions of 184 observed landed or offshore typhoon-scale storms are generated and archived for risk assessment. Moreover, this supplementary dataset of R_(max,s) and B_s enables the development of recursive models to facilitate both sub-region typhoon simulations and full track simulations. Since the present wind field model can only generate long-time-duration speed, say 10-min mean wind speed, Chapter 4 develops an algorithm to compute the gust factor curve by taking the non-stationary and non-Gaussian characteristics of typhoon winds into account. The real wind data of nine typhoons captured by the structural health monitoring system (SHMS) installed in Xihoumen Bridge were utilized to validate the proposed model. Then, the probability distributions of gust factor associated with any gust time duration of interest can be readily achieved after introducing the statistical models of skewness and kurtosis of typhoon winds. To predict the typhoon wind hazard along the coastal region of China, a geographically-weighted-regression (GWR) -based subregion model was proposed in Chapter 5. The storm genesis model was first applied to a circular boundary around the site of interest. Then, the typhoon forward model including the tracking model, intensity model, and wind field parameter model was developed utilizing the GWR method. A series of performance assessments were performed on the present subregion model before it was employed to predict the typhoon wind hazards around the coastal regions of China. Chapter 6 develops a framework to investigate the probabilistic solutions of flutter instability in terms of critical wind speed accounting for multiple resources of uncertainty to facilitate the development of the fragility curve of flutter issue of long-span bridges. The quantifications of structural uncertainties, as well as aerodynamic uncertainties or the randomness of flutter derivatives, were conducted using both literature survey and experimental methods. A number of probabilistic solutions of flutter critical wind speed for two bridges, say a simply supported beam bridge and the Jiangyin Suspension Bridge were achieved by introducing different sources of uncertainty utilizing both 2D step-by-step analysis and 3D multimode techniques. To examine the flutter failure probability of long-span bridge due to typhoon winds, a case study of a 1666-m-main-span suspension bridge located in the typhoon-prone region was performed. The fragility curves of this bridge in terms of critical wind speed and the typhoon wind hazards curves of the bridge site as the probability of occurrence with respect to any years of interest were developed, respectively by exploiting the techniques achieved in previous chapters. Then a limit state function accounting for the bridge-specific flutter capacity and the site-specific mean typhoon wind hazard as well as the gust factor effects was employed to determine the flutter failure probabilities utilizing Monte Carlo simulation approach

Investigation of wind characteristics of typhoon boundary layer through field experiments and CFD simulations

High-resolution observations of typhoon boundary layer above 100 m are rare as traditional wind towers are generally below 100 m, which limits the study of typhoon boundary layer and engineering applications such as wind-resistant design of tall buildings and wind turbines in typhoon-prone regions. In this study, boundary layer winds of super typhoon Lekima (2019) are observed, simulated and analyzed. Together with traditional wind tower, Doppler wind lidar is utilized for observations of typhoon boundary layer in order to obtain measured data above 100 m. Besides, Computational Fluid Dynamics (CFD) simulation based on Large Eddy Simulation (LES) method is conducted to further investigate the impact of complex terrain on the near-surface wind characteristics. The results show that the power law fits the mean wind speed profile well below 100 m. However, before and after the typhoon lands, a local reverse or low-level jet occurs in the mean wind speed profile at the height of 100–300 m, which cannot be depicted by the power law. Meanwhile, the turbulence intensity increases with height and experiences larger fluctuations. In addition, there is a significant negative correlation between the ground elevation and power exponents of the fitted mean wind speed profiles. This study provides useful information to better understand wind characteristics of the typhoon boundary layer

Impact of steering flow on tropical cyclone predictability

Although tropical cyclone (TC) track forecasts in numerical weather prediction models have improved considerably over the past few decades, there remain cases with large uncertainty. Typhoon Haiyan (2013) and Typhoon Hagupit (2014) are examples of two high-impact storms where, despite similarities in the observed track and intensity, the predictability of the storms differed greatly. Ensemble forecasts showed large uncertainty in the track of Hagupit, whereas the ensemble spread for Haiyan was considerably less. Using the Met Offce's Unifed Model, 5-day global and convection-permitting (CP) ensemble forecasts are analysed for both storms. Global forecasts show Haiyan was located on the southern periphery of the subtropical high and embedded in a strong easterly flow. In contrast, the steering flow of Hagupit was weak as the TC became located between two anticyclones. We show that Hagupit's position between the anticyclones, the strength of the anticyclones, the interactions between the TC outflow and its environment, and the upper-level geopotential height directly to the south of the TC contributed to whether Hagupit would make landfall over the Philippines or turn to the north. The track forecasts in the CP ensembles of both storms produced errors which were not present in the global forecasts. For Haiyan, CP forecasts predicted the motion of the TC to be too slow, whilst for Hagupit the CP forecasts predicted the TC to make a systematic south-west turn, away from the actual storm path. For a third high-impact TC, Hurricane Florence (2018), CP forecasts predicted the storm to move too far to the west before turning north. Analysis of these forecast busts shows differences in how TCs interact with upper-level steering winds, particularly in periods of strong vertical wind shear, can cause differences in the global and CP track forecasts

An Exploration of Wind Stress Calculation Techniques in Hurricane Storm Surge Modeling

As hurricanes continue to threaten coastal communities, accurate storm surge forecasting remains a global priority. Achieving a reliable storm surge prediction necessitates accurate hurricane intensity and wind field information. The wind field must be converted to wind stress, which represents the air-sea momentum flux component required in storm surge and other oceanic models. This conversion requires a multiplicative drag coefficient for the air density and wind speed to represent the air-sea momentum exchange at a given location. Air density is a known parameter and wind speed is a forecasted variable, whereas the drag coefficient is calculated using an empirical correlation. The correlation’s accuracy has brewed a controversy of its own for more than half a century. This review paper examines the lineage of drag coefficient correlations and their acceptance among scientists

Thermal response to sequential tropical cyclone passages: Statistic analysis and idealized experiments

The cold wake caused by a tropical cyclone (TC) extends for hundreds of kilometers and persists for several weeks, thus influencing the surface response for any subsequent TCs that might pass over it. It is commonly accepted that sea-surface temperature (SST) cooling, as produced by a single TC, occurs primarily through vertical mixing. However, when there are sequential TCs, the earlier TC can dramatically change the thermal structure of the upper ocean, which may influence the subsequent development of a latter-occurring TC (LTC). Therefore, the contribution of horizontal advection and vertical mixing to SST-cooling during the passage of LTCs is of great interest. Using a 19-year-long observational dataset and the heat budget analysis of an idealized numerical simulation, the SST change during the passage of sequential TCs is investigated. The results demonstrate that, on average, the SST cooling caused by the LTC shows an overall decreasing trend with enhanced lingering wakes. Budget analysis of the model simulations suggests that an earlier TC can suppress the vertical mixing induced by an LTC mainly through an alteration of dynamics within the deepened mixed layer and that the contribution of vertical mixing to the SST cooling is weaker due to the intensification of the earlier TC. The weakened vertical mixing dominates the decreased SST cooling induced by an LTC. In contrast, the cold wake generated by an earlier TC can produce more cold water on the right side of the TC’s track, which contributes to stronger horizontal advection upon the arrival of the LTC. In general, the effects of the earlier TC can suppress the sea-surface thermal response to an LTC. If the contribution of the horizontal advection to SST cooling is neglected, the SST cooling induced by an LTC could be reduced by about 40%. As for the response of the sub-surface water to the passage of an LTC, the weakened warm anomaly induced by vertical mixing and the enhanced cooling anomaly caused by the vertical advection explain the reduced tendency for the mixed layer to deepen. As a result, the tendency for the mixed layer depth (MLD) to increase is suppressed during the passage of an LTC. These results highlight the importance of optimally depicting cold wakes in numerical simulations to improve the prediction of the upper ocean’s response to sequential TCs

Oceanic response to the consecutive Hurricanes Dorian and Humberto (2019) in the Sargasso Sea

Understanding the oceanic response to tropical cyclones (TCs) is of importance for studies on climate change. Although the oceanic effects induced by individual TCs have been extensively investigated, studies on the oceanic response to the passage of consecutive TCs are rare. In this work, we assess the upper-oceanic response to the passage of Hurricanes Dorian and Humberto over the western Sargasso Sea in 2019 using satellite remote sensing and modelled data. We found that the combined effects of these slow-moving TCs led to an increased oceanic response during the third and fourth post-storm weeks of Dorian (accounting for both Dorian and Humberto effects) because of the induced mixing and upwelling at this time. Overall, anomalies of sea surface temperature, ocean heat content, and mean temperature from the sea surface to a depth of 100 m were 50 %, 63 %, and 57 % smaller (more negative) in the third-fourth post-storm weeks than in the first-second post-storm weeks of Dorian (accounting only for Dorian effects), respectively. For the biological response, we found that surface chlorophyll a (chl a) concentration anomalies, the mean chl a concentration in the euphotic zone, and the chl a concentration in the deep chlorophyll maximum were 16 %, 4 %, and 16 % higher in the third-fourth post-storm weeks than in the first-second post-storm weeks, respectively. The sea surface cooling and increased biological response induced by these TCs were significantly higher (Mann-Whitney test, p < 0.05) compared to climatological records. Our climatological analysis reveals that the strongest TC-induced oceanographic variability in the western Sargasso Sea can be associated with the occurrence of consecutive TCs and long-lasting TC forcing

Ocean observations in support of studies and forecasts of tropical and extratropical cyclones

© The Author(s), 2019. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Domingues, R., Kuwano-Yoshida, A., Chardon-Maldonado, P., Todd, R. E., Halliwell, G., Kim, H., Lin, I., Sato, K., Narazaki, T., Shay, L. K., Miles, T., Glenn, S., Zhang, J. A., Jayne, S. R., Centurioni, L., Le Henaff, M., Foltz, G. R., Bringas, F., Ali, M. M., DiMarco, S. F., Hosoda, S., Fukuoka, T., LaCour, B., Mehra, A., Sanabia, E. R., Gyakum, J. R., Dong, J., Knaff, J. A., & Goni, G. Ocean observations in support of studies and forecasts of tropical and extratropical cyclones. Frontiers in Marine Science, 6, (2019): 446, doi:10.3389/fmars.2019.00446.Over the past decade, measurements from the climate-oriented ocean observing system have been key to advancing the understanding of extreme weather events that originate and intensify over the ocean, such as tropical cyclones (TCs) and extratropical bomb cyclones (ECs). In order to foster further advancements to predict and better understand these extreme weather events, a need for a dedicated observing system component specifically to support studies and forecasts of TCs and ECs has been identified, but such a system has not yet been implemented. New technologies, pilot networks, targeted deployments of instruments, and state-of-the art coupled numerical models have enabled advances in research and forecast capabilities and illustrate a potential framework for future development. Here, applications and key results made possible by the different ocean observing efforts in support of studies and forecasts of TCs and ECs, as well as recent advances in observing technologies and strategies are reviewed. Then a vision and specific recommendations for the next decade are discussed.This study was supported by the National Oceanic and Atmospheric Administration and JSPS KAKENHI (Grant Numbers: JP17K19093, JP16K12591, and JP16H01846)

Oceanic Internal Waves and Internal Tides in the East Asian Marginal Seas

Oceanic internal waves (IWs) at frequencies from local inertial (e.g., near-inertial internal waves) to buoyancy frequencies (nonlinear internal waves or internal solitary waves), sometimes including diurnal and semidiurnal tidal frequencies, play an important role in redistributing heat, momentum, materials, and energy via turbulent mixing. IWs are found ubiquitously in many seas, including East Asian marginal seas (Indonesian Seas, South China Sea, East China Sea, Yellow Sea, and East Sea or Japan Sea), significantly affecting underwater acoustics, coastal and offshore engineering, submarine navigation, biological productivity, and the local and global climate. Despite decades of study on the IWs in some regions, our understanding of the IWs in the East Asian marginal seas is still in a primitive state and the mechanisms underlying every stage (generation, propagation, evolution, and dissipation) of IWs are not always clear. This Special Issue includes papers related to all fields of both low- and high-frequency IW studies in the specified region, including remote sensing, in situ observations, theories, and numerical models

The Southern Ocean Observing System (SOOS)

[in “State of the Climate in 2014” : Special Supplement to the Bulletin of the American Meteorological Society Vol. 96, No. 7, July 2015