Typhoon Kai-tak Research Articles

Simulation of extreme waves using coupled atmosphere-wave modeling system over the South China Sea

A real-time two-way coupled atmosphere-wave model in the South China Sea (SCS) has been established based on the atmosphere model WRF and the wave model SWAN in this study. We implement a nested computational grid along the SCS in the SWAN model and utilize the WRF model to provide high-resolution mesoscale wind field and pressure field over the SCS under the influence of typhoons. The numerical simulation of the typhoon waves generated by typhoon Kai-tak (1213), Rammasun (1409) and Hato (1713) had been carried out. The results of the numerical simulation are compared with the results of the field observation and the satellite altimeter. The verification results show that the coupled WRF-SWAN model has a high accuracy in the simulation of typhoon waves under the influence of typhoons in the SCS. The results of the simulation reveal the asymmetrical distribution of wind speed and typhoon wave height generated by typhoons in the space. The intensity of the typhoon on the right is greater than that on the left of the typhoon track. The real-time two-way coupled atmosphere-wave model based on the WRF model and the SWAN model can accurately simulate the dynamic process of typhoon and the temporal and spatial distribution of the typhoon waves generated by typhoons. According to the results of the coupled WRF and SWAN model, a corresponding relationship between typhoon wind speed and wave height was obtained, which was combined with the statistical data of typhoon occurring in the SCS during 1949–2019, the distribution characteristics of typhoon wave in the SCS had been simulated. The statistical data grid with a spatial accuracy of 0.5 degree of typhoon waves in the SCS had been established, and then the statistical wave height of typhoon in the SCS based on the Pearson-III distribution had been simulated. Eight characteristic locations have been identified in the SCS, the highest frequency and the hugest height of typhoon wave is near the Luzon Strait, the average of typhoon wave height is reached 9.2 m. The second is southern Taiwan in the eight different selected positions, and the statistical average value of typhoon wave height is close to 8.0 m. Comparison of the Paracel Islands, Spratly Islands and Dongsha Islands, the smallest statistical value of the typhoon wave height is the waves in Spratly Islands, with a mean value of 3.0 m, and the statistical mean value of typhoon wave height near the Paracel Islands and Dongsha Islands are about 6.0 m. Based on the statistical results, combined with the frequency of typhoon in the region, the distribution of typhoon wave height can be calculated at any location in the SCS, and it can be provide to some references for the design of the coastal and ocean engineering.

Numerical investigation of Typhoon Kai-tak (1213) using a mesoscale coupled WRF-ROMS model — Part Ⅱ: Wave effects

Continuing along the path of previous papers in this series, this paper describes numerical work to investigate seven different computational schemes of wave impact on the typhoon processes. Investigations showed that the wave dynamics factor is the most severe impact factors and should therefore be considered for coupling atmosphere-ocean interaction during a typhoon. The simulation results reveal the temporal and spatial distribution of typhoons field, pressure field, typhoon waves, storm surges, wind-induced flow field, sea surface stress and heat flux. Typhoon waves have a spatial asymmetrical distribution, the storm surges and flow field have asymmetrical distribution in space and has lag in time. This paper is a companion paper to “Numerical investigation of Typhoon Kai-tak (1213) using a mesoscale coupled WRF-ROMS model”.

Sensitivity of WRF simulated typhoon track and intensity over the South China Sea to horizontal and vertical resolutions

To determine the grid resolutions of the WRF model in the typhoon simulation, some sensitivity analysis of horizontal and vertical resolutions in different conditions has been carried out. Different horizontal resolutions (5, 10, 20, 30 km), nesting grids (15 and 5 km), different vertical resolutions (35-layers, 28-layers, 20-layers) and different top maximum pressures (1 000, 2 000, 3 500, 5 000 Pa) had been used in the mesoscale numerical model WRF to simulate the Typhoon Kai-tak. The simulation results of typhoon track, wind speed and sea level pressure at different horizontal and vertical resolutions have been compared and analyzed. The horizontal and vertical resolutions of the model have limited effect on the simulation effect of the typhoon track. Different horizontal and vertical resolutions have obvious effects on typhoon strength (defined by wind speed) and intensity (defined by sea level pressure, SLP), especially for sea level pressure. The typhoon intensity simulated by the high-resolution model is closer to the real situation and the nesting grids can improve computational accuracy and efficiency. The simulation results affected by vertical resolution using 35-layers is better than the simulation results using 20-layers and 28-layers simulations. Through comparison and analysis, the horizontal and vertical resolutions of WRF model are finally determined as follows: the two-way nesting grid of 15 and 5 km is comprehensively determined, and the vertical layers is 35-layers, the top maximum pressure is 2 000 Pa.

Numerical Investigation of Fresh and Salt Water Distribution in the Pearl River Estuary during a Typhoon Using a Fully Coupled Atmosphere-Wave-Ocean Model

Typhoons are major marine dynamic disasters that affect the coastal ocean areas of China. During a typhoon, the coupling dynamic factors, such as wind, waves, storm surges, and river runoff, greatly enhance the mass and energy exchange at the various interfaces of the ocean. A fully coupled atmosphere-wave-ocean model in the South China Sea (SCS) was established based on the WRF, SWAN, and ROMS models. The variation of sea surface salinity (SSS) and ocean subsurface salinity caused by Typhoon Kai-tak (201213) was analyzed by the fully coupled model, and the basic characteristics of the response of the upper ocean to the typhoon are given in this paper. The simulation results demonstrate that the salinity of the sea surface showed a sharp change during Typhoon Kai-tak, and it changed gradually after entering the recovery period. During the passage of Typhoon Kai-tak, the disturbance caused by strong winds strengthened the mixing process of the water in the Pearl River Estuary (PRE) and its adjacent waters. As the typhoon developed, under the influence of Ekman pumping, the mixing effect between the subsurface and the bottom and the upper water was obvious. Before the impact of Typhoon Kai-tak, the salinity had obvious stratification characteristics along the water depth. Due to the influence of the storm surge, the surface water with increased salinity was transported to the estuary, which led to an increase in the salinity of the estuary’s surface water. In this condition, it is highly likely for there to be saltwater intrusion. The salinity distribution characteristics of three schemes (ROMS model only, coupled WRF-ROMS model, and fully coupled WRF-SWAN-ROMS model) were compared in this study. In the fully coupled WRF-SWAN-ROMS model, the disturbance of the bottom water was the most obvious, and the salinity value was greater than that of the coupled WRF-ROMS model, which indicates that under the influence of waves, the mixing and exchange abilities were strengthened.

Numerical investigation of Typhoon Kai-tak (1213) using a mesoscale coupled WRF-ROMS model

The Typhoon Kai-tak that occurred in August 2012 produced strong winds, heavy rain, extreme wave, and storm surge, which also had a significant impact on the coastal areas of China and Vietnam. Studying the formation and tracking the movement mechanism of this typhoon will help reduce future coastal disasters, as well as have important scientific significance. The impact of a typhoon creates strong mass transport and energy exchange between the atmosphere and the ocean which produces a strong interaction between the wind field and the flow field. A coupled atmosphere-ocean model in the South China Sea was established based on the mesoscale atmospheric model WRF and the regional ocean model ROMS. Typhoon Kai-tak was simulated using this model. The simulation results show that the coupled WRF-ROMS model indicate high simulation accuracy with respect of storm surge in the South China Sea under the influence of Typhoon Kai-tak. The simulation results also reveal the temporal and spatial distribution of Typhoon Kai-tak's field, storm surge, and wind-induced flow fields. The spatial asymmetry and time lag in the spatial–temporal distribution of sea surface temperature during Typhoon Kai-tak have been discussed. The heat exchange at the air-sea interface was very strong under the influence of Typhoon Kai-tak, and the latent heat generated by water vapor evaporation plays a dominant role in the heat exchange at the air-sea interface, which shows that the heat carried by the vaporization of the sea surface is one of the important factors for the decrease of sea temperature under the influence of a typhoon.

Three-Dimensional Temperature Field Change in the South China Sea during Typhoon Kai-Tak (1213) Based on a Fully Coupled Atmosphere–Wave–Ocean Model

Studying the sea–air interaction between the upper ocean and typhoons is crucial to improve our understanding of heat and momentum exchange between the atmosphere and the ocean. There is a strong heat flux exchange between the atmosphere and the ocean during the impact of a typhoon, and the physical fields, such as the wind field, wave field, flow field, and SST field, also interact with each other. A fully coupled Atmosphere–Wave–Ocean model in the South China Sea was established by the mesoscale atmospheric model WRF, wave model SWAN, and the regional ocean model ROMS based on the COAWST model system. Typhoon Kai-tak was simulated using this fully coupled model and some other coupled schemes. In this paper, the variation of sea surface temperature (SST) and ocean subsurface temperature caused by Typhoon Kai-tak is analyzed by the fully coupled model, and the basic characteristics of the response of the upper ocean to the typhoon are given. The simulation results demonstrate that the fully coupled WRF-SWAN-ROMS model shows that the typhoon passes through the sea with obvious cooling. In the cold eddy region, the sea surface temperature cools 4 to 5 °C, and the cooling zone is concentrated on the right side of the track. The change of sea surface temperature lags more than 12 h behind the change of sea surface height. The decrease of SST on the left side of the track was relatively small: ranging from 1.5 to 2.5 °C. The disturbance of typhoon causes the subsurface water to surge to the surface, changes the temperature distribution of the surface, and causes the mixing layer to deepen about 40 m to 60 m. The simulation results reveal the temporal and spatial distribution of sea temperature and mixed layer depth. The sea surface temperature field has an asymmetrical distribution in space and has a lag in time. The heat exchange at the air–sea interface is very strong under the influence of the typhoon. The heat exchange between the air and sea is divided into latent heat and sensible heat, and the latent heat generated by water vapor evaporation plays a dominant role in the heat exchange at the air–sea interface, which shows that the heat carried by the vaporization of the sea surface is one of the important factors for the decrease of sea temperature under the influence of the typhoon.

Simulation of the storm surge in the South China Sea based on the coupled sea-air model

Under global warming and climate change, coastal regions, including China, are increasingly vulnerable to typhoon damages. With typhoon gales, heavy rainfall, and storm surge, typhoons cause great economic losses and casualties to the coastal communities of China every year. Therefore, it is of great academic and social values to study the generation mechanism and movement mode of typhoons. However, accurately predicting the potential storm surge that a typhoon in growing can cause is a very challenging task. During the evolution of typhoons, the ocean-atmosphere (sea-air) interface has strong exchange of mass and energy. At the same time, various physical fields, such as wind, pressure, flow and wave fields interact strongly with each other. To accurately simulate the typhoons and the corresponding storm surge, the air-sea interaction is not to be neglected. Hence, it is very important to establish a reliable prediction model of sea-air coupling. In this paper, we established a fully two-way coupled sea-air model for South China Sea (SCS) by applying the mesoscale atmospheric model named WRF, the regional ocean model named ROMS, and the model coupling toolkit (MCT). In this complicated coupled sea-air model, WRF transfers the sea surface stress, net heat flux, sensible heat flux, latent heat flux, shortwave radiation flux, and longwave radiation flux to ROMS through MCT coupler. In turn, ROMS transfers sea surface temperature (SST) to WRF. By applying the fully coupled WRF-ROMS model, the storm surge induced by Typhoon Kai-tak , 2012 in the SCS is simulated and analyzed in detail. The computational capability and reliability of the coupled WRF-ROMS model are well verified by comparing the simulated typhoon track, and corresponding intensity and strength with the observed values in the field. The verification data show that the two-way coupled model has better accuracy and advantage in simulating typhoon dynamics than the one-way coupled model. Subsequently, the validated two-way coupled model is used to systematically study the atmospheric dynamics and the storm surge induced by typhoons in SCS. By analyzing the numerical simulation results, the wind and pressure field, as well as the spatial and temporal distribution characteristics of storm surge and wind-induced current, are obtained. We found that the spatial distribution of the wind, storm surge and flow fields caused by Typhoon Kai-tak is asymmetrical. The intensity of the typhoon on the right side of track is stronger than that on the left. Moreover, compared with the temporal evolution of typhoon, there is some delay in the evolution of the storm surge and the wind-induced current in the near shore.

Evaluation of numerical wave model for typhoon wave simulation in South China Sea

The simulating waves nearshore (SWAN) model has typically been designed for wave simulations in near-shore regions. In this study, the model's applicability to the simulation of typhoon waves in the South China Sea (SCS) was evaluated. A blended wind field, consisting of an interior domain based on Fujita's model and an exterior domain based on Takahashi's model, was used as the driving wind field. The waves driven by Typhoon Kai-tak over the SCS that occurred in 2012 were selected for the numerical simulation research. Sensitivity analyses of time step, grid resolution, and angle resolution were performed in order to obtain optimal model settings. Through sensitivity analyses, it can be found that the time step has a large influence on the results, while grid resolution and angle resolution have a little effect on the results.

Typhoon Kai-Tak: An Ocean’s Perfect Storm

Abstract An unusually intense sea surface temperature drop (ΔSST) of about 10.8°C induced by the Typhoon Kai-Tak is observed in the northern South China Sea (SCS) in July 2000. Observational and high-resolution SCS model analyses were carried out to study the favorable conditions and relevant physical processes that cause the intense surface cooling by Kai-Tak. Upwelling and entrainment induced by Kai-Tak account for 62% and 31% of the ΔSST, respectively, so that upwelling dominates vertical entrainment in producing the surface cooling for a subcritical storm such as Kai-Tak. However, wind intensity and propagation speed alone cannot account for the large ΔSST. Prior to Kai-Tak, the sea surface was anomalously warm and the main thermocline was anomalously shallow. The cause was a delayed transition of winter to summer monsoon in the northern SCS in May 2000. This produced an anomalously strong wind stress curl and a cold eddy capped by a thin layer of very warm surface water west of Luzon. Kai-Tak was the ocean’s perfect storm in passing over the eddy at the “right time,” producing the record SST drop and high chlorophyll-a concentration.

Modeled Oceanic Response and Sea Surface Cooling to Typhoon Kai-Tak

An ocean response to typhoon Kai-Tak is simulated using an accurate fourth-order, basin-scale ocean model. The surface winds of typhoon Kai-Tak were obtained from QuikSCAT satellite images blended with the ECMWF wind fields. An intense nonlinear mesoscale eddy is generated in the northeast South China Sea (SCS) with a Rossby number of O (1) and on a 50-100 km horizontal scale. Inertial oscillation is clearly observed. Advection dominates as a strong wind shear drives the mixed layer flows outward, away from the typhoon center, thus forcing upwelling from deep levels with a high upwelling velocity (＞ 30m day^(-1)). A drop in sea surface temperature (SST) of more than 9℃ is found in both observation and simulation. We attribute this significant SST drop to the influence of the slow moving typhoon, initial stratification and bathymetry-induced upwelling in the northeast of the SCS where the typhoon hovered.

Comparison of phytoplankton blooms triggered by two typhoons with different intensities and translation speeds in the South China Sea

Two phytoplankton blooms in the South China Sea (SCS), triggered by 2 typhoons with different intensities and translation speeds, were compared using remotely sensed chlorophyll a (chl a), sea surface temperature (SST), vector wind field, and best-track typhoon data. Typhoon Ling-Ling in 2001 was strong, with a maximum sustained surface wind speed of 59 m s(-1), and fast-moving with a mean translation speed of 4.52 m s(-1). Typhoon Kai-Tak in 2005 was weak with a maximum sustained surface wind speed of 46 m s(-1), and slow-moving with a mean translation speed of 2.87 m s(-1). The weak, slow-moving typhoon Kai-Tak induced phytoplankton blooms with higher chl a concentrations, while the strong, fast-moving typhoon Ling-Ling induced blooms over a larger area. On average, about 7 typhoons per year affect the SCS, among which 41% are strong (> 50 m s(-1)) and 59% are weak, while 64% are fast-moving (>4.4 m s(-1)) and 36% are slow-moving. We conservatively estimate that typhoon periods may account for 3.5% of the annual primary production in the oligotrophic SCS.

Foraminiferal shells in sediment traps: Implications of biogenic particle transport in the Kao-ping submarine canyon, Taiwan

Foraminifera collected from sediment traps deployed at two depths in the Kao-ping submarine canyon were analyzed to provide information on biogenic particle transport. The discovery of benthic foraminiferal shells in the collecting cups was unexpected because the sediment traps were deployed at levels of 54 and 104 m above the seafloor of 290 m depth. The presence of shelf-originated benthic foraminifera captured by the sediment traps suggests that the canyon is not only a conduit for delivering terrestrial materials into the ocean but also acts as a passage allowing particles of marine-origin to be transported toward the shore. Furthermore, during the passing of Typhoon Kai-Tak, the cups at both upper and lower levels collected a higher diversity of benthic foraminifera species than at other time intervals, while there was also an increased similarity in the species collected by all cups. Most of the benthic taxa found in collecting cups were also present at as forms living in surface sediments from the shelf and slope as determined by staining, supporting a shore-wards transport particles of marine origin in the submarine canyon. Nevertheless, the stable isotopic compositions of the benthic foraminifera Cibicides wuellerstorfi displayed a similar range of variation as the other planktonic foraminifera ( Globigerina bulloides, Globigerinoides sacculifer and G. ruber), indicating these species dwelled at a relatively shallow depth (<50 m). The similarity of δ 18O− δ 13C compositions for both planktonic and benthic foraminifera collected from the sediment traps and also from the underlying surface sediments implies that most of the foraminiferal shells precipitated their shells locally and were transported by either settling out of the water column from the adjacent shelf or resuspension from the underlying sea floor.