Typhoon Chan-hom Research Articles

Influence of region-dependent error growth on the predictability of track and intensity of Typhoon Chan-hom (2020) in high-resolution HWRF ensembles

Enhancing the predictive capabilities for tropical cyclones (TCs) necessitates comprehensive investigations into their predictability. This study employs convection-allowing ensemble forecasts using the Hurricane Weather Research and Forecasting model, integrating perturbed initial conditions to examine error growth and initial condition error-related predictability (IE-predictability) of TCs. Distinctively, our research concentrates on how the predictability of TC track and intensity is influenced by initial errors in various zones: (1) the inner core and outer rainbands (0–350 km), (2) the near environment (350–1300 km), and (3) the far environment (1300–3500 km).Contrary to many prior studies, our findings suggest that the most critical region for initial errors affecting TC track forecasts is within the TC inner structures. For Typhoon Chan-hom (2020), characterized by significant track forecast discrepancies, the initial inaccuracies in the inner core and outer rainbands substantially affect the TC's proximate environment and track, more so than errors in the near and far environments. Regarding TC intensity, the inner core emerges as the most sensitive area. The surface wind configuration of the TC inner core at larger scales (wavenumbers 0–2) maintains predictability for over three days, whereas its structure at finer scales is only predictable for a few hours. Plain language summaryThe forecasting of tropical cyclones (TCs) has seen significant advancements over recent decades. A critical question arises: to what extent can we further enhance the accuracy of TC predictions? Addressing this requires in-depth exploration of TC predictability. In our study, we utilized convection-allowing ensemble forecasts based on the Hurricane Weather Research and Forecasting (HWRF) model, incorporating perturbed initial conditions to analyze the development of forecast errors and the predictability related to these initial errors in TCs. A novel aspect of this research is its emphasis on the impact of initial errors in different regions on the predictability of TC track and intensity. Our key findings are twofold: (1) The region most impacted by initial errors for TC track predictions may not always be the surrounding environment. Instead, errors within the TC's inner core and outer rainbands, owing to their pronounced interplay, can lead to significantly greater track forecast errors compared to those in the near and far environments. (2) As for TC intensity, the inner core emerges as the most susceptible region. The large-scale components (wavenumbers 0–2) of the TC's inherent vortex flow can be forecasted accurately for over four days, largely due to the predictability of the synoptic-scale environment. In contrast, the smaller-scale components (wavenumbers >2) demonstrate predictability for only a few hours.

Typhoon Chan-Hom (2015) induced sediment cross-shore transport in the mud depo-center of the East China Sea inner shelf

The erosion, transport, and deposition processes of sediments that are influenced by typhoons are significant constituents of the sedimentary source-sink processes in marginal seas. Nevertheless, the genesis of storm deposit layers in the continental shelf and their subsequent development and preservation after typhoons have not been comprehensively investigated. In this study, we have conducted a systematic investigation of the processes related to sediment transport and deposition, which were primarily induced by Typhoon Chan-Hom, and analyzed the evolution and preservation of storm deposits utilizing grain-size analysis, radionuclides (137Cs, 210Pbex and 7Be), and organic geochemistry (TOC, TN and δ13C). Sedimentary 137Cs, 210Pbex, and 7Be inventories measured three days after the typhoon suggest that the sediments in the water depth < 30 m were eroded, resuspended, and then transported offshore to deposit in the deeper areas (water depth > 50 m) of the Muddy Deposition Center of Zhejiang-Fujian Coast (MDC-ZFC), resulting in the formation of significant storm deposits (thickness of 8–16 cm) with high 210Pbex and 7Be. In addition to sediment transport, the offshore area also received organic matter generated by typhoon-induced phytoplankton blooms. Hence, in the present study area, the features of marine organic matter that possess high total organic carbon (TOC) content (>0.4%), low carbon to nitrogen (C/N) ratio (<7.5), and high δ13C value (> − 22‰) are crucial for identifying the storm deposit formed by cross-shore sediment transportation. In contrast, the commonly used indicators for identifying typhoon sedimentary records that rely on grain-size coarsening are not suitable for identifying storm deposits at the MDC-ZFC edge due to the influence of relict sands. Furthermore, the radionuclides and organic geochemical signals of the sediments revealed that no significant disturbance or transport of the storm sediments occurred three weeks after the passage of Typhoon Chan-Hom due to the high water depth, indicating that these storm deposits could be effectively preserved in this area. This study provides a valuable basis and reference for accurately identifying and interpreting typhoon sedimentary records in the MDC-ZFC.

The Influence of Typhoon-Induced Wave on the Mesoscale Eddy

The strong wind-induced current and sea level have influences on the wave distribution in a tropical cyclone (TC). In particular, the wave–current interaction is significant in the period in which the TC passed the mesoscale eddy. In this study, the wave fields of Typhoon Chan-hom (2015) are hindcastly simulated using a coupled oceanic model that utilizes a nested triangle grid, i.e., the finite-volume community ocean model-simulating waves nearshore (FVCOM-SWAVE) model. The forcing wind field is composited from the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis data and the simulation using a parametric Holland model, denoted as H-E. The open boundary fields include tide data from TPOX.5 and the hybrid coordinate ocean model (HYCOM) global datasets, including sea surface temperature (SST), sea surface salinity, sea surface current, and sea level data. The simulated oceanic parameters (e.g., the significant wave height, SWH) are validated against the measurements from the Jason-2 altimeter, yielding a root mean square error (RMSE) of 0.58 m for the SWH, a correlation (COR) coefficient of 0.94, and a scatter index (SI) of 0.23. Similarly, the simulated SSTs are compared with the remote sensing products of the remote sensing system (REMSS) and the measurements from Argos, yielding an RMSE of &lt;0.8 °C, a COR of &gt;0.95, and an SI of &lt;0.04. The significant zonal asymmetry of the wave distribution along the typhoon track is observed. The Stokes drift is calculated from the FVCOM-SWAVE simulation results, and then the contribution of the Stokes transport is estimated using the Ekman–Stokes numbers. It is found that the ratio of the Stokes transport to the total net transport can reach &gt;80% near the typhoon center, and the ratio is reduced to approximately &lt;20% away from the typhoon center, indicating that Stokes transport is an essential aspect in the water mixing during a TC. The mesoscale eddies are detected by the sea level anomalies (SLA) fusion data from AVISO. It is found that the significant wave heights, Stokes drift, and Stokes transport inside the eddy area were higher than those outside the eddy area. These parameters inside the cold mesoscale eddies were higher than t inside the warm mesoscale eddies. Otherwise, the SST mainly increased within the cold mesoscale eddies area, while decreased within the warm mesoscale eddies area. The influence of mesoscale eddies on the SST was in proportion to the eddy radius and eddy EKE.

Dynamics of Upper-Frequency-Range Infrasonic Wave Generation in the Northwestern Part of the Sea of Japan

The Pacific Northwest region is significantly affected by tropical cyclones, which pose a threat to humans due to strong winds and intense precipitation. Observations have shown that international meteorological agencies lower the hazard class of typhoons too early, or even stop tracking them altogether. We studied the results of in situ measurements of infrasonic microseismic oscillations in the upper frequency range, generated in the shelf area of the northwestern part of the Sea of Japan as a result of the long-term influence of strong tropical cyclones on the sea area. The measurement data were obtained using coastal laser strainmeters, which are components of the seismoacoustic hydrophysical measuring complex. The dynamics of upper-frequency-range infrasonic microseismic signals during the movement of Typhoon Chan-Hom 2015 in the Sea of Japan are shown. The same infrasonic microseismic oscillations excited by Typhoon 2022 Hinnamnor, which had no impact on the measuring station, have a difference in the width of the signal spectrum and frequency shift with maximum amplitude compared to the signals from Typhoon Chan-Hom.

Intermittent migration of mixing front driven by typhoon events on the inner shelf of the East China Sea: A FVCOM modeling study

Mixing fronts on the inner shelf of the East China Sea (ECS) are common under normal summer conditions, yet its migration during extreme weather events, such as typhoons, has been poorly understood due to limited availability of observational data. In this study, we used a well-validated Finite-Volume Community Ocean Model (FVCOM) to investigate the intermittent migration of mixing fronts on the inner shelf of the ECS during the passage of Typhoon Chan-hom in July 2015. The sedimentary and ecological effects of the front migration were also revealed. Results suggested that the potential energy anomaly (PEA) of 10 J/m3 could be a critical value to determine the location of the mixing front, which was consistent with that derived from the Simpson-Hunter (SH) parameter. Accordingly, the mixing front showed a rapid seaward migration from 15-m isobaths to 60-m isobaths under the drastic ocean turbulence during the typhoon and recovered shortly after its passage. This intermittent migration of the mixing front had the potential to facilitate cross-shelf material transport during the typhoon. On the one hand, the front migration could not only open the access for the seaward transport of nearshore resuspended sediments but also trap them within a certain area, causing an offshore deposition between 20 and 60 m isobaths south of the typhoon track. The mixing fronts had a boundary effect constraining the typhoon-induced sedimentation. On the other hand, unlike sediments that would settle down with weakened ocean dynamics, the offshore movement of coastal diluted water triggered by the typhoon remained after the front recovery. These nutrient-rich waters could be transported to the middle or even outer shelf of the ECS by the subsequent southwest summer monsoon, enhancing local primary production and potentially trigering significant algal blooms.

Influence of Multiple Interactions of Three Typhoons and a Mid-Latitude Cloud Band-Associated Trough in the North West Pacific upon Severe Tropical Storm Linfa

Multiple interactions of three typhoons and a mid-latitude cloud band-associated with a trough (MLCT) were investigated from 1 July to 10 July 2015, using the Korea Communication, Ocean, and Meteorological Satellite (COMS) satellite images and two kinds of meteorological models, such as UM-KMA (U.K.) and WRF-3.6 (U.S.A.), to generate the horizontal structure of wind and relative humidity, streamline, and moisture flux. As severe tropical storm (STS) Linfa moved toward the warmer area with a sea surface temperature (SST) of 31 °C in the northern South China Sea, it obtained not only more moisture by thermal convection of water vapor from the sea surface toward the lower atmosphere but also more momentum by its multiple interactions with both the MLCT and Typhoon (TY) Chan-Hom. Through their mutual interactions, mutual feedback of moisture and momentum fluxes could accelerate the formation of clouds in their systems and an asymmetric structure of their circulations. After Linfa weakened due to the increased friction of the shallower sea bottom close to the Chinese coast and its disconnection from the MLCT, later it became re-intensified with the increased wind speeds by a stronger interaction with more intensified TY Chan-Hom entering the path of the Kuroshio Current of SST 31 °C, which could supply additional moisture through thermal convection of water vapor into its system. Then, further interaction between the rapidly developed TY Nangka following behind and the MLCT enhanced the transfer of moisture and momentum fluxes from Chan-Hom into Linfa. Finally, after STS Linfa made landfall on the Chinese coast, it decayed into a weak low-pressure system before its dissipating, due to the weakening of its cyclonic circulation through the increased friction by the shallower sea bottom and the surrounding lands.

Suspended sediment dynamics in macrotidal turbid Hangzhou Bay during Typhoon Chan-hom

Typhoons are extreme weather events that cause serious social and economic losses in coastal and estuarine areas worldwide. Understanding the impact of typhoons on sediment dynamics is essential for protecting coastal areas from these marine disasters. Hangzhou Bay is macro-tidal, turbid, and frequently affected by strong typhoons. In this study, we established and validated a three-dimensional model coupling waves, current, and sediment to investigate the sediment dynamics in Hangzhou Bay during Typhoon Chan-hom. The results showed that high suspended sediment concentration (SSC) areas during the typhoon were mainly located at the bay head and near the southern shore near the Andong tidal flat. The maximum bottom SSC was at least twice that near the surface. The peak bottom SSC values at the peak flood (6 g/L) were larger than those at the peak ebb (5 g/L) owing to the stronger current velocity at peak flood. The SSC near the northern shore was larger at the peak flood than at other times, and the SSC was high at the southern shore at peak ebb at the cross-section near the Andong tidal flat. The typhoon impacted SSC by changing the bottom stress. SSC was most influenced by wave action, followed by wind stress action, and was least influenced by air pressure action, which contributed 71.3%, 69.9%, and 1.8% to the bottom stress, respectively. Our findings are scientifically important for research on geomorphological evolution and are practically meaningful for coastal management.

Numerical Study of Circulation and Seasonal Variability in the Southwestern Yellow Sea

A nested-grid ocean circulation modelling system (NGMS-swYS) is used for examining the impact of tides and winds on the three-dimensional (3D) circulation, hydrography and seasonal variability over the southwestern Yellow Sea (swYS). The modelling system is based on the Princeton Ocean Model (POM) and uses a nested-grid setup, with a fine-resolution (~2.7 km) inner model nested inside a coarse-resolution (~9.0 km) outer model. The domain of the outer model covers the China Seas and adjacent deep ocean waters. The domain of the fine-resolution inner model covers the swYS and adjacent waters. The NGMS-swYS is driven by a suite of external forcings, including the atmospheric forcing, tides, freshwater discharge and currents specified at the lateral open boundaries. A comparison of model results with observations and previous numerical studies demonstrates the satisfactory performance of the NGMS-swYS in simulating tides, seasonal mean circulation and distribution of temperature and salinity. Five additional numerical experiments were conducted using NGMS-swYS with different combinations of external forcing. Analysis of model results demonstrates that the monthly mean circulation over the swYS is affected significantly by tides and winds, with large seasonal variability. The northward Subei Shoal Current occurred in both winter and summer months in 2015, with persistent strong southeastward mean currents induced by tides along the 50 m isobath. Model results also demonstrated that strong wind-induced currents occurred with large sea surface cooling during Typhoon Chan-Hom.

Investigation of Storm Tides Induced by Super Typhoon in Macro-Tidal Hangzhou Bay

Typhoon-induced storm tides can cause serious coastal disasters and considerable economic losses. Understanding the mechanisms controlling storm surges helps the prevention of coastal disasters. Hangzhou Bay (HZB), a typical macro-tidal estuary, is located on the east coast of China, where typhoons frequently occur. The funnel-shaped topography makes this macro-tidal bay even more sensitive to storm tides. Super Typhoon Chan-hom was used as an example to study the characteristics and dynamic mechanisms of storm surges using a well-validated numerical model. The model considers the two-way coupling of waves and tides. The wind strength for the model was reconstructed using multi-source wind data and was refined by considering different rotating and moving wind fields. The Holland–Miyazaki model was used to reconstruct the local wind-field data with a good performance. The model results show that the total water level of HZB during typhoon Chan-hom was mainly dominated by tides, and the storm surge was closely related to the wind field. Surface flow was mostly influenced by winds, followed by tides. The spatial and temporal distributions of the significant wave height were controlled by the wind and local terrain. Wind stress was the largest contributor to storm surges (91%), followed by the pressure effect (15%) and the wave effect (5%). Both wind and wave-induced surges occurred during low slack waters. The tide-surge interaction changes (enhance or suppress) the surge by approximately 0.5 m during the typhoon, comprising approximately 50% of the total surge. Tides interacted with surges through various mechanisms, from the bay mouth (local acceleration and friction) to the bay head (friction and advection). The Coriolis force had a relatively minor effect. The findings of this study provide useful information for studies on sediment dynamics and coastal structures under extreme weather conditions.

Impact of Typhoon Chan-hom on sediment dynamics and morphological changes on the East China Sea inner shelf

The East China Sea (ECS) inner shelf is highly impacted by tropical cyclones. Due to the limited availability of observations, the sediment dynamics and associated sedimentary process under the impact of typhoons are still poorly understood. In this study, a high-resolution unstructured-grid FVCOM model was employed to investigate the rapid responses of sediment dynamics on the ECS inner shelf to the passage of Typhoon Chan-hom in July 2015. The results indicated that there was an evident asymmetry in sediment dynamics around the typhoon track. During the typhoon, sediments were resuspended in nearshore area and transported downwind with increasing southwestward and seaward sediment flux. The gear-like sediment delivery presented a ratchet effect that induced an evident spatial asymmetry in responses of sediment dynamics and shelf sedimentation to the short-lived typhoon event. The coastal upwelling normally existing in summer was intermittently blocked by the passage of typhoon, and rapidly transited to downwelling, which substantially enhanced the cross-shelf sediment transport. As a result, severe erosions were mainly occurred near the typhoon track within 20-m isobaths, in contrast to newly formed sedimentary layers (~10 cm in thickness) in areas far south of the typhoon track and around the 50-m isobaths. The spatial asymmetry of sedimentary response to the typhoon passage is therefore of importance to locating sediment records that well preserved the typhoon signals. The ratchet effect we proposed provided reasonable interpretations of the spatially asymmetric responses of shelf sedimentations in the northern Gulf of Mexico and the central Great Barrier Reef shelf to tropical cyclones, respectively.

A Study of Wave-Induced Effects on Sea Surface Temperature Simulations during Typhoon Events

In this work, we investigate sea surface temperature (SST) cooling under binary typhoon conditions. We particularly focus on parallel- and cross-type typhoon paths during four typhoon events: Tembin and Bolaven in 2012, and Typhoon Chan-hom and Linfa in 2015. Wave-induced effects were simulated using a third-generation numeric model, WAVEWATCH III (WW3), and were subsequently included in SST simulations using the Stony Brook Parallel Ocean Model (sbPOM). Four wave-induced effects were analyzed: breaking waves, nonbreaking waves, radiation stress, and Stokes drift. Comparison of WW3-simulated significant wave height (SWH) data with measurements from the Jason-2 altimeter showed that the root mean square error (RMSE) was less than 0.6 m with a correlation (COR) of 0.9. When the four typhoon-wave-induced effects were included in sbPOM simulations, the simulated SSTs had an RMSE of 1 °C with a COR of 0.99 as compared to the Argos data. This was better than the RMSE and COR recovered between the measured and simulated SSTs, which were 1.4 °C and 0.96, respectively, when the four terms were not included. In particular, our results show that the effects of Stokes drift, as well as of nonbreaking waves, were an important factor in SST reduction during binary typhoons. The horizontal profile of the sbPOM-simulated SST for parallel-type typhoon paths (Typhoons Tembin and Bolaven) suggested that the observed finger pattern of SST cooling (up to 2 °C) was probably caused by drag from typhoon Tembin. SST was reduced by up to 4 °C for cross-type typhoon paths (Typhoons Chan-hom and Linfa). In general, mixing significantly increased when the four wave-induced effects were included. The vertical profile of SST indicated that disturbance depth increased (up to 100 m) for cross-type typhoon paths because the mixing intensity was greater for cross-type typhoons than for parallel-type typhoons.

Assimilation of GPM Microwave Imager Radiance for Track Prediction of Typhoon Cases with the WRF Hybrid En3DVAR System

The impact of assimilating radiance data from the advanced satellite sensor GMI (GPM microwave imager) for typhoon analyses and forecasts was investigated using both a three-dimensional variational (3DVAR) and a hybrid ensemble-3DVAR method. The interface of assimilating the radiance for the sensor GMI was established in the Weather Research and Forecasting (WRF) model. The GMI radiance data are assimilated for Typhoon Matmo (2014), Typhoon Chan-hom (2015), Typhoon Meranti (2016), and Typhoon Mangkhut (2018) in the Pacific before their landing. The results show that after assimilating the GMI radiance data under clear sky condition with the 3DVAR method, the wind, temperature, and humidity fields are effectively adjusted, leading to improved forecast skills of the typhoon track with GMI radiance assimilation. The hybrid DA method is able to further adjust the location of the typhoon systematically. The improvement of the track forecast is even more obvious for later forecast periods. In addition, water vapor and hydrometeors are enhanced to some extent, especially with the hybrid method.

Effects of nonlinear terms and topography in a storm surge model along the southeastern coast of China: a case study of Typhoon Chan-hom

Based on Finite Volume Coastal Ocean Model (FVCOM), this study constructed a numerical model covering the Bohai Sea, Yellow Sea, and East China Sea. National Centers for Environmental Prediction’s Climate Forecast System Reanalysis (NCEP-CFSR) data were used to drive the model to simulate a large storm surge generated by Typhoon Chan-hom. The model was validated by multiple in situ observations of water levels taken at tidal gauge stations. The effects of the nonlinear terms and topography on the modeling of storm surges were then studied. First, the tide–surge interaction during the storm surge process was analyzed. The results show that the tide–surge interaction can suppress the storm surge at the climax of the astronomical tide and benefit the growth of the storm surge at the ebb of the astronomical tide. The tidal constituents M2, S2 and K1 were added to analyze the influences of the amplitudes and periods of tides on the nonlinear reaction. The results indicate that the nonlinear effect will be enhanced by an increased tidal height; additionally, the nonlinear interaction of semidiurnal tides is more significant than that of diurnal tides. The semidiurnal fluctuation that appears near the peak-value time is also attributed to the tide–surge interaction. Moreover, the tide–surge interaction can be influenced by the water depth and position of the area of interest. According to the numerical results, topography has a certain impact on the storm surge: The peak value of the storm surge will decrease with increasing slope. The existence of the Ryukyu Islands reduces the area influenced by the storm surge on the southeast coast of China but expands the high-value storm surge area.

Assimilation of GPM Microwave Imager Radiance data with the WRF hybrid 3DEnVar system for the prediction of Typhoon Chan-hom (2015)

The impact of assimilating radiances from the global satellite precipitation program (GPM) Microwave Imager (GMI) on the track prediction of typhoon Chan-hom was studied with a hybrid three-dimensional ensemble-variational (3DEnVar) data assimilation (DA) system. It is found that the data assimilation of clear-sky GMI radiance is able to depict the structure of typhoon well by warming the typhoon inner core area. The bias correction coefficients obtained by the off-line model along with the variational bias correction method can reduce the bias. GMI radiance data assimilation experiment can effectively adjust the core area of Typhoon “Chan-hom” and systematically correct the typhoon position in the background of the model. The sensitivity of the hybrid 3DEnVar DA behaviors on the choice of the ensemble members is also investigated. Results show that the 3DEnVar Global Ensemble Forecast System (3DEnVar_GEFS)-based ensemble spread is essentially dominated in the vicinity of Typhoon Chan-hom, which is less disturbed by random errors. In general, the track forecasts from 3DEnVar_RCV match better with the best track by adding Gaussian noise randomly based on the background error than those from the 3DVAR, and 3DEnVar_GEFS.

Wave Simulation by the SWAN Model and FVCOM Considering the Sea-Water Level around the Zhoushan Islands

In this study, the numerical wave model Simulating Waves Nearshore (SWAN), which resolves nearshore wave processes, and a hydrodynamic model, the Finite-Volume Community Ocean Model (FVCOM), were coupled to simulate waves and currents during Typhoon Fung-wong (2014) and Typhoon Chan-hom (2015) around the Zhoushan Islands. Both of these models employ the same unique unstructured grid. In particular, the influence of sea-surface currents, e.g., typhoon-induced and tidal currents, as well as the sea-water level, on wave simulation was studied. The composite wind field, which is derived from the parametric Holland model and European Centre for Medium-Range Weather Forecasts (ECMWF) winds (H-E winds), was taken as the forcing field. TPXO.5 tide data, sea-surface temperatures from the HYbrid Coordinate Ocean Model (HYCOM), HYCOM sea-surface salinity, and HYCOM sea-surface current were treated as open-boundary conditions. The comparison of sea-surface-current speed between the FVCOM simulation and the National Centers for Environmental Prediction (NCEP) Climate Forecast System Version 2 (CFSv2) data revealed a root-mean-square error (RMSE) of about 0.1206 m/s, with a correlation (Cor) more than 0.8, while the RMSE of the simulated sea-water level when compared with the HYCOM data was around 0.13 m, with a Cor of about 0.86. The validation indicated that the simulated results in this study were reliable. A sensitive experiment revealed that the sea-water level affected the typhoon-induced wave simulation. Validation against the measurements from the moored buoys showed an RMSE of &lt;0.9 m for the sea-water level, which specifically reflected less overestimation during the high-sea state. Moreover, the significant-wave-height (SWH) difference (SWH without the sea-water level minus SWH with the sea-water level) was as great as −0.5 m around the Zhoushan Islands during the low-sea state. Furthermore, we studied the typhoon-induced waves when Typhoon Fung-wong passed the Zhoushan Islands, revealing that the reduction of SWH could be up to 1 m in the Yangtze Estuary and tidal flats when the maximum waves occurred.

Impacts of Multiple Radiance Data Assimilation on the Simulation of Typhoon Chan-Hom

With the module of assimilating AMSU-A (Advanced Microwave Sounding Unit-A) and AIRS (Atmospheric Infrared Sounder) data in the WRFDA (Weather Research and Forecasting Model Data Assimilation) system, the impacts of joint assimilation of the radiance observations from two satellites on the simulation of typhoon Chan-hom (2015) are addressed. For comparison, experiments with the assimilation of solely GTS (Global Telecommunications System) data, AMSU-A data, or AIRS data are also performed. The results show that, compared to other experiments, the analysis field after assimilating multiple radiance data is closer to the observation. The simulated steering flow in its forecast field is conductive to the northeast twist of the typhoon. In addition, the simulated rainband and the FSS (fraction skill score) calculated from the experiment with assimilating multiple radiance data are better. In the deterministic forecast, better performance is obtained from the simulation with multiple radiance data in the forecast of track, MSLP (minimum sea level pressure), and MSW (maximum surface wind).

Comparison of Assimilating All-Sky and Clear-Sky Satellite Radiance for Typhoon Chan-Hom and Nangka Forecasts

The impacts of assimilating all-sky satellite radiance from the Advanced Microwave Scanning Radiometer 2 (AMSR2) on typhoon Chan-hom and Nangka are evaluated over traditional clear-sky radiance assimilation. Results show that more AMSR2 radiance data around typhoon core area are assimilated in all-sky experiment than clear-sky, which improves the utilization of satellite radiance data. Community Radiative Transfer Model (CRTM) brightness temperature simulation under all-sky conditions is in better agreement with observations than in the case of clear-sky conditions. In a cycle assimilation experiment, all-sky assimilation reduces typhoon track forecast errors by 14.84%, and intensity errors by approximately 16.89%. Wind, temperature and humidity analysis are clearly improved in all-sky assimilation, as evaluated using the European Center for Medium-Range Weather Forecasts (ECMWF) reanalysis data. All-sky assimilation better captures the structures of typhoons, with a stronger warm core and tighter circulation around the typhoon eye. This study explores the contributions to the improvements in all-sky assimilation. These improvements are attributed to the enhancements in initial geopotential height, temperature and moisture in the typhoon core areas. Moreover, assimilating cloud- and precipitation-affected radiance data improves hydrometer simulations, which leads to higher hydrometeor concentrations than clear-sky radiance and conventional data assimilation. The results demonstrate that assimilation of all-sky AMSR2 data improves the analysis and forecast of multiple typhoons.

Numerical Study of Storm Surge Inundation in the Southwestern Hangzhou Bay Region During Typhoon Chan-Hom in 2015

Storm surge inundation is a major concern in marine hazard risk assessment during extreme weather conditions. In this study, a high-resolution coupled model (the ADVanced CIRCulation model + the Simulating WAves Nearshore model) was used to investigate the storm surge inundation in the southwestern Hangzhou Bay region during Typhoon Chan-hom in 2015. The simulated hydrodynamic processes (sea surface wave and storm tide) were validated with measured data from wave buoys and tide gauges, indicating that the overall performance of the model was satisfactory. The storm surge inundation in the coastal area was simulated for several idealized control experiments, including different wave effects (wave-enhanced wind stress, wave-enhanced bottom stress, and wave radiation stress). Dike overflowing cases with different dike heights and dike breaking cases with different dike breach lengths were considered in the simulation. The results highlight the necessity of incorporating wave effects in the accurate simulation of storm surge inundation. Dike height significantly influences the magnitude and phase of the maximum inundation area in the dike overflowing cases, and dike breach length is an important factor impacting the magnitude of the maximum inundation area in the dike breaking cases. This study may serve as a useful reference for accurate coastal inundation simulation and risk assessment.

Distribution and source of heavy metals in the sediments of the coastal East China sea: Geochemical controls and typhoon impact

The present study conducted a comprehensive study on the distribution and source of heavy metals (Cu, Zn, Ni, Pb, Cd) in the sediments of the coastal East China Sea (ECS), one of the most developed regions in China with very active land-sea interactions, using 119 surface sediment samples and a 2-m sediment core collected after super Typhoon Chan-hom in 2015. Heavy metals in the surface sediments exhibited metal-dependent and regional distribution patterns, showing higher levels in the southern inner shelf (SIS) than the Yangtze River estuary (YRE), and generally being evaluated as unpolluted to moderately polluted in the coastal ECS (except few sites adjacent to Xiangshan Harbor were strongly polluted by Cd). Based on the organic carbon isotope compositions (δ13C) data as well as the strong correlations between heavy metals and natural major elemental contents (Al2O3, Fe2O3, and SiO2), we suggest natural weathering detritus as the major source of heavy metals in the YRE region and the spatial distributions were highly controlled by sediment grain size and organic matter. In contrast, the spatial distributions of heavy metals in the SIS region were less correlated with sediment properties, due to more complex sources and stronger hydrodynamic impacts. The vertical distribution of heavy metals in the sediment core indicated significant enrichments since 1950s, but showed unusual gradually decreasing trends in top layer (30 cm–0.5 cm), attributing to the strong disturbance of super Typhoon Chan-hom on sediment transportation and metal partitioning. Besides, we also observed that heavy metal levels in shallow water regions of Zhejiang coast were reduced due to the passage of typhoon. Such strong impacts of super Typhoon Chan-hom on heavy metal distributions in the ECS indicates that the impacts of extreme hydrodynamic events should raise more concern when assessing the distribution and potential risks of contaminants in coastal regions.

A comparative study of storm surge and wave setup in the East China Sea between two severe weather events

The coastal regions of the East China Sea are frequently threatened by the increased sea level induced by intense tropical cyclones. In this study, a coupled wave-circulation model ADCIRC + SWAN is used to investigate the spatial and temporal characteristics of storm surges and wave setup heights in the southeastern coastal area of China during two severe weather events with different tracks: Typhoon Saomai, which made direct landfall, and the bypassing Typhoon Chan-hom. By definition, the storm surge is attributable to the wind forcing, while the wave setup results from the wave radiation stress. The simulated sea levels agree well with the observations, with improved results when the wave setup was considered. The simulation results showed clearly different spatial patterns depending on the track type. The maximum storm surge resulted from the cumulative effect of the local onshore wind forcing, occurring on the right side of the Typhoon Saomai track and the left side of the Typhoon Chan-hom track. Significant surge levels along the coast on the left side of the typhoon track were well observed in both cases, resulting from the coastal-trapped shelf waves. The Saomai track type is more likely to cause extremely high storm surges along the coast due to the stronger cumulative effect of the onshore wind forcing. The maximum wave setup was governed by the swell and slope of the sea floor. The locations of the maximum wave setup and surge level were spatially close during Typhoon Saomai, but they were separated during Typhoon Chan-hom.