Severe sea state, which features huge ocean waves under strong surface wind and often comes along with heavy rainfall, storm surge, and coastal flooding, is associated to a range of disastrous ocean dynamic environments, including storm surge, disastrous wave, internal wave, etc., and bring serious economic damage and safety threat to coastal area. In China, previous studies have shown that the storm surge had the highest frequency and the most economically destructive among all marine natural disasters(Le, 1996; Yang, 2000). Taking Guangdong Province, located on the northern coastline of the South China Sea, as an example, on July 22th, 1980, typhoon No. 8007 induced a highest sea level of 594 cm at N and u Station at western Guangdong, which not only was the highest tidal observation record in both China and West Pacific countries but also ranked No. 5 in the world. In 2005, a series of storm surges induced by typhoon Washi, severe tropical storm Huaning, tropical storm Vicente, and typhoon Damrey, led to a direct economic loss of 794 million RMB Yuan and the disaster-affected population reached 2.52 million(Zhang et al., 2008).

In Chinese coastal area, typhoons and their subsequent ocean dynamic processes are the most typical causes of severe sea state. Usually, several dynamic processes will be induced in the coastal ocean under the impact of typhoon. First of all, the storm surge will be induced in the nearshore area. Secondly, in addition to the sea level anomaly, the typhoon wave will also be generated and its height is even more than 6 m. Thirdly, the storm could induce the near-inertial(oscillations)internal waves in some areas. And the near-inertial internal wave is the most active internal wave in the ocean, which also plays an important role in the mass and energy transport in both the horizontal and vertical directions.

The study on China coastal seas under severe sea state has been carried out for dozens of years. At present, Chinese researchers are interested in mainly two directions: the first one is to improve the accuracy of remote sensing of oceanographic information under severe sea state; the other one is to develop the numerical modeling and forecasting capability of costal seas based on further understanding to the dynamics of disastrous oceanic processes. Both of the directions ultimately point to prevention and mitigation of marine disasters induced by severe sea state. The articles in this special issue are some representative results in this aspect.

The remote sensing measurements of sea surface temperature(SST), sea surface height(SSH) and sea surface wind retrieved by satellite-borne microwave sensors, including microwave radiometer, scatterometer, altimeter and SAR, are widely used in the monitoring, forecasting, and research of coastal marine environments since 1990’s. However, there are still some defects for microwave remote sensing in coastal area and under severe sea state(Jiang and Song, 2010): microwave sensors cannot accurately retrieve geophysical information due to sidelobe effect and atmospheric refraction; Ku b and fails under heavy rainfall; wind direction fuzzy and accuracy loss under high wind speed for scatterometer; overestimation of huge wave heights for altimeter, etc.

The project Microwave Remote Sensing Information Retrieving Technique of Marine Dynamic Environment and Its Application, which is a part of a major project of National High Technology Research and Development Program of China(863 Program), is focused on three issues: to improve microwave remote sensing retrieving technique under severe sea state; to develop geophysical information retrieving technique for China’s microwave remote sensing satellite HY-2; and to develop spatial and temporal extension technique of marine remote sensing measurements. The articles in fi rst half of this special issue are some of the research results supported by this project.

Surface wind speed retrieval is of the greatest interest in the microwave remote sensing community. The retrievals encounter more difficulties under severe sea states, such as typhoon wind conditions, because the surface scattering properties can be contaminated by rainfall. Yao et al.(2015)studied the sensitivity of AMSR2 brightness temperatures(TB)to rain and wind speed, and established a channel combination of brightness temperature for the development of an algorithm for hurricane wind speed retrieval for rainfall conditions. Wang et al.(2015)developed an algorithm to retrieve the seasurface wind speeds from TBs measured by Aquarius’ L-b and radiometer. They suggested the feasibility of using Aquarius as a new tool for the remote sensing of wind speed, even in rainy conditions.

The significant wave height(SWH)detected by the altimeter were well validated under calm to moderate wind conditions, but their reliability under severe weather conditions has not been well quantified. Liu et al.(2015) studied the effect of wave spectrum width on the probability density distribution of windwave heights, their results indicate an overestimation in the traditional prediction of SWH, and provides a potential method for improving the accuracy of the SWH remote sensing retrieval algorithm, critical for extremely large waves under severe sea states. The detected wave height is useful in the study of swell decay rate of a moving typhoon(Wang and Chen, 2015). The retrieved wind and wave parameters can be related to the surface wave breaking properties, which would be useful in the study of the impact of sea spray on upper ocean temperature during typhoon passage(Zhang et al., 2015).

The wave spectrum reflects the inner structure of ocean waves.Chen et al.(2015)proposed a new modulation transfer function for ocean wave spectra retrieval from X-b and marine radar imagery. Chen et al.(2015)studied the effect of Gaussian and non- Gaussian sea surface on the low incidence radar backscatter, and improved the inversion performance of wave spectrum from ocean wave spectrometer by taking account into the effects of non-Gaussian properties of the sea surface slope.

In China, the numerical simulation of storm surge was developed rapidly in 1980s. Several storm surge models were developed, such as the Five Basin Model(FBM)developed by Xinian WANG, the typhoon storm surge prediction model(CTS model) and the temperate storm surge prediction model(CES model)developed by Yu et al.(2002). Numerous numerical experiments of storm surge had been designed in the marginal seas of China for studying the infl uences of different dynamic factors(Duan et al., 1997). Abundant meaningful results were achieved, and some models had been used in the real-time forecast(Wang et al., 1997). However, the accuracy of the storm surge prediction is not only related to the accuracy of atmosphere prediction, but also affected by other factors, such as the uncertainty of some parameters.

At present, the numerical forecast of storm surge has not considered the factor of the generation of the huge storm surge(e.g., the parameterization of super typhoon), and its characteristic, mechanism and interaction with huge wave under the condition of climate change. In order to improve the model performance, these issues shall be specifi ed urgently.

The generation and shape of the abnormal surface waves, and its potential damaging on the building in the ocean, are studied mainly with numerical model and lab experiments(Liu, 2004; Rui, 2004). Because of the specialty of typhoon weather, the studies on the generation, development and propagation of the typhoon wave is very limited. Therefore, the study on the observation and forecast of the typhoon wave is an urgent work too.

Many observations have shown that there is remarkable near-inertial oscillation in the wake of typhoon(e.g., Shay and Elsberry, 1987). Obvious waves are generated immediately in the upper ocean after typhoon, and propagated downward, which could last for several to more than 10 days. Using the POM model, Chu et al.(2000)simulated the upper ocean’s response to the typhoon Emie, and the results showed the existence of the surface divergent flow of the nearinertial oscillation in the South China Sea. The nearinertial response to typhoon in the South China Sea(Zhang et al., 2007) and the adjacent seas of Hainan Isl and (Zhu and Li, 2007)were reported, using buoy and mooring observations, respectively. Base on the continuous observation, Sun(2008)studies the upper ocean’s response to typhoon Faith in 1998, typhoon Kai-Tak in 2000, typhoon Chanchu and tropical depression Pablo in 2004 in South China Sea. His research showed that the main response of current to typhoon Faith was the near-inertial oscillation and the oscillating frequency was close to inertial frequency but slightly higher(blue shift). The near-inertial current could be found at each layer(the deepest layer locates at 270 m)obviously. Overall, research on the nearinertial oscillation in South China Sea remains weak.

The program Study on formation Mechanism and Forecasting Method of Disastrous Ocean Dynamic Processes in the South China Sea, which is a Key Regional Joint Research Program of the National Natural Science Foundation of China(NSFC)with Guangdong Province, is focused on the dynamics and numerical modeling issues of some most important disastrous oceanic processes under severe sea state: storm surges, huge waves, and near-inertial and soliton internal waves. The second half of this special issue presents the achievement of this program.

The oceanic response to tropical cyclones is among the most important part of the study on severe sea state. Liu et al.(2015)explored the physical and biological responses of the upper ocean to three typhoons in the central Luzon Strait, and find that during the passage of typhoons, the Ekman velocity is comparable to the geostrophic flow and upwelling is noticeable which leads to phytoplankton bloom consequently. Yang et al.(2015)investigated the oceanic near-inertial response to 2008 typhoon Hagupit on the northern continental shelf finding that the oceanic response is dominated by second baroclinic mode near-inertial waves. Lin et al.(2015)examined the interactions between internal tides and near-inertial waves after Hagupit and found that the vertical shear of the near-inertial currents in combination with the vertical current of internal tides generated motions in frequencies fD1 and fD2. Yang et al.(2015)investigated the surface wave spectrum under typhoon conditions based on in-situ observation and they proposed a spectrum with three parameters, i.e., the dimensionless lowest moment of spectrum, dimensionless peak frequency and spectrum width.

Efforts are also made to improve the wave forecast accuracy in the South China Sea. An ensembleoptimal- interpolation-based(EnOI)wave assimilation scheme is established by embedding the EnOI module into wave model WAVEWATCH III and this scheme is well tested by assimilating the altimeter significant wave heights by Qi and Cao(2015). Meanwhile, this scheme with a sampling strategy for a stationary ensemble is applied into the South China Sea in summer and achieved improvements similar to those of an optimal-interpolation-based scheme in the subsequent paper by Cao et al.(2015).

Oceanic mesoscale eddy in the South China Sea is an issue worth discussion. By using in-situ CTD data, Argo fl oats data and multi-satellite remote sensing data during May to August 2009, mesoscale eddy formation and surface flow field structure in the Luzon Strait area are examined and where the vigorous water exchange between the Kuroshio water and the South China Seawater emerges is address by Liu et al.(2015). Further, the spatial and temporal distribution characteristics of mesoscale eddies on the frequency, intensity and lifetime for 21 years are revealed by Xia and Shen(2015), using mesoscale eddies in the SCS obtained by an automatic eddy detection method.

We thank the National High Technology Research and Development Program of China(863 Program)(No. 2013AA09A505)Microwave Remote Sensing Information Retrieving Technique of Marine Dynamic Environment and Its Application and the NSFC Guangdong Joint Key Program(No. U1133001)Study on Formation Mechanism and Forecasting Method of Disastrous Ocean Dynamic Processes in The South China Sea for financial supports. Finally, this special issue would not have been possible without the dedication and inspirational comments of the reviewers.