Assimilation of Ocean Surface Wind Data by the HY-2B Satellite in GRAPES: Impacts on Analyses and Forecasts

The ocean surface wind(OSW)data retrieved from microwave scatterometers have high spatial accuracy and represent the only wind data assimilated by global numerical models on the ocean surface,thus playing an important role in improving the forecast skills of global medium-range weather prediction models.To improve the forecast skills of the Global/Regional Assimilation and Prediction System Global Forecast System(GRAPES_GFS),the HY-2B OSW data is assimilated into the GRAPES_GFS four-dimensional variational assimilation(4DVAR)system.Then,the impacts of the HY-2B OSW data assimilation on the analyses and forecasts of GRAPES_GFS are analyzed based on one-month assimilation cycle experiments.The results show that after assimilating the HY-2B OSW data,the analysis errors of the wind fields in the lower-middle troposphere(1000-600 hPa)of the tropics and the southern hemisphere(SH)are significantly reduced by an average rate of about 5%.The impacts of the HY-2B OSW data assimilation on the analysis fields of wind,geopotential height,and temperature are not solely limited to the boundary layer but also extend throughout the entire troposphere after about two days of cycling assimilation.Furthermore,assimilating the HY-2B OSW data can significantly improve the forecast skill of wind,geopotential height,and temperature in the troposphere of the tropics and SH.

Current Status and Main Application Achievements of Ocean Satellites

Ocean satellites have realized multi-satellite networked operation.The HY-1D satellite launched in June 2020 realized networked with HY-1C satellite,and completed the construction of ocean color satellite constellation.The HY-2D satellite launched in May 2021 is networked with the on orbit HY-2B and HY-2C satellites to complete the construction of marine dynamic environment satellite constellation.The 1 mC-SAR satellite 01 launched in November 2021 is networked with GF-3,which initially forms the marine monitoring satellite constellation.This year,the networking of 1 mC-SAR satellite 02 with satellite 01 and GF-3 is realized,and the construction of marine monitoring satellite constellation is completed.At present,the ocean satellites have the operational application capabilities of remote sensing investigation,monitoring,evaluation and supervision of marine ecology,marine disaster prevention and reduction,global oceans and Polar Regions,Sea Islands,rights and interests maintenance.

Applications of China's Ocean satellites in 2009

Considerable progress was achieved in China's ocean satellite missions in 2009. The ground application system for ocean satellites operates well,and the applications of ocean satellites have been further extended. The HY-1B satellite operates stably in orbit.We received 2018 orbits of measurements from the HY-1B satellite during 2009 and the accumulated raw data reached 8472GB;2778 orbits of data from the EOS/MODIS satellite

FY-3A/MERSI, ocean color algorithm, products and demonstrative applications

A medium resolution spectral imager （MERSI） on-board the first spacecraft of the second generation of Chinas polar-orbit meteorological satellites FY-3A, is a MODIS-like sensor with 20 bands covering visible to thermal infrared spectral region. FY-3A/MERSI is capable of making continuous global observations, and ocean color application is one of its main targets. The objective is to provide information about the ocean color products of FY-3A/MERSI, including sensor calibration, ocean color algorithms, ocean color prod- ucts validation and applications. Although there is a visible on-board calibration device, it cannot realize the on-board absolute radiometric calibration in the reflective solar bands. A multisite vicarious calibration method is developed, and used for monitoring the in-flight response change and providing post-launch cal- ibration coefficients updating. FY-3A/MERSI ocean color products consist of the water-leaving reflectance retrieved from an atmospheric correction algorithm, a chlorophyll a concentration （CHL1） and a pigment concentration （PIG1） from global empirical models, the chlorophyll a concentration （CHL2）, a total sus- pended mater concentration （TSM） and the absorption coefficient of CDOM and NAP （YS443） from Chi- na＇s regional empirical models. The atmospheric correction algorithm based on lookup tables and ocean color components concentration estimation models are described. By comparison with in situ data, the FY-3A/MERSI ocean color products have been validated and preliminary results are presented. Some suc- cessful ocean color applications such as algae bloom monitoring and coastal suspended sediment variation have demonstrated the usefulness of FY-3A/MERSI ocean color products.

China-France Oceanography Satellite (CFOSAT) simultaneously observes the typhoon-induced wind and wave fields

The China-France oceanography satellite(CFOSAT)developed by the China National Space Administration(CNSA)and Centre National D’Etudes Spatiales(CNES)was successfully launched into its orbit on October 29,2018.The Chinese wind scatterometer(SCAT)with swath width of about 1000 km and French wave spectrometer(Surface Wave Investigation and Monitoring,SWIM)with swath width of about 180 km onboard the CFOSAT are in line with all requirements and performing operationally.Thus,it is the first time that CFOSAT provides simultaneous and co-located observations of wind and wave fields with high spatial resolutions of 12.5 km×12.5 km for the winds and 70 km×90 km for the wave directional spectrum.The real-time and large-scale monitoring of wind and wave fields are of great significance for navigation and human activities on the sea(Xu et al.,2010;Tan et al.,2018;Sun et al.,2019),especially during severe typhoon processes when violent winds and hazardous waves occur(Walsh et al.,2002;Zhou et al.,2008).

Ocean Observation from Haiyang Satellites: 2012–2014

During 2012 and 2014, China has two Haiyang(which means ocean in Chinese, referred to as HY) satellites operating normally in space which are HY-1B and HY-2A. HY-1B is an ocean color environment satellite which was launched in April 2007 to observe global ocean color and sea surface temperature, and HY-2A is an ocean dynamic environment satellite which was launched in August 2011 to obtain global marine dynamic environment parameters including sea surface height,significant wave height, ocean wind vectors, etc. Ocean observation data provided by HY-1B and HY-2A have been widely used by both domestic and international users in extensive areas such as ocean environment protection, ocean disaster prevention and reduction, marine environment forecast,ocean resource development and management, ocean investigations and scientific researches, etc.

Results About Ocean Satellite Observation

Ocean observing satellites have become an important part of the China's three-dimensional marine observation system.They have played more and more important roles in marine pollution monitoring,marine environment and marine disaster monitor and forecasting,marine resource investigation and marine scientific research.In this paper,the author will give a brief review of China's operational and scientific activities in satellites ocean observation during 2008 to 2010.These activities include the application of the HY-1B for red tide and green tide detecting and monitoring,sea ice monitoring,fishery resources assessment at coastal zone and ocean,marine water quality assessment,sea surface temperature monitoring and forecasting,ocean primary productivity and carbon cycle research,and the development in the ocean dynamic environment parameter application technology before launch of HY-2.The achievements we have made in the ocean observation and monitoring during the past 3 years indicate that,the development of the ocean satellite series and the application of the ocean satellite can greatly promote the development of China's three-dimensional marine observation system and earth-observation system from space,accordingly we can improve the ability of acquiring the marine environmental parameter to sustain the marine economy development and the sea defense construction.

Ocean Observation from Haiyang Satellites

In 2018,China successfully launched three new Haiyang(which means ocean in Chinese,referred to as HY)satellites which are an ocean color observation satellite HY-1C(operational),an ocean dynamics environment satellite HY-2B(operational)and the China-France ocean satellite CFOSAT(experimental).In 2019,all the three satellites had finished their commissioning phases and were declared operational.HY-2A satellite continues to operate in-orbit,and its operational status is basically normal.So in 2020,China has 4 Haiyang satellites in-orbit,China’s ocean satellites enter into a new operational application phase.The operation of the ground application system of Chinese ocean satellites is stable.In 2019,Beijing,Hainan,Mudanjiang,and Hangzhou ocean satellite ground stations had received the data of HY-1C,HY-2A,HY-2B,and CFOSAT 5012 orbits and 26.46 TB data had been distributed to both domestic and international users.Chinese ocean satellite data has played an important role in marine disaster prevention and mitigation,development and management of marine resources,maintenance of marine rights and interests,marine environment protection,scientific researches,and blue economy development.

Preliminary results of the global ocean tide derived from HY-2A radar altimeter data

The HY-2A satellite,which is equipped with a radar altimeter and was launched on August 16,2011,is the first Chinese marine dynamic environmental monitoring satellite.Extracting ocean tides is one of the important applications of the radar altimeter data.The radar altimeter data of the HY-2A satellite from November 1,2011 to August 16,2014 are used herein to extract global ocean tides.The constants representing the tidal constituents are extracted by HY-2A RA data with harmonic analysis based on the least squares method.Considering tide aliasing issues,the analysis of the alias periods and alias synodic periods of different tidal constituents shows that only the tidal constituents M_(2),N_(2),and K_(2)are retrieved precisely by the HY-2A RA data.The derived tidal constants of the tidal constituents M_(2),N_(2)and K_(2)are compared to those of tidal gauge data and the TPXO tide model results.The comparison between the derived results and the tidal gauge data shows that the RMSEs of the tidal amplitude and phase lag are 9.6 cm and 13.34°,2.4 cm and 10.47°,and 8.1 cm and 14.19°for tidal constituents M_(2),N_(2),and K_(2),respectively.The comparisons of the semidiurnal tides with the TPXO model results show that tidal constituents have good consistency with the TPXO model results.These findings confirm the good performance of HY-2A RA for retrieving semidiurnal tides in the global ocean.

Validation and accuracy analysis of wind products from scatterometer onboard the HY-2B satellite

The Chinese marine dynamic environment satellite HY-2B was launched in October 2018 and carries a Ku-band scatterometer.This paper focuses on the accuracies of HY-2B scatterometer wind data during the period from November 2018 to May 2021.The HY-2B wind data are validated against global moored buoys operated by the U.S.National Data Buoy Center and Tropical Atmosphere Ocean,numerical model data by the National Centers for Environmental Prediction,and the Advanced Scatterometer data issued by the Remote Sensing System.The results showed that the wind speeds and directions observed by the HY-2B scatterometer agree well with these buoy wind measurements.The root-mean-squared errors(RMSEs)of the HY-2B wind speed and direction are 0.74 m/s and 11.74°,respectively.For low wind speeds(less than 5 m/s),the standard deviation of the HY-2B-derived wind direction is higher than 20°,which implies that the HY-2B wind direction for low wind speeds is less accurate than that for moderate to high wind speed ranges.The RMSE of the HY-2B wind speed is slightly larger in high latitude oceans(60°–90°S and 60°–90°N)than in low latitude regions.Furthermore,the dependence of the residuals on the cross-track location of wind vector cells and the stability of the HY-2B scatterometer wind products are discussed.The wind stability assessment results indicate that a clear yearly oscillation is observed for the HY-2B wind speed bias which is due to seasonal weather variations.In general,the accuracy of HY-2B winds meets the operational precision requirement and is consistent with other wind data.

Classification of ice and water in the Arctic using radar altimeter and microwave radiometer data from HY-2B satellite

Several Chinese marine satellites have been launched in recent years.Monitoring sea ice and the ocean in the Arctic is of great importance for climate research.Sea ice in the Arctic has changed rapidly during the past few decades with respect to the extent and thickness.In this study,we applied combined passive and active microwave data from the Chinese HaiYang-2B(HY-2B)satellite to classify ice and sea water in the Arctic.We use data from a radar altimeter(RA)and a calibration microwave radiometer(CMR)to discriminate between ice and water by applying several approaches(1)the single parameter threshold criteria,(2)the multi-parameters linear segmentations and(3)the K-means clustering.The results yielded by these methods were in good agreement(classification accuracy>95%)with the Satellite Application Facility on Ocean and Sea Ice products between November and April.For other months(May–October),however,the agreement was less good(lowest classification accuracy approximate 85%in summer).A hybrid approach combined with graphical ice edges detection and microwave radar waveform analysis is therefore developed.A visual comparison with SAR images suggested the hybrid approach results greatly improved the ice and water discrimination in summer.This study demonstrated that multi-sensors(RA and CMR)configurations from HY satellites can offer comparable polar earth observation to the European Space Agency and NOAA satellite products.

Study of coexisting upstream solitary wave packet and downstream wakes induced by Diaoyu Dao and its affiliated islands detected by satellite sun glitter

Taking the advantage of sun glitter(SG)observed from high-resolution satellites Gaofen-1(GF-1)and Gaofen-6(GF-6),a new method named Shield and Vortex 1-2(SAV1-2)for extracting spatial texture information from sun glitter was established.Sea surface texture detail information around Diaoyu Dao and its affiliated islands was extracted using SAV1-2.Meanwhile,the dynamic characteristics of the water environment were analyzed by combining the water color,temperature and dynamic data.The results show the following information.(1)Stable westward shield effect and eastward Karman vortex streets,extracted from SG,appear upstream(westward)and downstream(eastward)of the Diaoyu Dao and its affiliated islands respectively.(2)The dominant direction of the Kuroshio Current in the Diaoyu Dao and its affiliated islands area is east,the Diaoyu Dao and its affiliated islands interacting with Kuroshio,inducing western shield and eastern shelter and wake.Furthermore,wave intervals of the shield in front of the island range from hundreds to thousands of metres,and extend wider than the island itself.(3)Combining with high-resolution sea surface temperature(SST)and chlorophyll a(Chl a)concentration,the directional changes of water color elements are mutually supported and proved with the stable shield(wave packet)effect in the west and vortex street effect in the east.The change of SST upstream of the island(western shield area)is not significant,the temperature in the downstream shelter area decreases slightly,and the SST in the downstream wake area shows a general trend of obvious decrease.The wake area is dominated by Karman vortex streets in shape,supplementing by vortices.The concentration of Chl a in the west shield of island is as low as that in downstream shelter area,while it increases significantly in the downstream wake area with shape in eddy or ribbon.(4)The SAV methods of extracting sea surface texture detail using SG can be widely used in different sea areas and water quality.This paper can provide reference for the protection and development of Diaoyu Dao and its affiliated islands and its surrounding waters.It is suggested that some kind of current buffer,such as marine wind farm,should be established in the western waters of Diaoyu Dao and its affiliated islands to protect the islands,and marine pasture should be developed in the downstream of eastern waters of Diaoyu Dao and its affiliated islands.

Networking Observation and Applications of Chinese Ocean Satellites

This paper presents the networking observation capabilities of Chinese ocean satellites and their diverse applications in ocean disaster prevention,ecological monitoring,and resource development.Since the inaugural launch in 2002,China has achieved substantial advancements in ocean satellite technology,forming an observation system composed of the HY-1,HY-2,and HY-3 series satellites.These satellites are integral to global ocean environmental monitoring due to their high resolution,extensive coverage,and frequent observations.Looking forward,China aims to further enhance and expand its ocean satellite capabilities through ongoing projects to support global environmental protection and sustainable development.

Inversion method of deflection of the vertical based on SWOT wide-swath altimeter data

The deflection of the vertical(DOV)is the key information in the study of ocean gravity field.However,in most areas,the precision of the prime component of DoV is significantly lower than that of the meridian component.To obtain higher accuracy and resolution of ocean gravity information,researchers have proposed a novel altimeter called the wide-swath altimeter.This altimeter allows for the simultaneous acquisition of high-precision and high-resolution two-dimensional measurements of sea surface height(SSH).In this paper,the Surface Water and Ocean Topography(SWOT)mission with a wide-swath altimeter on board is selected for research.One cycle of swoT sea surface height data is simulated to inverse the DOV in the Arabian Sea(45°E—80°E,0°-30°N),and the inversion results are compared with those of conventional altimeter data.The results demonstrate that the difference between the meridian and prime components derived from the inversion of swoT wide-swath data is minimal,significantly outperforming the inversion results of conventional nadir altimeter data.The advantage of swoT wide-swath altimeter lies in its ability to use the multi-directional geoid slope at any sea surface measurement point to invert the components in the meridian and prime directions.To investigate the impact of this advantage on inversion precision,this paper employs a method to calculate the gradient of the geoid in multiple directions to invert DoV components.The improvement effect of calculating the gradient of the geoid in multiple directions on the precision of DoV component is analyzed.It is found that the accuracy of DoV inversion has significantly improved with the increase of geodetic gradient calculation direction.In addition,the effects of various errors and grid spacing in SwoT wide sea surface height data on the precision of Dov inversion are also analyzed.

An assessment of arctic sea ice concentration retrieval based on “HY-2” scanning radiometer data using field observations during CHINARE-2012 and other satellite instruments

A retrieval algorithm of arctic sea ice concentration （SIC） based on the brightness temperature data of “HY-2” scanning microwave radiometer has been constructed. The tie points of the brightness temperature were selected based on the statistical analysis of a polarization gradient ratio and a spectral gradient ratio over open water （OW）, first-year ice （FYI）, and multiyear ice （MYI） in arctic. The thresholds from two weather filters were used to reduce atmospheric effects over the open ocean. SIC retrievals from the “HY-2” radiom-eter data for idealized OW, FYI, and MYI agreed well with theoretical values. The 2012 annual SIC was calcu-lated and compared with two reference operational products from the National Snow and Ice Data Center （NSIDC） and the University of Bremen. The total ice-covered area yielded by the “HY-2” SIC was consistent with the results from the reference products. The assessment of SIC with the aerial photography from the fifth Chinese national arctic research expedition （CHINARE） and six synthetic aperture radar （SAR） images from the National Ice Service was carried out. The “HY-2” SIC product was 16% higher than the values de-rived from the aerial photography in the central arctic. The root-mean-square （RMS） values of SIC between “HY-2” and SAR were comparable with those between the reference products and SAR, varying from 8.57% to 12.34%. The “HY-2” SIC is a promising product that can be used for operational services.

Validation of Chinese HY-2 satellite radar altimeter significant wave height

Chinese Haiyang-2（HY-2） satellite is the first Chinese marine dynamic environment satellite. The dual-frequency （Ku and C band） radar altimeter onboard HY-2 has been working effective to provide operational significant wave height （SWH） for more than three years （October 1, 2011 to present）.We validated along-track Ku-band SWH data of HY-2 satellite against National Data Buoy Center （NDBC） in-situ measurements over a time period of three years from October 1, 2011 to September 30, 2014, the root mean square error （RMSE） and mean bias of HY-2 SWH is 0.38 m and （-0.13±0.35） m, respectively. We also did cross validation against Jason-2 altimeter SWH data, the RMSE and the mean bias is 0.36m and （-0.22±0.28） m, respectively. In order to compare the statistical results between HY-2 and Jason-2 satellite SWH data, we validated the Jason-2 satellite radar altimeter along-track Ku-band SWH data against NDBC measurements using the same method. The results demonstrate the validation method in this study is scientific and the RMSE and mean bias of Jason-2 SWH data is 0.26 m and （0.00±0.26） m, respectively. We also validated both HY-2 and Jason-2 SWH data every month, the mean bias of Jason-2 SWH data almost equaled to zero all the time, while the mean bias of HY-2 SWH data was no less than -0.31m before April 2013 and dropped to zero after that time. These results indicate that the statistical results for HY-2 altimeter SWH are reliable and HY-2 altimeter along-track SWH data were steady and of high quality in the last three years. The results also indicate that HY-2 SWH data have greatly been improved and have the same accuracy with Jason-2 SWH data after April, 2013. SWH data provided by HY-2 satellite radar altimeter are useful and acceptable for ocean operational applications.

Ocean color products retrieval and validation around China coast with MODIS

Waters along China coast are very turbid with high concentrations of suspended sediment nearly all the time,especially at the Hangzhou Bay,the Changjiang （Yangtze） River Estuary and the shoal along Jiangsu Province.In these turbid and optically complex waters,the standard MODIS ocean color products tend to have invalid values.Because the water-leaving radiances in the near-infrared （NIR） are significant resulting from the strong scattering of suspended particles,the standard MODIS atmospheric correction algorithm often gets no results or produces significant errors.And because of the complex water optical properties,the OC3 model used in the standard MODIS data processing tends to get extremely high chlorophyll-a （Chl-a） concentrations.In this paper,we present an atmospheric correction approach using MODIS short wave infrared （SWIR） bands based on the fact that water-leaving radiances are negligible in the SWIR region because of the extreme strong absorption of water even in turbid waters.A regional Chl-a concentration estimation model is also constructed for MODIS from in situ data.These algorithms are applied to MODIS Aqua data processing in the China coastal regions.In situ data collected in the Yellow Sea and the East China Sea in spring and autumn,2003 are used to validate the performance.Reasonably good results have been obtained.It is noted that water-leaving reflectance in the NIR bands are significant in waters along the China coast with high sediment loadings.The satellite derived and in-situ reflectance spectra can match in the turbid waters along China coast,and there is relatively good linear relationship between satellite derived and in-situ reflectance.The RMSE value of Rrs（λ） is 0.0031 sr ？1 for all the nine ocean color bands （412 to 869 nm）.The satellite-derived Chl-a value is in the reasonable range and the root mean square percentage difference is 46.1%.

Assessment of the initial sea surface temperature product of the scanning microwave radiometer aboard on HY-2 satellite

HY-2 satellite is the first satellite for dynamic environmental parameters measurement of China,which was launched on 16th August 2011.A scanning microwave radiometer（RM） is carried for sea surface temperature（SST）,sea surface wind speed,columnar water vapor and columnar cloud liquid water detection.In this paper,the initial SST product of RM was validated with in-situ data of National Data of Buoy Center（NDBC） mooring and Argo buoy.The validation results indicate the accuracy of RM SST is better than 1.7 C.The comparison of RM SST and WindSat SST shows the former is warmer than the latter at high sea surface wind speed and the difference between these SSTs is depend on the sea surface wind speed.Then,the relationship between the errors of RM SST and sea surface wind speed was analyzed using NDBC mooring measurements.Based on the results of assessment and errors analysis,the suggestions of taking account of the affection of sea surface wind speed and using sea surface wind speed and direction derived from the microwave scatteromter aboard on HY-2 for SST product calibration were given for retrieval algorithm improvement.

Satellite SAR observation of the sea surface wind field caused by rain cells

Rain cells or convective rain,the dominant form of rain in the tropics and subtropics,can be easy detected by satellite Synthetic Aperture Radar（SAR） images with high horizontal resolution.The footprints of rain cells on SAR images are caused by the scattering and attenuation of the rain drops,as well as the downward airflow.In this study,we extract sea surface wind field and its structure caused by rain cells by using a RADARSAT-2 SAR image with a spatial resolution of 100 m for case study.We extract the sea surface wind speeds from SAR image by using CMOD4 geophysical model function with outside wind directions of NCEP final operational global analysis data,Advance Scatterometer（ASCAT） onboard European Met Op-A satellite and microwave scatterometer onboard Chinese HY-2 satellite,respectively.The root-mean-square errors（RMSE） of these SAR wind speeds,validated against NCEP,ASCAT and HY-2,are 1.48 m/s,1.64 m/s and 2.14 m/s,respectively.Circular signature patterns with brighter on one side and darker on the opposite side on SAR image are interpreted as the sea surface wind speed（or sea surface roughness） variety caused by downdraft associated with rain cells.The wind speeds taken from the transect profile which superposes to the wind ambient vectors and goes through the center of the circular footprint of rain cell can be fitted as a cosine or sine curve in high linear correlation with the values of no less than 0.80.The background wind speed,the wind speed caused by rain cell and the diameter of footprint of the rain cell with kilometers or tens of kilometers can be acquired by fitting curve.Eight cases interpreted and analyzed in this study all show the same conclusion.

Application of MODIS data in monitoring suspended sediment of Taihu Lake,China

Application of MODIS in ocean color is mainly based on bands 8-16 with the spatial resolution of 1 000 m.This spatial resolution,however,can not meet the application demand of inland waters where the areas are relatively small.With the assumption of the black water at shortwave infrared(SWIR) wavelengths(>1 000 nm),we first propose an atmospheric correction method for bands 1 and 2 with their spatial resolution of 250 m,and we then establish a quantitative retrieval model for suspended sediment concentration retrieval using the in-situ data collected in Taihu Lake.We also use MODIS data to retrieve the suspended sediment concentration of Taihu Lake with the retrieval model.The comparison between the retrieved and measured suspended sediment concentrations confirms that our algorithm can provide reliable data for monitoring the suspended sediment in Taihu Lake.