Retrieval of Antarctic sea ice freeboard and thickness from HY-2B satellite altimeter data

Antarctic sea ice is an important part of the Earth’s atmospheric system,and satellite remote sensing is an important technology for observing Antarctic sea ice.Whether Chinese Haiyang-2B(HY-2B)satellite altimeter data could be used to estimate sea ice freeboard and provide alternative Antarctic sea ice thickness information with a high precision and long time series,as other radar altimetry satellites can,needs further investigation.This paper proposed an algorithm to discriminate leads and then retrieve sea ice freeboard and thickness from HY-2B radar altimeter data.We first collected the Moderate-resolution Imaging Spectroradiometer ice surface temperature(IST)product from the National Aeronautics and Space Administration to extract leads from the Antarctic waters and verified their accuracy through Sentinel-1 Synthetic Aperture Radar images.Second,a surface classification decision tree was generated for HY-2B satellite altimeter measurements of the Antarctic waters to extract leads and calculate local sea surface heights.We then estimated the Antarctic sea ice freeboard and thickness based on local sea surface heights and the static equilibrium equation.Finally,the retrieved HY-2B Antarctic sea ice thickness was compared with the CryoSat-2 sea ice thickness and the Antarctic Sea Ice Processes and Climate(ASPeCt)ship-based observed sea ice thickness.The results indicate that our classification decision tree constructed for HY-2B satellite altimeter measurements was reasonable,and the root mean square error of the obtained sea ice thickness compared to the ship measurements was 0.62 m.The proposed sea ice thickness algorithm for the HY-2B radar satellite fills a gap in this application domain for the HY-series satellites and can be a complement to existing Antarctic sea ice thickness products;this algorithm could provide long-time-series and large-scale sea ice thickness data that contribute to research on global climate change.

Extraction of ocean tidal information based on global equal-area grid and satellite altimeter data

Harmonic analysis of satellite altimetry data based on a global regular grid is affected by the grid spatial tessellation and placement of the grids.With the increase of latitude,the traditional lat/lon grid deforms greatly,resulting in uneven distribution of satellite altimeter data with latitude,which affects the extraction of tidal information.Alternatively,Hexagonal grids have been proved to be advantageous due to their isotropic,uniform neighbourhood,equal-area and more.Considering the merits above,the purpose of this paper is to use the global equal-area hexagonal grid to conduct a harmonic analysis of satellite altimeter data.First,the Icosahedron Snyder Equal Area projection method is used to construct a global equal-area hexagonal grid,Then the time series data of 19.8 years of Jason series satellite altimeter data are obtained.Finally,the harmonic constants of eight constituents(the M2,S2,N2,K2,K1,O1,P1,Q1)are extracted by harmonic analysis.By analysing the results,we conclude that the harmonic constants extracted from the global equal-area hexagonal grid have considerable accuracy and are consistent with the tidal characteristics of the eight components.Meanwhile,the accuracy of harmonic constants extracted from equal-area hexagonal grids is better than that of lat/lon grids.

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.

Feasibility of maintaining satellite altimetry calibration site based on qianliyan islet at the Yellow Sea

The calibration of the sea surface height(SSH)measured by satellite altimeters is essential to understand altimeter biases.Many factors affects the construction and maintenance of a permanent calibration site.In order to calibrate Chinese satellite altimetry missions,the feasibility of maintaining a calibration site based on the Qianliyan islet in Yellow Sea of China is taken into account.The related calibration facilities,such as the permanent tide gauge,GNSS reference station and meteorological station,were already operated by the Ministry of Natural Resources of China.The data could be fully used for satellite altimeter calibration with small fiscal expenditure.In addition,the location and marine environments of Qianliyan were discussed.Finally,we used the Jason-3 mission to check the possibility of calibration works.The result indicates that the brightness temperatures of three channels measured by microwave radiometer(MWR)and the derived wet tropospheric correction varies smoothly,which means the land contamination to MWR could be ignored.The high frequency waveforms at the Qianliyan site present no obvious difference from the normal waveforms received by satellite radar altimeter over the open ocean.In conclusion,the Qianliyan islet will not influence satellite altimetry observation.Following these analyses,a possible layout and mechanism of the Qianliyan calibration site are proposed.

Eddy properties in the Pacific sector of the Southern Ocean from satellite altimetry data

Mesoscale eddies play a key role in the ocean dynamics of the Southern Ocean, and eddy response to the climate changes has also been widely noted. Both eddy kinetic energy（EKE） and eddy detection algorithm are used to study the eddy properties in the Pacific sector of the Southern Ocean. Consistent with previous works,the maps of the EKE illustrate that higher energy confines to the Antarctic Polar Frontal Zone（APFZ） and decreases progressively from west to east. It also shows that the most significant increase in the EKE occurs in the western and central parts of the Pacific sector, where the baroclinicity of the Antarctic Circumpolar Current（ACC） is much stronger. Statistical eddy properties reveal that both of the spatial pattern and interannual variation of the EKE are primarily due to the eddy amplitude and the eddy rotational speed, rather than the eddy number or the eddy radius. In general, these results furtherly confirm that anomalous westerly wind forcing associated with the positive Southern Annular Mode（SAM） index enhances the Southern Ocean eddy activity by strengthening the eddy properties.

Influence of Range Gate Width on Detection Probability and Ranging Accuracy of Single Photon Laser Altimetry Satellite

The influence of the single photon laser altimeter range-gate width on the detection probability and ranging accuracy is discussed and analyzed,according to the LiDAR equation,single photon detection equation and the Monte Carlo method to simulate the experiment.The simulated results show that the probability of detection is not affected by the range gate,while the probability of false alarm is relative to the gate width.When the gate width is 100 ns,the ranging accuracy can accord with the requirements of satellite laser altimeter.But when the range gate width exceeds 400 ns,ranging accuracy will decline sharply.The noise ratio will be more as long as the range gate to get larger,so the refined filtering algorithm during the data processing is important to extract the useful photons effectively.In order to ensure repeated observation of the same point for 25 times,we deduce the quantitative relation between the footprint size,footprint,and frequency repetition according to the parameters of ICESat-2.The related conclusions can provide some references for the design and the development of the domestic single photon laser altimetry satellite.

Obtaining accurate measurements of the sea surface height from a GPS buoy

A dedicated GPS buoy is designed for calibration and validation(Cal/Val)of satellite altimeters since 2014.In order to evaluate the accuracy of the sea surface height(SSH)measured by the GPS buoy,twelve campaigns have been done within China sea area between 2014 and 2021.In six of these campaigns,two static Global Navigation Satellite System stations were installed at distances of<1 km and 19 km from the buoy to assess how the baseline length influenced the derived SSH from the buoy solutions.The GPS buoy data was processed using the GAMIT/GLOBK software+TRACK module and CSRS-PPP tool to achieve the SSH.The SSH was compared with conventionally tide gauge(TG)data to evaluate the accuracy of the buoy with the standard deviation of the height element.The results showed that the difference in the standard deviation of the SSH from the buoy and the TG was less than 16 mm.The SSHs processed with different ephemeris(Ultra-Rapid,Rapid,Final)were not significantly different.When the baseline length was 19 km,the SSH solution of the GPS buoy performed well,with standard bias of less than 26 mm between the heights measured by the buoy and TG,meaning that the buoy could be used for Cal/Val of altimeters.The bias between the Canadian Spatial Reference System-precise point positioning tool and the TRACK varied a lot,and some of them were over 130 mm.This deemed too high to be useful for Cal/Val of satellite altimeters.Moreover,the GPS buoy solutions processed by GAMIT/GLOBK software+TRACK module were used for in-orbit Cal/Val of HY-2B/C satellites in ten campaigns.The SSH and significant wave height of the altimeters showed good agreements with the GPS buoy solutions.

Observation of continental shelf wave propagating along the eastern Taiwan Strait during Typhoon Meranti 2016

This study is the fi rst to depict typhoon-induced continental shelf wave(CSW)propagation in the eastern Taiwan Strait(TWS)during the passage of Typhoon Meranti in 2016 using tidal gauge data and along-track satellite altimeter data.The strong amplitude response of sea level oscillations(within the range of 0.30–0.54 m)as a free,barotropic CSW after Meranti,which impacted the TWS,was clearly detected in time and frequency(in bands of 64–81 h)using wavelet and cross-wavelet analyses.The measured group and phase speeds were consistent with the dispersion curves for CSW with the fi rst-mode derived from the cross-shelf sections of the eastern TWS,with the mean speeds reaching 3 and 5.6±0.7 m/s,respectively.Coincidentally,the typhoon-induced sea level anomaly(SLA)was also captured by the satellite altimeter before this CSW entered into the TWS.Using the theoretical cross-shore CSW modes to fi t the SLA data,the results indicated that the fi rst three wave modes can interpret this CSW event appeared in the southern TWS very well,with the fi rst mode being the dominant one.

Annual and interannual variations of sea-level anomaly in the Bay of Bengal and the Andaman Sea

Annual and interannual variations of sea-level anomaly （SLA） in the Bay of Bengal and the Andaman Sea are investigated using altimeter - derived SLA data from 1993 to 2003. It is found that the SLA annual variation in the study area can be divided into three phases with distinctive patterns. During the southwest monsoon （May -September）, positive SLA presents in the equatorial region and extends northward along the eastern boundary of the bay, and the SLA distribution in the interior bay appears to be high in the east and low in the west with two cyclonic ceils developing in the north and south of the western bay respectively, between which an anticyclonic cell exists. During the early northeast monsoon （ October - December） , the whole bay is dominated by a large cyclonic cell with the pattern of high SLA in the east and low in the west still retained, and the SLA distribution outside the bay is changed in response to the reversal of the Indian Monsoon Current （IMC） in November. During the late northeast monsoon （January -April） , a large anticyclonic cell of SLA develops in the bay with negative SLA prevailing in the equatorial region and extending northward along the eastern boundary of bay. Therefore, the SLA distribution in the interior bay reverses to be high in the west and low in the east. It is suggested that the SLA annual variation in the bay is primarily driven by the local wind stress curl, involving Sverdrup balance while the abrupt SLA variation during the peak of northeast monsoon may be partly caused by the semiannual fluctuation of wind in the equatorial region. This fast adjustment in the interior bay is induced by the upwelling coastal Kelvin wave excited by the decay of Wyrtki jet during December through January. Besides the annual variation, in the bay, there are obvious SLA fluctuations with the periods of 2 and 3 - 7 a, which are driven by the interannual variability of large - scale wind field in the equatorial region. The coastal Kelvin wave also provides an important link for the SLA interannual variation between the equatorial region and the interior bay. It is found that the E1 Nino -Southern Oscillation （ENSO） -induced influence on the SLA interannual variation in the interior bay is stronger than the Indian Ocean dipole （IOD） with the associated pattern of low sea-level presenting along the periphery of the bay and high sea-level in the northeast of Sri Lanka.

Statistical characteristics and mechanisms of mesoscale eddies in the North Indian Ocean

The statistical characteristics and mechanisms of mesoscale eddies in the North Indian Ocean are investigated by adopting multi-sensor satellite data from 1993 to 2019.In the Arabian Sea(AS),seasonal variation of eddy characteristics is remarkable,while the intraseasonal variability caused by planetary waves is crucial in the Bay of Bengal(BOB).Seasonal variation of the eddy kinetic energy(EKE)is distinct along the west boundary of AS,especially in the Somali Current region.In the BOB,larger EKE occurs at the northwest basin from March to May,to the east of Sri Lanka from June to September,and along the east coast of India from November to December.The wind stress work(WW)is further studied to figure out the direct influence of wind forcing on EKE.The WW exerts positive effects on EKE along the west boundary of AS and in the south of India/Sri Lanka during the two monsoon seasons.Besides,the WW also has impact on EKE along the east coast of India in November and December.Eventually,we investigate the characteristics and the driving mechanisms of long lifespan eddies.In the AS,long lifespan anti-cyclonic eddies(AEs)mainly generate in the Socotra,the West Indian Coastal Current and the East Arabian Current regions,while cyclonic eddies(CEs)are concentrated in the northwest region.In the BOB,long lifespan AEs mostly form near the west of Myanmar,while CEs are accumulated at the north and northwest basin.The instabilities caused by Rossby waves,coastal Kelvin waves,seasonal currents,together with wind stress forcing exert enormous efforts on the generation and evolution of these eddies.