Study On The Tropical Cyclone-generated Waves By SAR Observation And The Aliasing Elimination On Altimeter

The swells generated by tropical cyclone(TC)with significant energy,wide range of influence and making it difficult to observe.Thus,they have attracted much attention in atmospheric sciences,ocean sciences,and ocean engineering.Meantime,the asymmetric structure and changes of TCs are often accompanied by changes in TC intensity,and affect the generation and development of TC waves.This makes it extremely important to use real observations to track,determine and analyze the swells generated by TCs.In this study,the swell data from space-borne synthetic aperture radar(SAR)wave mode(Wa Ve mode,WV)products are employed to track the TC generated swells,and analyzed the spatial structure,formation mechanism of these swells.The correlation between the swells and the forecasting methods were also initially explored.Combining the information of the swells by the Sentinel-1A&B,TC properties and best track information from IBTr ACS TC database,and the wind field data from ECMWF,a new tracking scheme for moving TC's swell is presented.Swells generated by TCs in the Pacific Ocean from 2015 to 2018 are tracked and divided into 6-hours resolution initial swell field data sets.In the results,a total of 126 typhoons and 23736-hour typhoon track points have acquired swell field data.Subsequently,the comparison of Sentinel-1 wave mode data and buoy data showed that the wavelength of swell and direction deviations were less than-10.06 and 9.08,and the root mean square error(RMSE)distribution was less than 35.44 meters and 15.21 degrees,and its statistical characteristics of the wave have little change with the swell propagation.The statistical results compared with WW3 show that the deviations of swell wavelength,direction and significant wave height are-19.51,-10.34 and-0.29,respectively,and the RMSE is 83.91 meters,91.30 degrees and 0.81 meters,respectively.There are obvious errors in the typhoon generated swell parameters from the WW3.With the TC swells field datasets form Sentinel-1A&B wave mode,the relationship between TC intensity,translation speed and the wavelength of TC swells field are statistically analyzed.Results show that the wavelength of the swell field in the front of TC and the rear has obvious asymmetric characteristics.Among them,the swells in the front of TC determines the strength of the asymmetric feature,which is affected by the TC intensity,size and translation speed.In the medium-translation speed of TC,the difference of the swell wavelength in the front and rear of TC-heading can reach about 200 meters.No obvious asymmetry is found in the wavelength of the swell on the left and right.Besides,the relation between swell wavelength and fetch as well as that between swell wavelength and the integral kinetic energy of the TC in four quadrants are also further discusses.The results show that the asymmetric intensity of the swell wavelength can be explained by the status of wave development under the TC,and in line with the extended fetch theory.And the asymmetry of the swell wavelength is also proportional to the integral kinetic energy.Meantime,the errors in the WW3 are also analyzed.All of these are helpful to correct the parameters of the TC wave numerical model and improve the TC numerical forecast ability.In addition,this study also uses the fetch-law model of Kudryavtsev et al.(2015)to simulate and analyze the TC swells field wavelengths of four examples of TC KILO(2015)and TC HECTOR(2018).It can better simulate the wavelength characteristics of the TC swells in the RF quadrant and LR quadrant.Through the sensitivity analysis experiment,it is found that the length of the dimensionless fetch,inverse wave age,the location when the swell is generated and the 10 meters wind speed of determine the state of the swell when it leaves the typhoon intense wind region.When the length of dimensionless fetch( #)is 10!?10" or the ratio between the average distance from re-propagation trajectory to typhoon center()and the radius of maximum wind speed is 2?5,the swell wavelength can obtain the best simulation effect.The root square error is about 50 meters.These resulting rates from these observations can serve as a reference for TC forecasting,TC swell modeling,and related studies.In the end,this study further corrected the non-tidal aliasing errors in the altimeter sea level products of the Western Mediterranean Sea.Here,we employed the highfrequency tide gauge data to extract the altimeter non-tide aliasing correction in the west Mediterranean Sea.The spatial average method and EOF analysis method were adopted to track the high-frequency oscillation signals from 15 tide gauge records(TGs),and then were used to correct the aliasing errors in the Jason-1 and Envisat observations.The results showed that the EOF analysis method is better than the spatial average method in the altimeter data correction.After EOF correction,90% of correlation(COR)between TG and sea level of Jason-1 has increased 5%,and 3%increase for the Envisat sea level;for the spatial average correction method,only 70%of Jason-1 and Envisat data at the TGs location has about 2% increase in correlation.The EOF correction reduced the average percentage of error variance(PEL)by 30%,while the spatial average correction increased the average percentage of PEL by 20%.After correction by the EOF method,the altimeter observations are more consistent with the distribution of strong currents and eddies in the west Mediterranean Sea.The results prove that the proposed EOF method is more effective and accurate for the nontide aliasing correction.