An Evaluation Of FY-3C Radio Occultation Data Quality And Study Of Tropopause Characteristics

Radio occultation(RO)is a new atmospheric sounding technique to retrieve atmospheric parameter profiles.The refractivity profiles of the atmosphere variation with radius are retrieved from Doppler data obtained when radio signals pass through the atmosphere,by applying Abel funaction other relevant atmospheric parameters such as density,pressure and temperature can be derived.Compared with other existing atmospheric sounding methods,RO has the characteristics of global uniform distribution,all-weather,high precision,high vertical resolution and low cost.Fengyun-3C(FY-3C)satellite is the first satellite launched by mainland China that can use RO technology to retrieve atmospheric profiles.In this paper,the FY-3C RO is studied as follows:(1)This research evaluates the quality of FY-3C RO profiles including refractivity,temperature,and specific humidity by comparing with corresponding information from the European Centre for Medium-Range Weather Forecasts(ECMWF)Interim Reanalysis(ERA-Interim)data over the period of 2015-2018.The assessment is carried out by calculating and analyzing mean systematic differences between FY-3C and ERA-Interim profiles and corresponding standard deviations over a selected spatial and temporal domain.Results show that the FY-3C RO profiles are overall with a good agreement with the ERA-Interim data.Global mean refractivity systematic differences are within ±0.2% from 5 to 30 km altitude range with relative standard deviations of less than 2%.Global temperature mean systematic differences vary within ±0.2 K from the 10 to 20 km altitude range with standard deviations of less than 2 K.Global mean specific humidity differences are found to be within ±0.2 g/kg from the 2 to 20 km range with standard deviations of less than 1 g/kg.FY-3C profiles show visible latitudinal and altitudinal variations,while the seasonal variations are minor.Sampling errors of refractivity and temperature are also found to be larger at higher latitudinal regions due to RO events being less sampled in the polar region.(2)The ionospheric correction bending angle profiles corrected by dual-frequency signal difference are still affected by the ionospheric residual errors and thermal noises,and the bending angle profiles show the form of high frequency oscillation at the altitude of more than 50 km.Using the MISIE-90 model as the background data,the observed bending angle profiles of FY-3C were statistically optimized by using two methods.The high frequency oscillation of the bending angle profiles in the upper atmosphere can be effectively eliminated by either applying a static linearly weighted ionospheric correction method to the bending angle profiles with corresponding background ones,or according to the signal and noise covariance of the neutral atmosphere and the ionospheric atmosphere to determine the weight of L1 signal and background data for statistical optimization.Compared with ECMEF data,it is proven that the statistical optimization bending angle profiles and atmospheric refractivity profiles retrieved are more accurate,and the second method is more scientifically rigourous and effective.The results of statistical optimization of FY-3C using MSISE-90 data cannot further improve the existing data quality of FY-3C.(3)The atmospheric temperature profiles of FY-3C are used to calculate the tropopause height and temperature by calculating the atmospheric temperature gradient,and the annual,latitude and seasonal variations of the global tropopause height and temperature were studied.The results show that the global tropopause changes have annual periodicity.In general,the tropopause height increased by about 3.9 m and the temperature decreased by about 0.33 K from 2016 to 2018.The tropopause height in the low latitudes is between 16 and 17 km and the temperature is 195 K±3 K.At high latitudes,the tropopause height ranges from 8 km to 10 km,and the temperature varies greatly with the seasons,especially in the southern hemisphere.There are large gradients of tropopause height and temperature between 30°?50° in both of the northern and southern hemispheres.The change of tropopause in the low latitude zone is less affected by the season,followed by the middle latitude zone.The change of tropopause in the high latitude zone is more obvious with the season.In general,the tropopause height is lower in summer and higher in winter,while the tropopause temperature is higher in summer and lower in winter.The temperature and height of the tropopause in the northern and southern hemispheres have a certain asymmetry with time except that the tropopause height in the high latitudes has a similar trend with time in the northern and southern hemispheres.In this paper,the reliability and error characteristics of FY-3C data as well as the statistical optimization of bending angle profiles are studied,which provides a basis and reference for the improvement of FY-3C retrieve algorithms and the development of other RO missions.An the FY-3C RO data are used to enrich the study of global tropopause changes.