在低对流层,传统的闭环模式容易造成较大的信号追踪误差.开环模式通过内部或外部方式消除导航资料调制(navigation data modulation,NDM)对大气反演的影响,能够准确地恢复信号的相位.内部和外部方式的区别是,前者通过信号相邻采样的内在联系移除NDM,后者利用卫星导航位元数据进行处理.通过内部和外部两种方式处理COSMIC(constellation observing system for meteorology,ionosphere and climate)掩星数据,利用全谱反演方法获得弯曲角,通过Abel积分变换计算折射率.COSMIC掩星个案分析表明,在低对流层,内部方式可能会导致信号的半周跳现象,从而造成反演的折射率出现误差.分别用上述两种方式对2007年第71天至73天共约3130个COSMIC掩星开环数据进行处理.将折射率反演结果与ECMWF(european centre for medium-range weather forecasts)分析场资料进行统计比较,结果显示:外部处理方式的探测深度比内部方式低100—200 m.在热带的3 km以下,内部方式具有比外部方式稍大的折射率负偏差.因此,外部方式能够得到比内部方式略优的反演结果.
This paper presents the design of an observation operator for assimilation of global navigation satellite system(GNSS) radio occultation(RO) refractivity and the related operational implementation strategy in the global GRAPES variational data assimilation system.A preliminary assessment of the RO data assimilation effect is performed.The results show that the RO data are one of the most important observation types in GRAPES,as they have a significant positive impact on the analysis and forecast at all ranges,especially in the Southern Hemisphere and the global stratosphere where in-situ measurements are lacking.The GRAPES model error cannot be controlled in the Southern Hemisphere without RO data being assimilated.In addition,it is found that the RO data play a key role in the stable running of the GRAPES global assimilation and forecast system.Even in a relatively simple global data assimilation experiment,in which only the conventional and RO data are assimilated,the system is able to run for more than nine months without drift compared with NCEP analyses.The analysis skills in both the Northern and Southern Hemispheres are still relatively comparable even after nine-month integration,especially in the stratosphere where the number of conventional observations decreases and RO observations with a uniform global coverage dominate gradually.
在大气多路径和噪声的条件下,高斯白噪声会造成滑动频谱方法获得的弯曲角与真值之间的偏离,无法获得较好的反演结果。为此,文章提出了一种改进的滑动频谱方法,即利用信号的振幅和谱能量信息对滑动频谱方法进行修正,削弱了信号中噪声的影响,与真值较为接近。分别用改进前后两种方法对中华卫星三号计划(Constellation Observing System for Meteorology,Ionosphere and Climate,COSMIC)的掩星进行反演,并将其折射率计算结果和通过全谱反演方法获得的折射率一起,与欧洲中期天气预报中心(EuropeanCentre for Medium-Range Weather Forecasts,ECMWF)的分析场资料进行了统计比较。结果表明:改进的滑动频谱方法删除了信号中的部分噪声,减少了系统偏差;与全谱反演方法进行比较,发现两者具有相当的反演精度。
