区域极轨卫星ATOVS辐射偏差订正方法研究

A REGIONAL ATOVS RADIANCE-BIAS CORRECTION SCHEME FOR REDIANCE ASSIMILATION

近年来,卫星辐射资料在数值天气预报(NWP)系统中的直接同化研究取得了长足进展。为了利用TIROS业务垂直探测器(ATOVS)的辐射资料,必须对卫星观测辐射值的系统性偏差进行订正。在ECMWF原全球TOVS辐射偏差订正方案基础上,结合ATOVS资料特征和中国的实际情况,建立了适用于区域NOAA-15/16/17极轨气象卫星ATOVS辐射资料的偏差订正方案。该方案偏差订正分两步进行:首先进行扫描偏差订正,然后进行气团偏差订正。扫描偏差是临边测量相对于星下点测量的系统偏差,统计显示该种偏差具有一定的纬度依赖性,所以订正时按每10度的纬度带分别进行订正。气团偏差订正主要就是根据当时的天气条件进行订正,而天气条件一般用预报因子来定量表示。文中从中国国家气象中心T213背景场导出预报因子:(1)1000—300 hPa的厚度,(2)200—50 hPa的厚度,(3)模式地表温度,(4)总可降水量。模式预报因子的使用从观念上将对观测值的订正变为对计算前向辐射值的订正问题。试验结果表明,订正结果显著。

Recently, much progress has been made in the direct assimilation of satellite radiance measurements in numerical weather-prediction （NWP） system. In order to use radiances from the Advanced TIROS Operational Vertical Sounder （ATOVS） of NOAA-15 /16 /17 , biases between the observed radiances and those simulated from the model first guess must be corrected. In this study, a regional ATOVS radiance - bias correction scheme for NOAA-15 /16 /17 satellites is developed based on the global TOVS radiance - bias correction scheme of the European Center for Medlum-Range Weather Forecasts （ECMWF） and by taking into account both ATOVS instrument characteristics and weather conditions in China. This scheme includes two steps of bias corrections： scan bias correction and alr-mass bias correction. Scan bias is a relative bias between limb and nadir measurements and corrected for each latitude band of 10 degrees due to its statistically latitudinal dependency. Air-mass bias is corrected according the weather condition at the observation time and it is quantitatively represented with so- called ＂predictors＂. In this study, predictors are computed from the background field of global T213 from the National Meteorological Center （NMC）. Four predictors are used： （1） 1000- 300 hPa thickness, （2） 200 -50 hPa thickness, （3） model surface temperature and （4） total precipitable water. The use of model predictors in correction transforms conceptually the correction of the observations into the correction of the computed forward radiances. The preliminary test result is encouraging, and the scheme is expected to be used in the operational NWP data assimilation system in China.