偏差订正方法在IASI辐射率资料同化中的应用研究

Bias correction of infrared atmospheric sounding interferometer radiances for data assimilation

根据IASI（Infrared Atmospheric Sounding Interferometer）的资料特征和GRAPES（Global/Regional Assimilation and Prediction System）同化系统的具体情况,建立了适用于业务使用的关于IASI辐射率资料的偏差订正方案,该方案包括扫描偏差订正和气团偏差订正。统计表明,IASI资料的扫描偏差不像微波资料一样具有明显的纬度依赖性,但在2x2的像元内存在某种特殊的扫描偏差,临边测量相对于星下点的扫描偏差可以用＂扫描角＂作为自变量而消除,而2x2的像元内的偏差只能通过稀疏化来规避;气团偏差主要根据当时的天气条件进行订正,利用模式背景场作为预报因子定量给出天气条件,采用1 000~300 h Pa的厚度、200~50 h Pa的厚度、50~20 h Pa的厚度以及模式地表温度作为预报因子。订正方案的试验结果显示,偏差能够长时间维持在比较低的稳定水平,订正结果显著。

Taking into account both IASI instrument characteristics and the specific circumstance of GRAPES,a bias correction scheme applicable to operational system is developed for IASI radiances. The scheme utilizes a scan correction and an air-mass correction. Figures show that instead of latitudinal dependency for microw ave radiances bias,there is a different scan bias for each of the 2x2 IASI pixels. Scan bias betw een limb and nadir measurements can be removed w ith the view angle as independent variable,w hile the smaller amplitude pixel-to-pixel bias avoided by the w ay of thinning. Air-mass bias is corrected according to the w eather condition at that time,w hich can be quantitatively represented w ith predictors computed from the background field. Four predictors are used,including1 000 ~ 300 h Pa thickness,200 ~ 50 h Pa thickness,50 ~ 20 h Pa thickness,and model surface temperature. Verification results show that biases can be controlled stably at a relatively low level for a long time and the correction effects are significant.