偏差订正方法在IASI辐射率资料同化中的应用研究

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。

Bias correction of infrared atmospheric sounding interferometer radiances for data assimilation

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 microwave radiances bias,there is a different scan bias for each of the 2x2 IASI pixels.Scan bias between limb and nadir measurements can be removed with the view angle as independent variable,while the smaller amplitude pixel-to-pixel bias avoided by the way of thinning.Air-mass bias is corrected according to the weather condition at that time,which can be quantitatively represented with predictors computed from the background field.Four predictors are used,including 1000~300 hPa thickness,200~50 hPa thickness,50~20 hPa 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.