A new Infrared Atmospheric Sounding Interferometer channel selection and assessment of its impact on Met Office NWP forecasts
A new set of Infrared Atmospheric Sounding Interferometer (IASI) channels was re-selected from 314 EUMETSAT channels. In selecting channels, we calculated the impact of the individually added channel on the improvement in the analysis outputs from a one-dimensional variational analysis (1D-Var) for the Unified Model (UM) data assimilation system at the Met Office, using the channel score index (CSI) as a figure of merit. Then, 200 channels were selected in order by counting each individual channel’s CSI contribution. Compared with the operationally used 183 channels for the UM at the Met Office, the new set shares 149 channels, while the other 51 channels are new. Also examined is the selection from the entropy reduction method with the same 1D-Var approach. Results suggest that channel selection can be made in a more objective fashion using the proposed CSI method. This is because the most important channels can be selected across the whole IASI observation spectrum.
In the experimental trial runs using the UM global assimilation system, the new channels had an overall neutral impact in terms of improvement in forecasts, as compared with results from the operational channels. However, upper-tropospheric moist biases shown in the control run with operational channels were significantly reduced in the experimental trial with the newly selected channels. The reduction of moist biases was mainly due to the additional water vapor channels, which are sensitive to the upper-tropospheric water vapor.
Key wordsHyperspectral IR sounding channel selection 1D-Var data assimilation
本文从欧洲气象卫星的 314 个观测通道中重新选择了一系列新的大气红外探测器(IASI)通道. 在选择过程中, 用通道得分指数(CS))分别评估在一维变分(1D-Var)同化系统中独立增加每一个通道对英国气象局一体化模式(UM)的改进情况; 然后, 根据每个通道的CSI贡献依次筛选出200个通道, 和当前业务中使用的 183 个通道相比, 新选择的通道有 149 个与之相同, 其余的 51 个通道与之不同. 同时, 本文采用熵减法在相同 1D-Var 系统中选出的通道的进行对比研究, 其结果表明, 使用 CSI 方法进行通道选择更客观, 这是因为该方法可以从整个探 IASI 观测频谱中选择出最重要的通道. 在使用 UM 全球同化系统的预报实验中, 采用新的通道相比于当前业务通道, 在预报质量方面没有明显的改善, 但是新方案能明显减小, 由当前业务通道带来的对流层上层水汽误差. 这种改善主要是因为增加了对于对流层上层水汽敏感的额外水汽通道.
关键词大气红外探测器 通道选择 一维变分 联合模式
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