IMPACT OF DATA ASSIMILATION FOR THE IASI OBSERVATIONS ON THE FORECAST OF TYOHOON HONGXIA AND MOLANDI
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摘要: 红外高光谱大气探测仪IASI可提供高精度的大气垂直温度和湿度信息，能够探测台风结构特征，有效弥补台风影响区域观测资料稀缺的不足。以WRFDA三维变分同化系统为基础构建IASI同化试验平台，实现McNally提出的MW云检测方法，并调整参数形成大阈值的LMW云检测方法，以超强台风“红霞”（1506）和“莫兰蒂”（1614）为试验个例，对IASI观测资料进行同化对比试验。对于台风“红霞”，MW云检测方案对于高层通道299保留的观测数目仅为大阈值LMW云检测的16.2%和WRFDA系统默认的MMR云检测的9.2%，对于底层通道921分别为3.3%和2.6%。但是MW试验分析场强度最强，获得的72 h台风路径预报最接近真实路径，路径误差最小。两个台风个例试验结果相似，表明有效的云检测过程能提高IASI资料同化分析场的准确性，同化IASI资料有利于改善台风预报技巧。Abstract: The Infrared Atmospheric Sounder Interferometer (IASI) provides the temperature and humidity information with high precision about the atmosphere in the vertical direction. The IASI instrument can detect the characteristics of typhoon structure and make up for the shortage of the observation data distributed sparsely in typhoon-affected areas. In this study, a three-dimensional variational data assimilation for Weather Research and Forecasts (WRFDA) system was chosen as the basic assimilation system, and the MW cloud detection put forward by McNally was implemented in the WRFDA system for IASI cloud contamination detection and the cloud parameters were tuned for the research. All the IASI observations are assimilated after quality control and variational bias correction and the impact of the data assimilation on the forecasts of the super typhoon Hongxia (1506) and Molandi (1614) are assessed. The results from both the typhoon experiments are similar and indicate that the cloud detection influences the assimilation of the IASI observations very much. For the super typhoon Hongxia, the MW cloud detection scheme retained just 16.2% the number of observations by the large-threshold LMW cloud detection scheme and 9.2% of that by the MMR cloud detection scheme for the upper-level channel 299, and 3.3% and 2.6% respectively for the lower-level channel 921, but the analysis affected by the MW cloud detection scheme reduces the track error of typhoon Hongxia for the first 72 hours most remarkably and improves the path forecast most accurately. Generally the assimilation of IASI observations improves the skills of typhoon forecast.
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Key words:
- infrared hyper-spectral /
- IASI /
- Typhoon /
- data assimilation /
- cloud detection
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图 6 台风"红霞"MW（a、b）、LMW（c、d）和MMR（e、f）试验的海面分析场（a、c、e）及500 hPa分析场（b、d、f）的同化分析的温度、风速、气压场对比
红圈表示差异明显的区域。彩色色标表示不同气压高度大气的温度。
图 7 台风“红霞”的72 h路径（a）和路径误差（b）以及台风“莫兰蒂”72 h路径（c）的对比
a.、c.中绿色为控制试验；红色为MW试验；紫色为LMW试验；黄色为MMR试验对应的路径；蓝色为观测路径。
表 2 IASI观测资料质量控制步骤
步骤 检测标准 舍弃的要素 1 混合地表上空的IASI观测，isflag=4、5、6、7 该视场 2 观测视场为IASI扫描线两端处的5个视场 3 若某一观测视场的云水路径≥0.2 kg/m2 4 某一通道的波速>2 400 cm-1 该通道 5 某一通道观测增量|O-B|>15 K 6 某一通道观测增量|O-B|>观测误差的3倍 
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表 3 IASI偏差订正预报因子
参数序号 预报因子 P0 1 (常数) P1 1 000~300 hPa位势高度 P2 200~50 hPa位势高度 P3 表面温度 P4 总降水 P5 IASI扫描角θ P6 IASI扫描角的平方θ2 P7 IASI扫描角的立方θ3
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表 4 IASI同化试验方案
试验名称 同化系统 云检测方法 同化资料 CNTL试验 - - - MW试验 WRF-3DVar MW云检测方法 IASI LMW试验 WRF-3DVar LMW云检测方法 IASI MMR试验 WRF-3DVar MMR云检测方法 IASI
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表 5 通道299、327、354、921的云敏感高度
通道号 峰值高度/hPa 云敏感高度 299 138 71.775 6 327 814 75.049 2 354 433 74.586 5 921 1 085 100.990 0
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表 6 台风“莫兰蒂”24 h预报的中心气压和瞬时风速
“莫兰蒂”数据 中心气压/hPa 风速/(m/s) 观测OBS 910.0 65.0 MW 965.5 66.5 LMW 968.9 59.1 MMR 978.5 50.3 CNTL 959.3 68.2
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