全球数值模式中的台风初始化Ⅰ: 方案设计

The initializatio n of tropical cyclones in the NMC global model PartⅠ: Scheme design.

  • 摘要: 由于缺少大量有效的观测资料,台风初始化对数值天气预报业务模式而言,仍然是一个悬而 未决的难题。中国国家气象中心自从1996年将台风数值预报系统投入业务运行以来,一直使 用经验的人造bogus涡旋台风初始化技术。实际上,不同时期的台风有着不同的环流结构, 即使同一个台风在不同的生命期也具有不同的结构特征,而这些结构特征的差异并不能依靠 现有的bogus涡旋技术体现出来,这种主观方法的统一性与台风在时空上的差异性形成了强烈的反差。最近,基于国家气象中心全球资料分析同化预报循环系统,设计和发展了一套新的 台风初始化业务方案,它主要由初始涡旋形成、涡旋重定位和涡旋调整3部分过程组成。相 比于业务中使用的人造bogus涡旋台风初始化方案,新方案在很大程度上减少了人为因素对台风涡旋结构的影响,而更多地是依靠数值模式自身的动力和物理过程来协调约束产生三维 空间的涡旋结构。应用新方案,文中对生成于西北太平洋的2006年0605号台风格美(Kaemi) 进行了数值试验,初步分析表明,新方案在实现台风涡旋环流结构的初始化方面效果较好, 同时,对台风格美多个时次的预报结果也显示,相比于业务使用的bogus方案而言,新方案 对台风路径平均预报误差有了大幅度的降低。

     

    Abstract: Due to lack of valid observational data over the tropical oceans, TC (tropical c yclone) initialization for numerical models is one of major difficulties in accurate TC track forecasting. Since TC numerical prediction model at Natio nal Meteorological Center of China (NMC/China) was put into operation in 1996, it has always been applied an empirical and statistical method which imposes a scale-selective sophisticated filtering to remove shallow vortex from the initial field and inserts bogus vortex into it, constructed by a few observed parameters-such as central pressure, position, and maximum sustained wind. In fact, TC str ucture varies from one form to another for different TCs and even the same TC in different time of life, which is not directly related to these observed paramet ers. The spatial and temporal variability of TC structure are not represented by bogus vortex with a few observed parameters. Recently, a new TC initialization scheme based on NMC global analysisforecast cycle has been developed successfu lly. The new scheme is composed of three procedures: vortex formation, vortex relocation, and vortex modification. Specifically, an appropriate vortex (generated by global model forecast) is inserted into background field when TC occurs at first time (it is called vortex formation). In the next forecast time, there must be a little weak and misplaced vortex in the background (6-h forecast at first time), what all we need to do is to move the vortex to correct position and mo dify it close to observed parameters by relative dynamic formula (it is called v ortex relocation and modification). It can be seen that initial vortex of the ne w scheme is mostly formed with the integration of the global model and is compat ible with the dynamics and physics of the global model. Using the new scheme of TC initialization, a series of experimental forecasts were performed for TC Kaemi (0605) that occurred in the western North Pacific in 2006. From study it is fo und that the new scheme can produce the reasonable and realistic vortex in the initial field and produced much more accurate TC track prediction compared with operational bogus one. It indicates that mean track forecast biases can be reduce d effectively by new TC initialization scheme.

     

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