Planetary Wave Dynamics: Implications of Bimodality

最近对北半球中纬度季节内低频变率的研究强烈表明，行星长波（k = 2至4）的概率密度分布明显是双峰的。纬向平均气流的速度、水平和垂直切变均呈单峰概率密度分布。这种分区的物理基础还不清楚。双峰结构是同化过程中数据分层方法或数值截断的人工产物的可能性也没有被排除。本研究通过对观测数据和数值模式模拟的进一步诊断分析来解决这些问题。将审查北半球的数据，以描绘行星长波的两种模态之间转换所涉及的过程，并将分析扩展到包括南半球的数据。将分析一般环流模式模拟的选定长积分的输出，以确定模式行星长波的概率密度分布函数。对双峰性对全球数值预报的预测能力可能产生的影响进行了评价。这项工作将有助于我们对季节内气候变率中重要的动力过程的基本理解。

Recent studies of intraseasonal, low frequency variability in midlatitudes of the Northern Hemisphere have strongly suggested that the probability density distributions of the planetary long waves (k = 2 to 4) are significantly bimodal. In contrast, the speed, horizontal and vertical shear of the zonal mean flow possess unimodal probability density distributions. The physical basis for such partitioning has not yet clear. The possibility that the bimodal structures are an artifact of data stratification method or numerical truncation during assimilation has not yet been discounted either. The present study addresses these issues through further dignostic analyses of observational data and numerical model simulations. The Northern Hemisphere data would be examined to delineate the processes involved in transitions between the two modes for the planetary long waves and analysis extended to include the Southern Hemisphere data. Output from selected long integrations of a general circulation model simulations will be analysed to determine the probability density distribution functions of model planetary long waves. The possible influence of bimodality on the predictive skill of global numerical forecasts would be evaluated. This work would contribute to our basic understanding of the dynamical process important in intraseasonal climate variability.