Three-Dimensional Modal and Nonmodal Structures in Frontal Rainbands

已经观察到，中纬度降水通常与锋面系统有关，锋面降水被组织成带，表现出三维亚结构。在天气尺度上对锋面和相关降水的预报在过去十年中稳步改进；然而，在中尺度上对锋面亚结构和相关降水的预测却不太成功。这项研究的目的是探索新的不稳定机制，这将为一些观测到的三维亚结构提供可能的解释，包括嵌入锋面雨带的强风暴元素。作为一种可能的动力机制，对称不稳定性和条件不稳定性在解释某些观测到的锋面雨带的总体结构方面显示出许多有吸引力的特征。然而，在锋面雨带中经常观测到的三维亚结构超出了当前SI理论的描述。最近的研究表明，非线性SI环流在三维上是不稳定的，相关的次级不稳定模态类似于观测到的一些中尺度亚结构。到目前为止取得的结果仍然是非常初步的，但揭示了一些需要进一步调查的重要问题。这项研究的成功完成将促进对降水系统的理论认识，并可能导致更好的预报。

It has been observed that mid-latitude precipitation is commonly associated with frontal systems and frontal precipitation is organized into bands, which exhibit three-dimensional substructures. Prediction of fronts and associated precipitation on the synoptic scale has steadily improved over the past decade; however, prediction of frontal substructures and associated precipitation on the mesoscale has experienced less success. The goal of this research is to explore new instability mechanisms that will provide possible explanations for some observed three-dimensional substructures, including severe storm elements, embedded in frontal rainbands. As one of the possible dynamic mechanisms, symmetric instability (SI) and conditional SI have been shown to have many attractive features in interpreting the gross structures of some observed frontal rainbands. However, the often-observed three-dimensional substructures in frontal rainbands are beyond the description of current SI theories. Recent research by the Principal Investigator (PI) suggests that nonlinear SI circulations are unstable in three dimensions and the related secondary unstable modes resemble some observed mesoscale substructures. The results obtained so far are still very preliminary but shed light on a number of important issues, which need further investigation.Successful completion of this research will advance theoretical understanding of precipitation systems and potentially lead to better forecasts.