Three-Dimensional Modal and Nonmodal Structures in Frontal Rainbands

据观察，中纬度降水通常与锋面系统和锋面降水组织成带，其中表现出三维的子结构。 在过去的十年中，天气尺度上的锋面和相关降水的预测稳步提高，但是，锋面子结构和中尺度上的相关降水的预测却不太成功。 本研究的目标是探索新的不稳定机制，将提供可能的解释，一些观察到的三维子结构，包括严重的风暴元素，嵌入在锋面雨带。 对称不稳定（symmetric instability，SI）和条件不稳定（conditional SI）作为可能的动力机制之一，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子结构超出了现有SI理论的描述范围。 主要研究者（PI）最近的研究表明，非线性SI环流在三维空间是不稳定的，相关的次级不稳定模式类似于一些观察到的中尺度子结构。 到目前为止，所获得的结果仍然是非常初步的，但揭示了一些重要的问题，这需要进一步调查，成功完成这项研究将推进理论上的理解降水系统，并可能导致更好的预测。

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.