Collaborative Research: An Observational, Modeling, and Climatological Study of Sierra Rotors

本研究主要针对内华达山脉背风处的山波诱导旋翼进行研究。旋翼是一种强烈的低层水平涡旋，它沿着平行于山顶下游的轴线形成。虽然已知它们对航空非常危险，但对这一现象的定量观察和理论研究很少。因此，对转子结构和转子动力学的了解相对不完整，对转子的预测能力也缺乏技巧。本研究将采用观测和数值模拟的方法进行。观测结果将在欧文斯山谷的上游收集，欧文斯山谷位于南内华达山脉的正东。南内华达山脉的东坡是美国境内最高、最陡的准线性地形屏障。吹过内华达山脉的西风经常在欧文斯山谷上空产生振幅较大的山波和强烈的旋涡。本项目的两个主要目标是：(1)建立转子行为的定量特征，包括转子诱发山波事件的位置和频率分布；(2)评估当前运行的中尺度模式在多大程度上能够可靠地预测转子的发生。第一个主要目标将通过一项小型实地试验来完成，其中包括在欧文斯山谷安装一系列自动气象站和风廓线仪，并发射无线电探空仪对上游气流进行取样。支持第二个目标的数值模拟工作旨在确定模式预报的缺陷（如与观测数据比较所揭示的）是由于对击打山区的大尺度气流的描述不足，还是由于在山波和转子本身的详细计算方面存在困难。该项目的研究结果将为提高应用于陡峭地形的数值模式的预报能力提供依据。这种改进的结果之一将是对山区的严重航空危险提供更好的预报。

This research is focused on a study of mountain-wave induced rotors in the lee of the Sierra Nevada. Rotors are intense low-level horizontal vortices that form along an axis parallel to and downstream of a mountain crest. While they are known to be very hazardous to aviation, there have been few quantitative observational and theoretical studies of this phenomenon. As a result, understanding of rotor structure and rotor dynamics is relatively incomplete, and the ability to forecast rotors shows little skill.This study will be conducted using observations and numerical simulations. The observations will be collected in and upstream of the Owens Valley, which lies directly to the east of the southern Sierra Nevada. The eastern slopes of the southern Sierra Nevada are the tallest, steepest quasi-linear topographic barrier in the contiguous United States. The westerly winds that blow across the Sierra Nevada frequently generate large-amplitude mountain waves and strong rotors over the Owens Valley.Two major objectives of this project are to: (1) establish quantitative characteristics of the rotor behavior including the location and the frequency distribution of the rotor-inducing mountain-wave events, and (2) evaluate the extent to which current operational mesoscale models can reliably forecast the occurrence of rotors. The first major objective will be accomplished with a small field experiment involving the installation of an array of automatic weather stations and wind profilers in the Owens Valley and radiosonde launches to sample the upstream flow. The numerical modeling effort in support of the second objective is designed to determine whether deficiencies in the model forecasts (as revealed by comparison with the observational data) are due to an inadequate representation of the large-scale flow striking the mountains or to difficulties in the detailed computation of the mountain waves and rotors themselves.The results of this project will serve as a basis or improving the forecasting capability of numerical models applied to steep topography. One consequence of such improvement would be to provide better forecasts of severe aviation hazards in mountainous terrain.