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区的山波感生旋翼进行研究。旋风是沿着平行于山顶和山顶下游的轴线形成的强烈的低层水平涡旋。虽然众所周知，它们对航空非常危险，但对这一现象的定量观测和理论研究很少。因此，对转子结构和转子动力学的了解相对较不完整，对转子的预测能力也不是很强。观测将在欧文斯山谷及其上游收集，欧文斯山谷位于内华达山脉南部的正东部。内华达山脉南部的东坡是毗邻的美国最高、最陡峭的准线性地形屏障。这个项目的两个主要目标是：(1)建立旋翼行为的量化特征，包括旋翼诱发的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能够可靠地预测旋翼发生的程度。第一个主要目标将通过一项小型实地试验来实现，其中包括在欧文斯山谷安装一系列自动气象站和风廓线，并发射无线电探空仪对上游气流进行采样。支持第二个目标的数值模拟工作旨在确定模式预报中的不足之处(通过与观测数据的比较发现)是由于对冲击山脉的大范围气流的描述不充分，还是由于山波和旋翼本身的详细计算困难所致。该项目的结果将作为提高数值模式应用于陡峭地形的预报能力的基础。这种改进的一个结果是，对山区的严重航空危险提供更好的预测。

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.