The Multi-Scale Dynamics of Sundowner Wind Systems

该研究项目旨在更好地了解导致南加州日落纳风事件的机制。在没有非常强的地面风速的情况下，日落风通常与极端加热有关。夕阳风造成了有利于毁灭性的南加州野火的条件，使得预报它们变得至关重要，由于它们的快速和非常精细的规模开始。要检验的基本假设跨越多种运动尺度：在较大尺度(中-α/β)，日落纳风事件与：1)由上游沿海气压梯度驱动的低空沿海急流伴随着强烈的低层逆温，以及2)沿沿海山脉(如圣伊内斯)的浅临界层和浅记分器参数间断。这一低层沿岸急流的结果是：1)近岸冷海洋边界层与升高的内陆对流边界层之间的温度梯度向内陆延伸；2)上游极地急流的右侧出口区加强了沿海高地面气压(达到该温度梯度的西北方向)。在较小的尺度(中-伽马)，涉及内部重力波(IGW)破裂的波发展沿着沿海(Santa Ynez)山脉向西的斜坡发展，在海层上方触发一个短暂的旋风。旋翼放大和断裂，将较热和较干燥的空气迅速混合到海洋层中。这种混合是日落风快速变暖的第一个组成部分，因为寒冷的海洋层被消散，并被来自剩余大陆边界层的相对温暖的空气所取代。第二个分量随后迅速出现，伴随着IGW的拖尾暖池下降到沿海山区形成中伽马尺度的尾迹低，导致10-25℃的总变暖。本研究活动采用的方法主要包括数值模拟、观测数据分析和几个真实和理想化的数据实例的理论分析。天气研究和预报(WRF)模式将被用来模拟大量的日落风个例研究，范围从极端到边缘到零个例研究。这些事件将根据观测到的地面风、阵风和加热强度进行分类，数值模拟将涉及到一系列的情况。根据地形坡度、形状和大小的不同，水平网格的分辨率将在区域范围内从~10公里到日落风发生的特定位置的~666m，从而允许模拟场的尺度收缩。这一研究活动的智力价值在于了解日落风事件的多尺度机制及其对天气和多中尺度动力学的敏感性，包括海层和非常细尺度的地形结构。特别是，将更详细地了解更大规模的急流调整如何建立中尺度风和温度梯度，这些梯度可能会受到地形的扰动，从而产生中伽马尺度的山波、钻孔和尾流低。此外，海洋层、山波和更大规模的急流如何在加利福尼亚州圣巴巴拉附近火灾多发地区发生的日落风中相互作用。这项研究活动的更广泛影响将涉及该地区天气预报的进展，最值得注意的是预测导致极端野火危险的条件。由于业务计算机模型无法模拟日落风更精细的尺度特征，美国国家气象局(NWS)、私人预报员和当地应急人员更难警告/准备应对火灾事件。这一区域的公众极易受到伤害，根据这项研究所获得的知识改进预报，可以通过对预报员和应急人员进行有关关键前兆情况的教育，防止生命和财产损失。此外，来自代表性不足社区的实习生将在加利福尼亚州奥克斯纳德的NWS预测办公室就这一项目进行合作。

The research project seeks to gain a better understanding of the mechanisms that cause Sundowner wind events in Southern California. Sundowner winds are typically associated with extreme heating in the absence of very strong surface wind velocities. Sundowner winds cause conditions favorable for devastating Southern California wildfires making forecasting them critically important and extraordinarily difficult due to their fast and very fine scale onset. The basic hypothesis to be tested spans a multiplicity of scales of motion: At larger scales (meso-alpha/beta) Sundowner wind events are associated with: 1) a low-level coastal jet driven by an upstream coastal pressure gradient accompanying a strong low-level inversion as well as 2) a shallow critical layer and shallow Scorer Parameter discontinuity along the coastal mountains (e.g., Santa Ynez). This low-level coastal jet results from: 1) an inland extension of the thermal gradient between a cold near shore marine boundary layer and an elevated inland, heated convective boundary layer and 2) a reinforcement of coastal high surface pressure (to the NNW of this thermal gradient) by the right exit region of an upstream polar jet streak. At the smaller scales (meso-gamma), wave development involving an internal gravity wave (IGW) breaking develops along the westward-facing slopes of the coastal (Santa Ynez) Mountains, triggering a short-lived rotor above the marine layer. The rotor amplifies and breaks, mixing warmer and drier air rapidly into the marine layer. This mixing represents the first component of rapid Sundowner warming as the cold marine layer is dissipated and replaced by relatively warm air from the residual continental boundary layer. The second component occurs quickly thereafter as the trailing warm pool accompanying the IGW descends the coastal mountains forming a meso-gamma scale wake low resulting in 10-25C° total warming.The methods employed in this research activity primarily involve numerical modeling, observational data analyses and theoretical analyses of several real and idealized data case studies. The Weather Research and Forecasting (WRF) model will be employed to simulate numerous Sundowner wind case studies ranging from extreme to marginal to null case studies. The events will be classified based on observed surface wind gust and heating intensities and the numerical modeling will involve a complete spectrum of cases. The horizontal grids will range in resolution from ~10 km regionally to ~666 m in the specific locations of Sundowner wind genesis depending on terrain gradient shape and magnitude thus allowing the scale contraction of simulated fields.The intellectual merit of this research activity is to understand the multi-scale mechanisms causing Sundowner wind events and their sensitivity to synoptic and multiple mesoscale dynamics involving both the marine layer and very fine scale terrain structure. In particular, more detailed understanding will be derived of how larger scale jet adjustments establish the mesoscale wind and temperature gradients that can be perturbed by terrain to generate meso-gamma scale mountain waves, bores and wake lows. Furthermore, how the marine layer, mountain wave, and larger scale jets interact in the Sundowner wind that occurs in the fire-prone regions near Santa Barbara, California.The broader impacts of this research activity will involve weather forecasting advancements in this region, most notably predicting conditions that result in extreme wildfire danger. The inability of operational computer models to simulate the finer scale characteristics of Sundowner winds makes it much more difficult for the National Weather Service (NWS), private forecasters and local emergency responders to warn/prepare for fire events. The public is extremely vulnerable in this region and improved forecasts derived from the knowledge gained by this research may prevent the loss of life and property by educating forecasters and emergency responders concerning key precursor conditions. In addition, student interns from underrepresented communities will collaborate on this project at the NWS Forecast Office in Oxnard, California.