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。日落风造成的条件有利于毁灭性的南加州野火，使预测他们至关重要，非常困难，由于他们的快速和非常精细的规模开始。要检验的基本假设跨越了运动的多种尺度：在较大尺度（中α/β）下，日落风事件与以下因素有关：1）由上游海岸压力梯度驱动的低空海岸急流，伴随着强烈的低空逆温，以及2）沿海岸山脉沿着的浅层临界层和浅层Scorer参数不连续性（例如，Santa Ynez）。这种低层海岸急流的原因是：1）在寒冷的海岸海洋边界层和升高的内陆加热对流边界层之间的热梯度的内陆延伸; 2）上游极地急流条纹的右侧出口区域对海岸高表面压力（该热梯度的NNW）的加强。在较小的尺度（中伽马），波的发展，包括内部重力波（IGW）打破发展沿着向西的斜坡海岸（圣伊内斯）山脉，触发了一个短暂的转子以上的海洋层。转子放大并破裂，将温暖和干燥的空气迅速混合到海洋层中。这种混合代表了第一个组成部分的快速暮光变暖的冷海洋层消散，并取代了相对温暖的空气从残留的大陆边界层。 第二个分量则是伴随着IGW的尾流暖池沿海岸山脉下降，形成一个中-γ尺度的尾流低压，导致10- 25 C °的总体增温。本研究主要采用数值模拟、观测资料分析以及对若干真实的和理想化的资料个例的理论分析。天气研究和预报（WRF）模型将用于模拟大量的日落风案例研究，从极端到边缘到零案例研究。将根据观测到的地面阵风和加热强度对事件进行分类，数值模拟将涉及完整的情况谱。水平网格的分辨率范围将从区域的~10 km到日落风成因的特定位置的~666 m，这取决于地形梯度形状和大小，从而允许模拟场的尺度收缩。导致日落风事件的尺度机制及其对天气学和涉及海洋层和非常细尺度地形的多个中尺度动力学的敏感性结构 特别是，更详细的了解，将得出如何更大规模的射流调整建立中尺度风和温度梯度，可以扰动地形产生中伽马尺度山波，孔和尾流低。此外，如何海洋层，山波，和更大规模的射流在日落风发生在火灾易发地区附近的圣巴巴拉，加州的相互作用。这项研究活动的更广泛的影响将涉及天气预报在这一地区的进步，最显着的预测条件，导致极端的野火危险。操作计算机模型无法模拟日落风的更精细尺度特征，这使得国家气象局（NWS），私人预报员和当地紧急响应人员更难以警告/准备火灾事件。 该地区的公众极易受到影响，从这项研究中获得的知识所产生的更好的预报可能会通过教育预报员和应急人员了解关键的前兆条件来防止生命和财产损失。 此外，来自代表性不足社区的学生实习生将在加州奥克斯纳德的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.