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。在没有很强的地面风速的情况下，日落风通常与极端加热有关。晚风造成了有利于毁灭性南加州野火的条件，由于它们的快速和非常小的规模，因此预测它们非常重要和非常困难。待测试的基本假设跨越多种运动尺度：在较大尺度（中尺度α / β）上，日落风事件与：1)由上游海岸压力梯度驱动的低空海岸急流伴随强烈的低空逆温，以及2)沿海岸山脉（如圣伊内斯）的浅临界层和浅Scorer参数不连续有关。这一低空沿海急流是由以下原因造成的：1)寒冷的近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向内陆延伸；2)上游极地急流条纹的右出口区域加强了沿海高地表压力（向该热梯度的北西北方向）。在较小的尺度上（中尺度），波的发展涉及到一个内部重力波（IGW）破裂，沿着海岸（圣伊内斯）山脉向西的斜坡发展，在海洋层上方触发一个短暂的旋翼。旋翼放大和断裂，将温暖和干燥的空气迅速混合到海洋层中。当寒冷的海洋层消散并被来自残余大陆边界层的相对温暖的空气所取代时，这种混合代表了快速日落变暖的第一个组成部分。第二个分量随后迅速发生，伴随IGW的拖尾暖池沿海岸山脉下降，形成中伽马尺度尾流，导致10-25°c的总变暖。本研究活动采用的方法主要包括数值模拟、观测数据分析和若干实际和理想化数据案例的理论分析。天气研究和预报（WRF）模式将用于模拟大量的日落风案例研究，范围从极端到边缘到零案例研究。这些事件将根据观测到的地面阵风和加热强度进行分类，数值模拟将涉及所有情况。水平网格的分辨率范围从区域~10 km到~666 m，在特定的日落风发生地点，这取决于地形梯度的形状和大小，从而允许模拟场的尺度收缩。这项研究活动的智力价值在于了解导致晚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，这些中尺度动力学涉及海洋层和非常精细的地形结构。特别是，更详细地了解大尺度喷射调整如何建立中尺度风和温度梯度，这些中尺度风和温度梯度可以被地形扰动，从而产生中尺度山波、孔和尾流低。此外，在加州圣巴巴拉附近的火灾易发地区，海洋层、山波和更大规模的射流是如何在晚风中相互作用的。这项研究活动的更广泛影响将涉及该地区天气预报的进步，最显著的是预测导致极端野火危险的条件。操作计算机模型无法模拟日落风的精细尺度特征，这使得美国国家气象局（NWS）、私人预报员和当地紧急救援人员更难对火灾事件发出警告/做好准备。公众在这一地区极为脆弱，从这项研究获得的知识中获得的改进预报可以通过教育预报员和应急人员了解关键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预测办公室合作完成这个项目。

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.