Can a Spray Infused Boundary Layer Alter the Air-Sea Momentum Transfer Rates in High Winds?

最近的现场和实验室观察一致表明，在30-35米/秒的风速范围内，整体空气动力阻力系数趋于平稳。在这个阈值以上，观测到的趋势与有限数量的报告观测值存在差异。高度可重复的实验室阻力系数测量风速高达50米/秒显示不依赖于风速。相比之下，基于飞机的观测和地下速度剖面表明，阻力系数在较高的风速下减小。这两项研究都存在很大的不确定性。已经提出了降低阻力系数的潜在机制；与此相反，也有人提出，当风速大于40米/秒时，阻力系数会趋于平稳。这些理论模型将不同的喷雾对海气动量传递的影响作为预测阻力系数趋势的主要原因。这一实验成果将提供高喷雾浓度对阻力系数影响的直接测试。在一系列的实验室实验中，将在大风中向海气边界层注入一定量的喷雾。将测试范围广泛的喷雾液滴大小、浓度和流入速度。这将使我们能够完成两个主要目标，第一个是确定在大风中的阻力系数的趋势。第二项任务是阐明造成所观察到的趋势的机制。将注入足够的喷雾，形成多层流动，以测试所谓的三明治模型。通过改变入流速度和液滴大小，评估强化喷雾对表面短波的抑制效果及其对阻力系数的影响。来袭喷雾的沿风向速度将发生变化，以便与其他模型预测结果进行直接比较。海气相互作用咸水池（ASIST）设施将采用鲁棒控制体积方法，其中水面斜率用于确定总体剪切应力。即使在极端条件下（风速高达54米/秒），也可以可靠地确定阻力系数。重点将放在大风区（35-54 m/s），在此区域，先前实验室和现场观测的阻力系数随风速的变化趋势尚未建立，喷雾效应的模型也不一致。喷雾浓度、液滴大小、层厚和喷射（注入）速度对阻力系数的影响（如果有的话）将被量化。飓风强度预报技术目前缺乏改进，这表明对极端风中的海气热、水分和动量通量的认识存在根本性的缺陷。该研究有助于解决喷雾载荷对海气动量通量的影响问题。这对飓风的预期最大潜在强度具有重要意义。该项目将有助于限制预测飓风模型中使用的动量传递系数，从而有助于改进强度预报。该研究的其他影响将支持一名研究生的博士论文项目，该研究生已经在理解极端条件下的湿焓传递率方面取得了重大进展。社区外展一直是ASIST正在进行的一项重要活动。本研究中采用的批量方法在概念上很简单，可以纳入演示材料。这些资料将提供给参观设施（以及设施网站）的社区团体，以加强公众对飓风增强或衰退的基本过程的了解。

Recent field and laboratory observations have consistently shown that the bulk aerodynamic drag coefficient levels off in the range of winds from 30-35 m/s. Above this threshold there are differences in the observed trend from the limited number of reported observations. Highly repeatable laboratory drag coefficient measurements for wind speeds up to 50 m/s showed no dependence on wind speed. In contrast, aircraft-based observations and sub-surface velocity profiles indicated that the drag coefficient decreased at higher winds. There were large uncertainties inherent in both studies. Potential mechanisms for a decreasing drag coefficient have been proposed; while in contrast there also are proposed mechanisms that would lead to the drag coefficient leveling off for winds greater than 40 m/s. These theoretical models invoke different spray effects on the air-sea momentum transfer as the primary reason for the predicted drag coefficient trends.This experimental effort will provide a direct test of the effect of high spray concentrations on the drag coefficient. Controlled volumes of spray will be injected into the air-sea boundary layer in high winds in a series of laboratory experiments. A wide range of spray droplet sizes, concentrations and inflow velocities will be tested. This will enable us to accomplish two primary objectives, the first being to determine the trend of the drag coefficient in very high winds. The second will be to elucidate the mechanisms responsible for the observed trend. Sufficient spray will be injected to create a multi-layer flow to test the so-called sandwich model. By varying the inflow velocity and droplet size, surface short-wave suppression effects of enhanced spray and its corresponding effect on the drag coefficient will be evaluated. The along-wind velocity of the incoming spray will be varied to provide a direct comparison with other model predictions. A robust control volume approach will be used in the Air-Sea Interaction Saltwater Tank (ASIST) facility, wherein the water surface slope is used to determine the bulk shear stress. This will allow reliable determination of the drag coefficient, even in extreme conditions (winds up to 54 m/s). The focus will be on the high-wind regime (35-54 m/s) where the observed trend of the drag coefficient in previous lab and field observations with wind speed is not established and the models for the spray effect diverge. The effect (if any) of spray concentration, droplet size, layer thickness and ejection (infusion) velocity on the drag coefficient will be quantified. The current lack of improvement in skill in hurricane intensity forecasting demonstrates that there are fundamental weaknesses in understanding of the air-sea heat, moisture and momentum fluxes in extreme winds. This study will help to resolve the question of what effect spray loading has on the air-sea momentum flux. This has important implications for the expected maximum potential intensity of hurricanes. This project will help to constrain the momentum transfer coefficient used in predictive hurricane models, thereby contributing to improved intensity forecasts.Additional impacts of the research will be support of the PhD thesis project of a graduate student who has already made significant advances in understanding of the moist-enthalpy transfer rates in extreme conditions. Community outreach has been a significant ongoing activity at ASIST. The bulk methods employed in this research are conceptually simple to incorporate into presentation materials. These will be made available to the community groups that tour the facility (and the facility web site) to enhance public understanding of the fundamental processes related to hurricane intensification or decay.