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测一致表明，在风速为30-35 m/s的范围内，整体气动阻力系数趋于稳定。 超过这一阈值，观察到的趋势与有限数量的报告观察结果存在差异。风速高达50 m/s时的高重复性实验室阻力系数测量结果表明，风速不依赖于阻力系数。相比之下，基于飞机的观测和次表层速度剖面表明，阻力系数在较高的风速下下降。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;而相比之下，也提出了导致阻力系数在风速大于40 m/s时趋于平稳的机制。这些理论模型援引不同的喷雾对气-海动量传递的影响作为预测阻力系数趋势的主要原因，这项实验工作将提供高喷雾浓度对阻力系数影响的直接测试。在一系列实验室实验中，将在大风中向海气边界层注入受控量的喷雾。将测试各种喷雾液滴尺寸、浓度和流入速度。这将使我们能够实现两个主要目标，第一个是确定在非常高的风的阻力系数的趋势。第二个问题是阐明造成所观察到的趋势的机制。将注入足够的喷雾以产生多层流动来测试所谓的夹层模型。通过改变来流速度和液滴尺寸，将评估增强喷雾的表面短波抑制效果及其对阻力系数的相应影响。将改变来流喷雾的顺风速度，以提供与其他模型预测的直接比较。一个强大的控制体积的方法将被用于在海气相互作用盐水罐（ASIST）设施，其中水面斜率被用来确定整体剪切应力。这将允许可靠地确定阻力系数，即使在极端条件下（风速高达54 m/s）。重点将放在强风状态（35-54 m/s），在此情况下，未确定先前实验室和现场观察中观察到的阻力系数随风速的趋势，并且喷雾效应模型存在分歧。喷雾浓度、液滴尺寸、层厚度和喷射（注入）速度对阻力系数的影响（如果有的话）将被量化。目前在飓风强度预测方面缺乏改进的技能表明，在了解极端风中的海气热量、水分和动量通量方面存在根本性的弱点。这项研究将有助于解决喷雾负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影响。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，该研究生在理解极端条件下的焓-焓传递率方面已经取得了重大进展。社区外展一直是ASIST正在进行的一项重要活动。在这项研究中采用的批量方法在概念上很容易纳入演示材料。这些资料将提供给参观该设施（和该设施网站）的社区团体，以提高公众对与飓风增强或减弱有关的基本过程的了解。

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.