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直表明，总体气动阻力系数在30-35m/S的风速范围内趋于稳定。在这一阈值以上，观测到的趋势与报告的有限数量的观测结果存在差异。风速高达50米/S的高重复性实验室阻力系数测量表明，风速不依赖于风速。相比之下，基于飞机的观测和次表层速度分布表明，在更高的风速下，阻力系数降低。这两项研究都存在很大的不确定性。已经提出了减小阻力系数的可能机制，同时也提出了在大于40m/S的风速下使阻力系数趋于平稳的机制。这些理论模型引用了不同的喷雾效应作为预测阻力系数趋势的主要原因。这一实验工作将为高喷雾浓度对阻力系数的影响提供直接的测试。在一系列实验室实验中，将在大风中将受控数量的喷雾注入海-气边界层。将测试各种喷雾液滴的大小、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大的风中阻力系数的趋势。第二个将是阐明导致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，以测试所谓的三明治模型。通过改变入口速度和液滴尺寸，可以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。进入的喷雾的顺风速度将发生变化，以提供与其他模型预测的直接比较。海-气相互作用咸水池(ASIST)设施将使用一种稳健的控制体积方法，其中水面斜率用于确定整体剪应力。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米/S)。重点将放在大风区域(35-54m/S)，该区域的阻力系数随风速的变化趋势没有建立在实验室和现场观测的基础上，喷雾效应的模型也有分歧。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。目前在飓风强度预报技术方面缺乏改进，这表明在理解极端风中的海-气热量、水汽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载荷对海气动量通量有何影响的问题。这对飓风的预期最大潜在强度具有重要影响。这个项目将有助于限制用于飓风预测模型的动量传递系数，从而有助于改进强度预测。研究的其他影响将支持一名研究生的博士论文项目，他已经在理解极端条件下的湿热传递速率方面取得了重大进展。社区外展一直是本协会持续开展的一项重要活动。这项研究中使用的批量方法在概念上很简单，可以融入到演示材料中。将向参观该设施(以及该设施的网站)的社区团体提供这些资料，以加强公众对与飓风加剧或衰减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.