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.
The goal of this project was to determine the effect that sea spray has on the bulk momentum transfer coefficient between the air and water in very high winds. To achieve this goal a series of comprehensive laboratory experiments was designed to isolate and quantify the spray effects on the momentum transfer. To successfully conduct these experiments we implemented and refined the necessary observational technologies and analysis approaches in year two and in the no-cost extension year three of the project. We have tested new approaches for defining spray concentration profiles and have conducted observations with existing technology to better define the size distributions of the spray. We have expanded these observations to very small particles and have observed a well defined size spectrum. Through these observations conducted in collaboration with Chris Fairall at NOAA and Ivan Savelyev at NRL we have quantified the spray concentrations due to wind-wave interactions in very high winds for sizes ranging from 5x10-3 µm to ~600 µm. We have made observations of the impact of these spray concentrations on the drag coefficient and have studied their effects on the drag coefficient. Measurements of the drag coefficient behavior in varying wind and sea spray concentrations have been conducted. It was determined that the most reliable way to measure the mean laboratory surface slope (required to extract drag in high winds) was through the use of pressure sensors in both the air and water. However these measurements exhibited some drift in their baseline output voltages that has required extensive post-calibration. We are proceeding with this effort and are preparing a manuscript reporting the final outcome of the study. As we have proceeded toward that goal, three manuscripts have already been published that include spray measurements collected through this project. Collectively these works (Toffoli et al., 2011, Savelyev et al., 2011, Jeong et al., 2012) demonstrate the importance of understanding spray effects at the air-sea interface in high winds.This work will likely result in improved treatment of spray in hurricane models. We are already collaborating with researchers interested in incorporating new spray formulations in coupled wind-wave models. These highly resolved coupled models (e.g. Chen et al. , 2007) offer great promise for improving hurricane intensity forecasting, which has remained a significant challenge.
