High Wind Speed Air-Sea Gas Exchange

在之前的提案中，在京都大学的高速风浪箱和迈阿密SUSTAIN风浪箱中研究了高风速下的海气输送。在风速大于33 m/s （u_10）时，气体传递速度的增加大于摩擦速度的三次方。将气泡诱导的气体传递与自由表面上的气体传递区分开来是可能的。气泡诱导的气体交换在淡水中并不显著。在风速为80 μ m/s的海水中，对于He和SF6等溶解度较低的气体，其传递量是自由水面的1.7倍。对于二氧化碳和DMS，气泡效应即使在最高风速下也不占主导地位。该提案将研究新发现的在高风速下急剧增加的气体传递速度的原因。假设是，它要么是由与表面破碎过程相关的强烈增强的湍流引起的，要么是交换表面积的显着扩大，要么是两者的结合。短时风浪洞研究不能代表海洋条件。事实上，在无限取水的环形风浪箱——海德堡风浪箱——中，气泡的浓度大约高出十倍。这是一个迹象，表明急剧增加的高风速状态的开始可能会向下转移到大约两倍的低风速。海德堡风场的测量可以显著缩小线性风浪箱与海洋条件之间的差距：A)使用时间分辨率仅为10秒的快速气体交换测量，可以通过在打开风后直接进行气体交换测量来关闭“获取间隙”。波场在大约10分钟内形成，因此可以覆盖到无限远的整个提取范围。B)使用SF6大气，可以减小“波龄间隙”。那么在相同的风速下，可以达到2.2倍的摩擦速度。因此，由于同样的因素，高风速有效地缩小了，导致波浪年龄比其他实验室设施多一倍以上。Aeolotron中的实验还将包括高可溶性气体示踪剂，以测量空气侧传递速度，以便更好地了解表面的不同部分（白帽破碎波，封闭气泡表面，喷雾液滴和被破裂气泡和撞击液滴扰动的水面）如何促进气体交换。

In the previous proposal, air-sea gas transfer at high wind speeds was studied in the high-speed wind-wave tank of Kyoto University and the Miami SUSTAIN wind-wave tank. Beyond a wind speed of 33 m/s (u_10), a new regime is established where the gas transfer velocities increase stronger than with the friction velocity cubed. It was possible to separate bubble-induced gas transfer from transfer across the free surface. Bubble-induced gas exchange is not significant at all in fresh water. In seawater at a wind speed of 80\,m/s and for gases with low solubility such as He and SF6, it is 1.7 times higher than the transfer across the free water surface. For CO2 and DMS, the bubble effect is not dominant even at the highest wind speeds.This proposal will study the cause of the new found steeply increasing gas transfer velocities at high wind speeds. The hypothesis is that it is either caused by strongly enhanced turbulence associated with surface fragmentation processes, a significantly enlarged exchange surface area or a combination of both. Short fetch wind-wave tunnel studies are not representative for oceanic conditions. Indeed, bubble concentrations are about ten times higher at the infinite fetch annular wind-wave tank, the Heidelberg Aeolotron. This is an indication that the start of the steeply increasing high wind speed regime may be shifted down to about two times lower wind speeds.Measurements in the Heidelberg Aeolotron can narrow the gap between linear wind-wave tank and oceanic conditions significantly: A) Using fast gas exchange measurements with a temporal resolution of only 10 s, the ``fetch gap'' can be closed by performing gas exchange measurements directly after turning on the wind. The wave field develops in about 10 min, so that the whole fetch range up to infinity can be covered. B) Using an SF6 atmosphere, the ``wave-age gap'' can be reduced. Then 2.2 times higher friction velocities are achieved at the same wind speed. Thus, high wind speeds are effectively scaled down by the same factor, resulting in more than twice the wave age than in other laboratory facilities. The experiments in the Aeolotron will also include highly soluble gas tracers to measure the air-side transfer velocities in order to better understand how different parts of the surface (breaking waves with white caps, closed bubble surfaces, spray droplets and water surface disturbed by bursting bubbles and impacting droplets) contribute to gas exchange.