High Wind Speed Air-Sea Gas Exchange

在以前的方案中，在京都大学的高速风浪水槽和迈阿密维持式风浪水箱中研究了高风速下的海气输送。在超过33m/S(U_10)的风速范围内，建立了一个新的区域，此时气体的传输速度比摩擦速度的平方增加得更快。可以将气泡诱导的气体转移与跨越自由表面的转移分开。在淡水中，气泡诱导的气体交换并不显著。在风速为80m/S的海水中，对于He、SF6等低溶解度气体，其传输量是自由水面传输量的1.7倍。对于CO2和DMS，即使在最高风速下，气泡效应也不占主导地位。这项建议将研究新发现的在高风速下气体传输速度急剧增加的原因。假设它要么是由与表面碎裂过程相关的强烈增强的湍流引起的，要么是显著增大的交换表面积造成的，要么是两者兼而有之。短距离风浪风洞研究在海洋条件下不具代表性。事实上，在无限大的环形风浪水箱--海德堡气旋加速器上，气泡浓度大约高出十倍。在海德堡加速器上的测量可以显著缩小线性风浪水槽和海洋条件之间的差距：a)使用时间分辨率仅为10 S的快速气体交换测量，可以通过在启动风后直接进行气体交换测量来弥合“获取差距”。波场在10min左右发展，可以覆盖到无限远的整个提取范围。B)使用SF6大气，可以缩小“波-年龄差”。在相同的风速下，摩擦速度提高了2.2倍。因此，高风速被有效地缩小了相同的系数，导致的波浪年龄是其他实验室设施的两倍以上。Aeolotron的实验还将包括测量空气侧传输速度的高度溶解气体示踪剂，以便更好地了解表面的不同部分(白帽破碎波、闭合气泡表面、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.