Author(s): Hiroki Okachi; Tomohito J. Yamada; Yasunori Watanabe; Junichi Ohtsuka
Linked Author(s):
Keywords: Sea spray; Raindrop; Marine observation; Wind tunnel experiment; Momentum exchange
Abstract: Various fluxes are always exchanged at the air-sea surface. Considering flux exchanges in furious storm condition, there are much rain droplets and sea sprays generated from wave breaks. Sea sprays are instantly accelerated by local wind as they are generated. They have substantial mass. Therefore, momentum is conveyed by these droplets from the atmosphere to the ocean. Focusing on the raindrops, they have horizontal speed more than wind speed when reaching the near-sea surface. That is, raindrops accelerate horizontal wind speed, however, there is no confirmation. Additionally, sea spray transfers latent heat extracted from the atmosphere in the process of its phase change. In this research, parameterization of momentum exchange at the air-sea surface is reconsidered concerning about drag coefficient. The drag coefficient increases as wind speed increases. We propose an equation which includes the effect of raindrops on the equation of drag coefficient that takes into account of sea spray and the characteristics of momentum exchange coefficient related to rainfall intensity and wind velocity. We also perform marine observation from 2013 to 2016 at an observational tower in Wakayama-prefecture, Japan, where typhoon often hits. Several typhoons passed near the tower for four years. We further conduct wind tunnel experiments to directly observe sea sprays (diameter > 72 um) for six high wind speed conditions (U10=23~28m/s).
Year: 2018