Calculating Friction Velocity From Data By A Ship-borne Supersonic Anemometer

As well known, the friction velocity U* is closely related with the definitions of air-sea momentum, heat and gas fluxes (e.g. CO2 and vapor fluxes), so its measurement is the base of investigating air-sea fluxes. After reviewing the history and the present status of the studies in U* and air-sea interaction, we report a recently developed method to measure friction velocity from data by a ship-borne supersonic anemometer. The crucial point of the method is that with a suitably chosen frequency as the lower limit the signals measured by the ship-borne anemometer are high-passed by use of a fast filtering method (Xu, 2004) and then the high-passed signals are used as u ' (t) and w' (t) to calculate U*. Also reported in this paper are the results of U* calculated by the present method from the data measured by a supersonic anemometer installed at the foremast of"Dong Fang Hong-2", during 2005 Yellow Sea and South China Sea Cruise of the SOLAS. These results are consistent well with the results given by the empirical formula of SethuRamon & Rayner(1975) and Garratt (1977), as well as the relationship of CD VS U10 ,when U* is transferred into CD, from early articles by various authors, showing the feasibility of the present method. We also discussed how to choose the frequency limit of the higher and lower modes, and tested it by in-situ data.