Wind Turbulence over the Sea Surface Measured at the Tsuyazaki Station. : I. Method of Measurement and Preliminary Analysis

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  • 津屋崎沖観測塔で観測した海上風乱流 : I.計測法と予備解析
  • 津屋崎沖観測塔で観測した海上風乱流(1)計測法と予備解析
  • ツヤザキオキ カンソクトウ デ カンソク シタ カイジョウフウ ランリュウ 1 ケイソクホウ ト ヨビ カイセキ

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Abstract

As regards the wave dependence of the roughness length, turbulence of the wind above the sea surface was measured at a station 2 km off Tsuyazaki, Fukuoka, Japan, in the autumn of 2003. The measurement system was designed to provide independent estimates of the friction velocity u_* and the surface roughness z_0 based on different principles : (a) the profile method, (b) the eddy-correlation method, and (c) the inertial dissipation method. For this purpose a vertical array of anemometers were fixed to the station for the simultaneous measurement of the wind turbulence and mean wind profiles. The array consists of four cup-type anemometers and three ultrasonic anemometers in between. The data analyzed were (1) mean wind profiles over the sea surface, (2) Reynolds stress, (3) spectral level of wind turbulence in the inertial subrange of wind turbulence, and (4) wind-waves measured by a wave-gauge of capacitance type. The measurement system worked well. However, most data obtained were not so good in quality primarily due to the unfavorable wind direction and low wind speeds in autumn, though the stratification was neutral throughout the period of experiment. Here only a preliminary analysis was presented for data of a good quality. For those data all the methods yielded a good agreement, though the eddy-correlation and profile methods were liable to give a larger scatter. The drag coefficient C_d against U_<10> was found in a reasonable range of 1 × 10^<-3> ～ 2 × 10^<-3>. Most data of the nondimensional roughness were scattered in the range of 10^<-3> ～ 10^<-1> against the wave-wind parameter (inverse of the wave age) u^^～ ≡ ω_pυ_*/9 = 2 × 10^<-2> ～ 7 × 10^<-2>, where ω_p denotes the peak frequency of wind-wave spectra and g the gravitational acceleration. The nondimensional roughness in the central part of the scattered data was about 3 × 10^<-2>, which was a little larger than a standard value of the Charnock constant 1.8 × 10^<-2>. The nondimensional roughness z^^～_0 ≡ gz_0/u^2_* was observed to increase with the wave-wind parameter u^^～ like z^^～_0 ∝ u^^～ in accordance with other field observations. As a whole the present results are close to those of the HEXOS experiment. Though more careful analysis and further experiment are needed, the present results suggest that the nondimensional roughness is not so large or small as has often been reported from the observation in the sea, and that it is somewhat larger than the Charnock constant.

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