Influence of swell on the wind gradient in the marine atmospheric boundary layer

Wind profiles over sea have been analyzed. Measurements of the wind profiles and turbulence were used from the site Östergarnsholm in the Baltic Sea, together with directional wave buoy data outside the island in the Baltic Sea. It’s found that there are generally three types of wind profiles over the sea. Most commonly the wind profile is modified by the swell energy close to the surface. The effect of swell causes a decreased wind gradient. Consequently the roughness length (z0) will become very small.

During a two-day case study, it’s found that the normalized wind speed gradient (*) in the layer close to the surface decreases continuously with the increasing of wave age (c0/Uc) even when c0/Uc<1.2. It’s believed that it is the energy of swell (which also exists during growing sea) that influences the wind profile. A parameter E1/E (**)is derived from the wave spectrum to represent the effect of swell.

* The normalized wind speed gradient in the layer close to the surface: (U12 – U7 ) /U, where U=½(U12+U7), U12, U7 are the wind speeds at 12 m and 7 m respectively.

** E1/E, where E1 is the swell portion of the wave spectrum from 0 up to the frequency n_SWELL, which is the frequency where the wave speed reaches the wind speed, E is the total energy of the wave spectrum.