14.5a
Height Variation of Turbulent Parameters in the Marine Atmospheric Boundary Layer due to Wave Influence (Formerly paper number P5.16)
Anna Sjöblom, Uppsala University, Uppsala, Sweden; and A. S. Smedman
Long term tower measurements at three heights over the sea have been used to investigate the structure of the marine atmospheric boundary layer and the validity of Monin-Obukov similarity theory.
The measurements were performed at Östergarnsholm, a small island situated east of Gotland, Sweden, in the middle of the Baltic Sea. The flux footprint analysis and the sea floor slope confirms that Östergarnsholm represent open sea conditions for most of the time. Turbulence instruments are placed at about 10, 18 and 26 meters on the tower, and slow response instruments at five heights. A wave-rider buoy is also deployed in the flux footprint area.
Only measurements with an undisturbed over water fetch were used, leaving more than 1300 30 minutes averages, where more than 700 also contains wave information. The results show that turbulent parameters have a strong height dependency due to wave influence, and the validity of Monin-Obukov similarity theory can be questioned during mature sea and swell. The structure of the wind profile, and thus the normalised wind gradient in the lower parts of the boundary layer can be divided into three height layers, whose depth is a strong function of the wave age: a wave influenced layer close to the surface, a transition layer and an undisturbed constant-flux layer. In the transition layer, the normalised wind gradient becomes large, and during neutral conditions it is about 1.5 rather than 1.0, which is commonly assumed. But, the inertial subrange scale with height for neutral conditions, at least for growing and mature sea, so the normalised dissipation does not respond in the same way as the normalised wind gradient.
This will have consequences for the inertial dissipation method, which builds on a correct parameterisation of the turbulent kinetic energy budget. It is not possible to measure at just one height and then assume height constancy. The parameterisations used have to be adjusted to fit the specific layer in which the measurements take place. To conclude, the height of the measurements has to be considered during measurements in the atmospheric boundary layer over sea.
Session 14, The Marine BL
Friday, 19 July 2002, 8:30 AM-9:58 AM
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