6A.6
The turbulent kinetic energy budget over sea
Anna Sjöblom, Uppsala University, Uppsala, Sweden; and A. S. Smedman
The turbulent kinetic energy budget over sea was investigated in this study, using long term measurements.
Östergarnsholm (57°26' N, 18°54' E) is a small island situated east of Gotland, Sweden, in the middle of the Baltic Sea. The island is very flat, and has a sparse vegetation almost without any trees. On the southern most tip, semi-continuous measurement have been performed on a 30 meter tower since May 1995. The slope of the sea floor outside Östergarnsholm permits an undisturbed wave field up to at least 20 ms-1. Slow response instruments are placed at five heights on the tower at Östergarnsholm, with measurements of wind speed, wind direction and temperature. Turbulence instruments are placed at 8, 16 and 24 meters. In addition to these instruments, a wave-rider buoy is deployed 5 km south-east of Östergarnsholm, measuring sea surface (bucket) temperature, significant wave height, wave direction and the spectra of the wave field.
Shear production, buoyancy and the turbulent transport term was determined directly from the measurements, and the dissipation was computed from the inertial subrange of velocity spectra. The pressure transport term was determined as a residual. Only measurements with an undisturbed over water fetch were used, leaving about 800 30 minutes averages, where about 500 also contains wave information. The data set has been investigated according to stability, wave age and wind speed, at all three levels of turbulent measurements.
The turbulent transport is close to zero for all stabilities encountered here, and the pressure transport is increasing with -z/L. This leads to an imbalance between production and dissipation with increasing -z/L; dissipation exceeding production. Significant pressure transport also occurs for swell and wind speeds below 6 ms-1, while the turbulent transport is still small, so dissipation is exceeding production. During growing sea and high wind conditions the measurements indicate that production exceeds dissipation, especially the measurements at 8 meters.
The results indicate that the commonly used balance between production and dissipation is questionable, especially in swell conditions.
Session 6A, Marine and Oceanic Bls
Friday, 11 August 2000, 10:15 AM-4:45 PM
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