An Analysis of Low-Frequency Maritime Atmospheric Turbulence

Abstract

New data are presented for the spectrum of turbulent wind energy under maritime conditions in the frequency region 1.0?0.03 mHz. The corresponding measurements were made at five levels on a mast 46 m high on a small islet off the coast of central Norway. Twelve time series of length 10 h 40 min have been analyzed. The mean wind speeds of the series are in the range 11?19 m s?1, and the wind directions are westerly, which have maritime conditions upwind. Four of the time series were characterized by unstable atmospheric conditions (mean lapse rate ??T/?z in the range 12?20 K/km) and show little or no indication of a spectral gap for heights above 40 m. Four other lime series with stable to neutral conditions (mean lapse rate 3?7 K/km) do show a gap in the wind speed spectra around 0.5 mHz, in agreement with the Kansas (1972) and Minnesota (1978) experiments. The remaining series, with lapse rates fluctuating around the neutral value of 9.8 K/km, show intermediate behavior. The temperature spectra at 45-m height do not show a gap even for stable to neutral conditions.