Standard Deviations of Wind Speed and Direction from Observations over a Smooth Surface

Sylvain M. Joffre Finnish Meteorological Institute, Air Quality Department, 0810 Helsinki, Finland

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Tuomas Laurila Finnish Meteorological Institute, Air Quality Department, 0810 Helsinki, Finland

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Abstract

Measurements of wind speed and direction made every minute on a 15 m mast sited on a large expanse of sea ice were analyzed to study the behavior of their standard deviation. The large scatter normally observed under low wind speed and/or non-neutral conditions was related to autocorrelations at 1 min. It appeared that the wind speed was the main factor affecting the variability of the standard deviations; stability had much less effect. The normalized total wind speed (at z = 10 m) standard deviation σV/V is well approximated by 0.4/V m s−1 for lower wind speeds (V ≤ 5 m s−1) and by σV/V = 0.08 when V > 5 m s−1. The wind direction standard deviation σα follows a similar law with σα = 0.32/V m s−1 for V ≤ 5 m s−1 and σα = 0.065 for higher winds. It is also shown that vertical velocity standards deviations from a variety of experiments can be related to simultaneous horizontal velocity measurements.

Abstract

Measurements of wind speed and direction made every minute on a 15 m mast sited on a large expanse of sea ice were analyzed to study the behavior of their standard deviation. The large scatter normally observed under low wind speed and/or non-neutral conditions was related to autocorrelations at 1 min. It appeared that the wind speed was the main factor affecting the variability of the standard deviations; stability had much less effect. The normalized total wind speed (at z = 10 m) standard deviation σV/V is well approximated by 0.4/V m s−1 for lower wind speeds (V ≤ 5 m s−1) and by σV/V = 0.08 when V > 5 m s−1. The wind direction standard deviation σα follows a similar law with σα = 0.32/V m s−1 for V ≤ 5 m s−1 and σα = 0.065 for higher winds. It is also shown that vertical velocity standards deviations from a variety of experiments can be related to simultaneous horizontal velocity measurements.

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