Analysis of Long Time Series of Coastal Wind

Tore Heggem Department of Physics, Norwegian University of Science and Technology, Trondheim, Norway

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Rune Lende Department of Physics, Norwegian University of Science and Technology, Trondheim, Norway

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Jørgen Løvseth Department of Physics, Norwegian University of Science and Technology, Trondheim, Norway

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Abstract

A study of a 14-yr time series of wind speed recorded on the coast outside the city of Trondheim in middle Norway is presented. Analysis of the time series shows that in this area there is, in general, no gap in the wind speed power spectrum in the 1 h−1 region. For heights below 20–40 m, dependent on the exact distance from the shoreline, there is a dip in the spectrum around 2 mHz; for higher elevations, there is a monotonical rise in the spectral energy for decreasing frequency f until the weather peak. Far below the weather peak, the spectrum decreases as f.

Analyses of quasi-stationary 12-h time series indicate that in addition to weather (synoptic) variations and the turbulence due to ground friction peaking in the 0.01-Hz region, there are other mechanisms feeding energy into the spectrum in the mesoscalar (1 h) region, where the spectral gap conventionally is assumed to be found. As an example, a selected time series is analyzed that shows a pronounced peak in the spectrum at 0.4 mHz. Time series with a periodic structure are predominantly found for unstable atmospheric conditions over the sea, with air temperature 4 K or more below the sea temperature.

It is shown that unstable atmospheric conditions in the surface layer over the sea will quickly turn into stable to neutral conditions over land, thus the local lapse rate is not necessarily a good indicator for conditions causing mesoscale fluctuations.

Aliasing effects and sensor response are also studied.

Corresponding author address: Jørgen Løvseth, Dept. of Physics—Lade, NTNU, N-7034 Trondheim, Norway.

Email: jorgen.lovseth@phys.ntnu.no

Abstract

A study of a 14-yr time series of wind speed recorded on the coast outside the city of Trondheim in middle Norway is presented. Analysis of the time series shows that in this area there is, in general, no gap in the wind speed power spectrum in the 1 h−1 region. For heights below 20–40 m, dependent on the exact distance from the shoreline, there is a dip in the spectrum around 2 mHz; for higher elevations, there is a monotonical rise in the spectral energy for decreasing frequency f until the weather peak. Far below the weather peak, the spectrum decreases as f.

Analyses of quasi-stationary 12-h time series indicate that in addition to weather (synoptic) variations and the turbulence due to ground friction peaking in the 0.01-Hz region, there are other mechanisms feeding energy into the spectrum in the mesoscalar (1 h) region, where the spectral gap conventionally is assumed to be found. As an example, a selected time series is analyzed that shows a pronounced peak in the spectrum at 0.4 mHz. Time series with a periodic structure are predominantly found for unstable atmospheric conditions over the sea, with air temperature 4 K or more below the sea temperature.

It is shown that unstable atmospheric conditions in the surface layer over the sea will quickly turn into stable to neutral conditions over land, thus the local lapse rate is not necessarily a good indicator for conditions causing mesoscale fluctuations.

Aliasing effects and sensor response are also studied.

Corresponding author address: Jørgen Løvseth, Dept. of Physics—Lade, NTNU, N-7034 Trondheim, Norway.

Email: jorgen.lovseth@phys.ntnu.no

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