So-Kumneth Sim | Searcharxiv

arXiv2024

Distributions and correlation properties of offshore wind speeds and wind speed increments

We determine distributions and correlation properties of offshore wind speeds and wind speed increments by analyzing wind data sampled with a resolution of one second for 20 months at different heights above sea level in the North Sea. Distributions of horizontal wind speeds can be fitted to Weibull distributions with shape and scale parameters varying weakly with the vertical height separation. Kullback-Leibler divergences between distributions at different heights change with the squared logarithm of the height ratio. Cross-correlations between time derivatives of wind speeds are long-term anticorrelated, and the even parts of their correlation functions satisfy sum rules. Distributions of horizontal wind speed increments change from a tent-like shape to a Gaussian with rising increment lag. A surprising peak occurs in the left tail of the increment distributions for lags in a range $10-200\,{\rm km}$ after applying the Taylor's hypothesis locally to transform time lags into distances. The peak is decisive in order to obtain an expected and observed linear scaling of third-order structure functions with distance. This suggests that it is an intrinsic feature of atmospheric turbulence.

physics.ao-ph↗

arXiv2022

Signatures of geostrophic turbulence in power spectra and third-order-structure function of offshore wind speed fluctuation

We analyze offshore wind speeds with a time resolution of one second over a long period of 20 months for different heights above the sea level. Energy spectra extending over more than seven decades give a comprehensive picture of wind fluctuations, including intermittency effects at small length scales and synoptic weather phenomena at large scales. The spectra $S(f)$ show a scaling behavior consistent with three-dimensional turbulence at high frequencies $f$, followed by a regime at lower frequencies, where $fS(f)$ varies weakly. Lowering the frequency below a crossover frequency $f_{\rm\scriptscriptstyle 2D}$, a rapid rise of $fS(f)$ occurs. An analysis of the third-order structure function $D_3(\tau)$ of wind speed differences for a given time lag $\tau$ shows a rapid change from negative to positive values of $D_3(\tau)$ at $\tau\simeq 1/f_{\rm\scriptscriptstyle 2D}$. Remarkably, after applying Taylor's hypothesis locally, we find the third-order structure function to exhibit a behavior very similar to that obtained previously from aircraft measurements at much higher altitudes in the atmosphere. In particular, the third-order structure function grows linearly with the separation distance for negative $D_3$, and with the third power for positive $D_3$. This allows us to estimate energy and enstrophy dissipation rates for offshore wind. The crossover from negative to positive values occurs at about the same separation distance of 400 km as found from the aircraft measurements, suggesting that this length is independent of the altitude in the atmosphere.

physics.flu-dyn↗