When the wind speed decreases below a certain value (1-2 m s-1) meandering (low frequency horizontal wind oscillations) starts to prevail. In these conditions it becomes difficult to define a precise mean wind direction and to estimate the airborne dispersion. To study the wind and turbulence characteristics during meandering, two sonic anemometer datasets, containing hourly wind observations, were analysed: the first one, lasting one year, was recorded in complex terrain (Graz, Austria) and the second one, lasting about one month, was recorded in a rather flat area (at Tisby site, Sweden). It was obtained that meandering seems to exist under all meteorological conditions regardless the stability or wind speed and that it was confirmed that meandering sets a lower limit for the horizontal wind component variances. Further, it was found that the autocorrelation functions of the horizontal wind components, computed for the low wind cases, show an oscillating behaviour with the presence of large negative lobes. Two different relationships found in literature considering these oscillatory aspects, were fitted to the data. They contain two parameters: one, which can be associated with the classical integral time scale and the second one with meandering occurrence. Based on these relationships, expressions for the mean square displacement of particles were also derived.
An analysis of sonic anemometer observations in low wind speed conditions
2005
Abstract
When the wind speed decreases below a certain value (1-2 m s-1) meandering (low frequency horizontal wind oscillations) starts to prevail. In these conditions it becomes difficult to define a precise mean wind direction and to estimate the airborne dispersion. To study the wind and turbulence characteristics during meandering, two sonic anemometer datasets, containing hourly wind observations, were analysed: the first one, lasting one year, was recorded in complex terrain (Graz, Austria) and the second one, lasting about one month, was recorded in a rather flat area (at Tisby site, Sweden). It was obtained that meandering seems to exist under all meteorological conditions regardless the stability or wind speed and that it was confirmed that meandering sets a lower limit for the horizontal wind component variances. Further, it was found that the autocorrelation functions of the horizontal wind components, computed for the low wind cases, show an oscillating behaviour with the presence of large negative lobes. Two different relationships found in literature considering these oscillatory aspects, were fitted to the data. They contain two parameters: one, which can be associated with the classical integral time scale and the second one with meandering occurrence. Based on these relationships, expressions for the mean square displacement of particles were also derived.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
