Temperature auto‐correlation and spectra functions in low‐wind meandering conditions

In quiet, clear sky conditions at night a remarkable feature may occur with respect to the wind near the surface. Like river flows, the wind may spontaneously start to meander without need for a specific trigger to initiate such motion. In addition to this regular and ‘slow’ motion, air parcels also experience chaotic motion due to weak background turbulence. Besides wind, temperature is also affected. The slow and regular response of temperature can be separated from its fast chaotic response, by plotting the ‘temperature variance’ as a function of frequency (black line, see Figure). At low frequencies a peak is found, as a signature of the aforementioned meandering motion. The red curve represents a theoretical fit. The gray line represents corresponding fluctuations of vertical wind velocities. The observational data consist of high‐frequency temperature and wind measurements from a sonic anemometer operated at 5m above the surface near the city of Turin, Italy. Eulerian low‐wind temperature statistics are investigated through the analysis of sonic anemometer observations gathered in two experimental campaigns, the Urban Turbulent Project in Northern Italy and the Large‐Scale Biosphere‐Atmosphere Experiment in Amazonia (Brazil). The observed auto‐correlations and spectra functions are tested with the theoretical relationships previously proposed for the horizontal velocity components in low wind speed conditions. The comparison showed that the temperature field, similarly to the horizontal velocity field, presents a characteristic oscillatory behaviour with a distinct isolated spectrum peak frequency due to the wind meandering. The ratio between this frequency and the one associated to the horizontal velocity components is close to one. This, together with the similarity between the temperature and velocity spectra and auto‐correlation functions, suggests that a dynamical link between temperature and velocity oscillations exists.