Daily evolution and seasonal variation of atmospheric turbulence in open spaces in Western Sao Paulo State, Brazil

Abstract

This paper seeks to identify a pattern of air turbulence that reflects the effect of buildings on air turbulence characteristics measured in open spaces in Western São Paulo State, Brazil. Atmospheric turbulence was estimated using high-frequency observations of the three orthogonal wind components (u, v, w). A CSAT3 sonic anemometer from Campbell Scientific Inc. (CSI) was deployed on the roof of a building, and the turbulent components (u', v', w') were systematically measured and recorded at 0.1 s and 5 min intervals, respectively, over 1 year between March 2015 and February 2016 with a CR3000 automatic data acquisition system (CSI). The data analysis took into account the diurnal and nocturnal variability of the turbulence, and the experimental results revealed the existence of a daily vertical circulation pattern. The w component varied between 15 and 45 cm.s-1 on average, and the turbulent fluctuations observed indicated that an ascending component with a speed of up to 1 m.s-1 predominated during the middle of the day and early afternoon. Throughout the night until the early hours of the morning, a less robust, subsiding component with a speed of up to 0.5 m.s-1 was observed. The mean horizontal flow (u, v) was low-speed (around 1 m.s-1) and predominantly from the southeast. There was a consistent change in the direction of this wind, which changed to easterly during the morning as the ascending branch developed. We propose a model for air circulation close to the surface in which this change in direction of the wind is the main effect of the building on the observed turbulence. The conclusion was that other asymmetric patterns observed, as valley breezes, could be understood as secondary circulations that had as backdrop the effect of the building.