Turbulent Quantities Behavior During Planetary Boundary Layer Transition for a Vertical Multi-Level Micrometeorological Tower

The Planetary Boundary Layer (PBL), over land surface and under clear sky exhibits a well defined diurnal cycle. During day time, the PBL flows is dominated by turbulence. In this case, turbulent behavior is sustained by wind shear and heat flux from surface to atmosphere. On the other hand, during night time, the heat flux is inverted and becomes a sink of turbulence, that weakens and may shows a intermittent pattern. Between this completely different periods there is a transition time that occurs a couple of hours before and following sunset. This transition period is remarked by the turbulent activity decay. The main goal of this work is analyze the behavior different turbulent quantities, as turbulent kinetic energy (TKE), wind speed variances (σ_u², σ_v² and σ_w²), potential temperature variance (σ_θ²), sensible heat flux (<w′θ′>) and friction velocity (u_*), during PBL transition time. To evaluate this statistical properties of turbulent flow a dataset from a 140 m micrometeorological tower was employed. The tower is situated in Linhares, ES, Brazil (−19^∘31′53″; − 39^∘48′03″). This tower has 11 vertical levels with sonic anemometers sampling at 10 Hz (see more in ). The data was arranged in terms of astronomical sunset (reference time t = 0) due the lack of radiation measurements. The total sample is composed by 321 days between August 05, 2016 and August 31,2017, averaged on 1 min window sample. The period considered is 5 hours before and 2 hours after astronomical sunset (−300min ≤ t ≤ 120min). The final time series is the average for all days regarding temporal interval around sunset. The data analysis shows that turbulent variables related to temperature, as <w′θ′> and σ_θ² decay faster than those related only to turbulent wind field, as σ_u², σ_v², σ_w² and, consequently, TKE. Furthermore, σ_θ² revels a minimum value close <w′θ′> = 0 time (around 25min before sunset). The ratio 0.5σ_H² and TKE, where σ_H² = σ_u² + σ_v² exhibit a nearly constant answer to the turbulent decay in the levels close to the ground. Still, for higher the σ_H² becomes more important to TKE after transition.