Variabilidade horizontal de pequena escala e a influência de obstruções no escoamento de submeso na camada limite noturna

Abstract

The forcings responsible for the spatial variability of mean and turbulent quantities in the nocturnal boundary layer are not well understood, especially when the layer is in a very stable, highly stratified state. Mixing becomes weak and the turbulent eddies are spatially controlled by local features. The variability of mean quantities such as temperature, wind speed and humidity increases and the horizontal transport by non-turbulent modes of the flow, such as submeso, becomes important. In this study, results from two field campaigns, carried out in two distinct sites, are presented. The first experiment happened in moderately complex sites, with many meteorological stations deployed over distances of tens to hundreds of meters. Results show that when the layer is very stable, the variability of mean and turbulent quantities over short distances is triggered mainly by the large-scale mean wind speed. The wind speed value that triggers this transition at a given site depends strongly on the altitude, but also on the proximity to obstructions. The second experiment presents a novel idea. Some stations have been deployed at progressively increasing distances from specific obstacles, such as trees, shrubs or buildings. The aim was to quantify how the different obstacles affect the mean wind speed. To quantify the influence of the obstructions, a transmission factor was determined. From these factors, the decomposition of turbulent kinetic energy into temporal scales through the multiresolution decomposition method allowed determining the effects of the obstacles in different scales of the motion, such as turbulence or non-turbulent low-frequency fluctuations, such as submeso. Results indicate that smaller obstructions, such as trees and small buildings affect more intensely the turbulent scales of the flow, while topographic obstacles have larger impact on longer flow scales.