A Micrometeorological Analysis of the Vento Norte Phenomenon in Southern Brazil

Abstract

A sonic anemometer is employed to investigate turbulent statistical characteristics and meteorological properties associated with a local windstorm known as the Vento Norte (Portuguese for north wind), which is observed in central Rio Grande do Sul state situated in southern Brazil. The Vento Norte consists of persistent and warm northerly gusts of moderate to strong magnitude that occur just downstream from a modest zonally-oriented downslope. This flow, which generates a shear-dominated atmospheric boundary layer, is investigated through analysis of turbulent properties of the velocity components and temperature measured over two years from a meteorological tower. The overall micrometeorological analysis highlights the influence of the Vento Norte phenomenon on the daily cycle of turbulent energy and fluxes and on the surface energy budget. A more detailed investigation of Vento Norte characteristics is performed by analyzing an extended Vento Norte event that lasted for approximately 35 h. In this episode, a strong increase in turbulence activity is observed on the vertical velocity component and in the absolute turbulent heat flux, as well as a reduction in the relative humidity. Further, the cospectra of the heat-flux wavelet suggest that the Vento Norte flow perturbs the boundary-layer turbulence in both daytime and night-time conditions. The surface energy budget shows a positive night-time soil heat flux and an enhanced positive latent heat flux, induced by the low relative humidity and intense turbulent activity during the Vento Norte event. The onset and demise of the Vento Norte event are characterized by sub-mesoscale activity in the velocity components and temperature field.