Convergência vertical dos fluxos turbulentos de energia - implicação em variações de quantidades médias junto à superfície

Abstract

The present study is divided into three chapters. In the first chapter, a boundary layer method is proposed to estimate sensible and latent heat fluxes on a monthly scale, based on hourly observations of temperature and specific humidity near the surface, as well as in the vertical profiles in the morning period. It is assumed that advective effects can be neglected on this timescale so that tendecies of temperature and humidity are controlled solely by the heat flux convergence in the mixing layer. In the model, growth of the mixing layer is governed by the sensible heat flux and is opposite to the average thermal stratification observed from upper air soundings in a given month. The method is tested for three sites, in which flux estimates are compared to eddy covariance observations without the use of filters based on synoptic regimes, cloudiness or other meteorological conditions. In general, the method is shown to provide realistic monthly estimates of the turbulent fluxes which agree well with the observations. Chapter 2 addresses a necessary refinement in the flux estimation model, given that the evolution of temperature tends to diverge from the observations at 2 pm local time, which coincides with the period of maximum air temperature. The analysis of 170 AmeriFlux flux network sites reveals that the maximum temperature occurs before heat flux inversion, regardless of site characteristics. In addition, results from a Weather Research and Forecasting model numerical simulation are presented which show that the difference between the times of maximum temperature and the inversion in the sign of the sensible heat flux tends to be smaller than in the observations. Nevertheless, this difference cannot be explained by cloud cover or advection. Chapter 3 discusses a hypothesis for the observed decrease in temperature based on the vertical divergence of the sensible heat flux. At 5 sites classified into high and low towers, the existence of vertical divergence in the afternoon occurs consistently. However, it is not clear yet how flux divergence controls the maximum air the time of temperature near the surface. .