Estudo de Longo Prazo da Estimativa da Altura da Altura da Camada Limite Atmosférica Acima da Amazônia Central

Abstract

The height (zi) of the Atmospheric Boundary Layer (ABL) is an essential parameter for ap plications such as weather and climate forecasting, pollutant dispersion, and air quality. This study presents a detailed analysis of the variability of the daytime and nighttime boundary layer above an undisturbed tropical forest. Data were collected from the Amazon Tall Tower Obser vatory (ATTO) site in the central Amazon rainforest. Two datasets were used: a ceilometer to investigate the daytime phase of the ABL, known as the convective boundary layer (CBL), and a profile of 10 three-dimensional sonic anemometers, installed at multiple levels on the ATTO towers, to analyze the nighttime phase of the ABL, known as the nocturnal boundary layer (NBL). With the backscatter profile provided by the ceilometer, it was possible to identify the height where a strong gradient in the backscatter signal occurs, which was considered the top of the CBL. However, at night, the ceilometer is not suitable for estimating the NBL because, in the absence of convection, the ABL height is quite shallow, only a few hundred meters. The refore, for the nighttime period, the turbulent sensible heat flux profile obtained from the sonic anemometers was used. The NBL top was considered to be the height where turbulent fluxes f luctuate around zero. During the daytime phase, the analysis of six years of ceilometer data revealed that the maximum depth of the CBL ( 1400 m) occurs in the afternoon, around 15:00 local time, being higher in the dry season ( 1741 m) and during El Niño years, and lower in the rainy season ( 1263 m) and in LaNiñayears. At night, the NBL exhibited significant interannual and seasonal variations. Higher heights were observed during the rainy season in a La Niña year ( 181.9 m), while the lowest values occurred in the dry season of an El Niño year ( 133.3 m). The main factors influencing this variability were turbulence intensity, radiation balance, and atmospheric stability. The morning growth of the ABL was more intense during the dry period, influenced by higher solar radiation, lower cloud cover, and more favorable initial conditions for this growth. Additionally, a simple model was developed to estimate the maximum height of the ABL based on the morning growth rate. The model showed good agreement with the observed data, providing a practical tool for estimating ABL height in locations without spe cific instrumentation. The results presented here are the first to show a long-term assessment of the CBL and NBL height in the Amazon, as previous studies have only explored short-term experimental campaigns lasting a few days. We believe that these findings not only provide unprecedented information on NBL variability in the Amazon but also contribute to improving the parameterizations of regional and global atmospheric models, enhancing the understanding of atmospheric processes in tropical ecosystems.