Radiação difusa horária na transição Cerrado-Amazônia

Abstract

Knowledge about energy levels incident on the Earth's surface is essential for a wide range of purposes in a variety of areas, such as environmental, agricultural, civil engineering and architecture, among others. The ecotone Cerrado-Amazon in the state of Mato Grosso, intensely anthropically modified, is still unknown in terms of understanding the processes of energy conversion and energy balance, making the monitoring and modeling of diffuse radiation essential in this area. In view of this, the objective of this study was to analyze and model the hourly solar diffuse radiation on horizontal in the Mato-Grossense Cerrado-Amazon transition. For that, hourly measurements were obtained from the database of the Automatic Weather Station of the Federal University of Mato Grosso, Campus Sinop (-11.864°; -55.485°; 371m), between June 2011 and October 2016. The global and diffuse radiation was monitored by pyranometers positioned on a Melo-Escobedo shadow ring. Geometric, astronomical and anisotropic corrections were applied to these data; annual and seasonal estimation equations were generated at different intervals of the atmospheric transmissivity coefficient (Kℎ𝑇). In addition, it was evaluated 18 models of estimating the diffuse fraction (𝐾𝑑ℎ) described in the literature, subdivided into 45 equations. The behavior exhibited by diffuse radiation was similar to that of the global and at the top of the atmosphere radiation, with maximum values at noon. The seasonality is striking, influencing the values of the diffuse radiation and also of its radiometric fractions. In all the seasons the sky was cloudy in the early morning, with KT reduction at the end of the day, except in the dry season, due to the low values of the solar elevation angle. The established correlation Kd x Kℎ𝑇 presented a peculiar behavior, with the curve amplitude characteristic of regions located in low latitudes (closer to the Equator). The maximum values observed for 𝐾𝑑ℎ were 0.8 to 0.9 in the range of KT less than 0.2. The generated estimation equations tended to underestimate the values of the diffuse radiation, with better performance being presented by the annual models. Among the models of estimation evaluated, those generated in this study, together with those developed for tropical regions, presented better performance than the others, which confirms the influence of latitude on energy levels of diffuse radiation.