Determinação do saldo de radiação e radiação solar global com produtos do sensor Modis Terra e Aqua.

Abstract

The energy from the Sun is the most important component in the development of the physical processes that influence the conditions of weather and climate in the Earth. The net radiation - Rn is indispensable for assessment of evapotranspiration and is responsible for heating the soil and the lower portions of the atmosphere. The objective of this work is the determination of Rn according to a methodology proposed by Bisht et al. (2005), using satellite images. The Rn is estimated for the satellite overpass and for the daily period, under heterogeneous areas for clear sky days. The study was applied to Ceará State for five days with clear sky, using a total of 9 images (4 MODIS/Aqua and 5 MODIS/Terra). The results were validated with data collected in 9 weather stations, distributed spatially, and presented good agreement with the methodology proposed. The instantaneous solar global radiation was overestimated for the methodology with MODIS/Terra and MODIS/Aqua images, but presented precision of 83,4% and 86,9%, respectively. With relationship to the diurnal cycle of the global radiation, the values obtained for the determination coefficients by MODIS/Terra and MODIS/Aqua they were 0,931 and 0,925, respectively. The maximum values of approach in absolute percentage were 98,1% and 96,4% for MODIS/Terra and MODIS/Aqua, respectively. The instantaneous radiation balance calculated in the instant of the passage MODIS/Terra (Farm Frutacor located in Quixeré) it varied among 719 W m-² (85,7%) and 822 W m-² (99,4%), meaning differences of 96,7 W m-² and 4,1 W m-², respectively and for MODIS/Aqua it varied between 601,5 W m-² and 707,1W m-², values those that were satisfactory. The cycle of the day of the dear net radiation for the two satellites presented same correlation coefficient to 0,79 when it was compared with the observed data (without any correction of clouds), but when the data of net radiation observed schedules were corrected, for the model senoidal, the determination coefficient increased for 0,85. To the net radiation average daily the values varied among 74,5% (103,7 W m-²) and 98,8% (5,6Wm²) of precision.The applied methodology was capable to estimate the global solar radiation and the radiation balance, for satellite images, presenting acceptable instantaneous results during the drought period, in most of the stations. This study got to reduce, of five for three, the number of products MODIS that Bisht et al, (2005) they used to estimate the net radiation in surface, getting in that way, to reduce the necessary time to acquire the images and the necessary space in disk for storage of the same ones, turning most operational the process.