Publicações

Model GL version 1.2 runs operationally at CPTEC/INPE assessing daily solar radiation from GOES 10 VIS imagery. Besides basic hypotheses about atmospheric variables, the fundamental local variable is reflectance at a pixel observed by satellite (Ceballos et al. 2004). Daily irradiation is obtained by time integration of local irradiance, therefore a geostationary satellite is the best instrument for this task. Northeastern Brazil is equally observed by GOES 10 and MSG satellites, with similar (although opposite) viewing angles. This fact suggests that both satellites could assess solar radiation in this region with similar performance. This paper presents results of a comparison between those estimates. It is shown that GOES and MSG provide similar daily cycle of solar irradiance for several regional sites, compatible with pyranometer measurements. Both satellites provide close results for instantaneous irradiance in nearly cloudless sites. In cloudy conditions, absolute differences can be higher than 100 W/m**2 (about 10-20% or even more of assessed irradiance); this makes evident how delicate can be comparisons over small areas. On the other hand, daily totals (mean irradiance over 24 hours) are quite similar, with standard deviation of about 20 W/m**2 related to ground truth and absolute mean deviation of about 10-15 W/m**2 (5% or less than daily value). However, MSG results tend to be higher than GOES (it is suggested a systematic difference of about +20 W/m**2). The somewhat better fitting of MSG results suggest to improve reflectance assessed for GOES 10 VIS images, which need corrections due to VIS sensor degradation. Nevertheless, the overall results make evident the confidence for using GL model and MSG VIS imagery, in order to extend solar radiation assessments to Atlantic Ocean, poorly covered by GOES images.