[Comparative observation of diffuse radiation in Qianyanzhou during the spring of 2012].

Global radiation and diffuse radiation were measured from March to June of 2012 in Qianyanzhou Experimental Station of Red Soil and Hilly Land, Chinese Academy of Sciences by ising three types of pyranometers, including CMP11 attached with a shadow ring, SPN1 and RSR3, which were placed in parallel. The observations showed that both global radiation and diffuse radiation measured by these pyranometers had a good linear correlation. The global radiation measured by SPN1 and RSR3 was respectively 3. 0% and 20.5% lower than that measured by the CMP 11. The diffuse radiation measured by SPN1 and RSR3. was respectively 5.5% and 7.9% lower of than that measured by the CMP11. Under the sunny, cloudy and overcast conditions, the daily variations of diffuse radiation measured by the three pyranometers were similar, and hence, the diffuse radiation values at a specific solar elevation angle measured by the different pyranometers were also similar. There was difference in daily accumulative diffuse radiation measured by the different pyranometers. Monthly diffuse fractions of March, April and May were 0.56, 0.59 and 0.70, respectively. In the subtropical area of southern China, the diffuse radiation accounted for a relatively large proportion of the global solar radiation and varied considerably over time. It is necessary to conduct long-term continuous measurements to capture the variability of diffuse radiation over different underlying surfaces.