Abstract
Models of the diffuse fraction of daily solar radiation (KD) have been developed over India for different seasons (winter, spring/autumn, and summer/monsoon) using the clearness index, also known as atmospheric transmissivity (KT). Measurements of global (RG) and diffuse (RD) solar radiation made at four stations in different climates (Jodhpur—arid, New Delhi—semi-arid, Nagpur—subhumid, and Kolkata—humid) have been used to develop two types of empirical models. The “regional” models cover all climate zones and “local” models are climate specific. On an average, regional models perform better than local models over Indian subtropical regions. An exception is the arid climatic regions where local models exhibits the lowest Akaike's information criterion (AIC) and higher coefficient of determination, R2 (0.8–0.9). The applicability of regional models has been tested over an additional set of 16 stations well distributed in India. In winter, the majority of these stations exhibit R2 > 0.8. In spring/autumn, R2 was highest for Ranchi (R2 = 0.92). In summer/monsoon, 14 stations out of 16 have R2 > 0.8. The regional models perform well in all the seasons over the stations of India except Shillong. In Shillong for winter, spring/autumn, and summer/monsoon bias (R2) are 0.182 (0.06), 0.048 (0.36), and −0.05 (0.30), respectively. It is concluded that regional models of the KD can be applied for all seasons to the Indian subtropics except in higher latitude and mountain areas where errors are high.
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