Solar Radiation over Four Cities of India: Trend analysis using Mann-Kendall Statistical Test

Abstract

The present work deals with the study of the solar radiation trend over four major cities of India. The four cities Jodhpur, New Delhi, Nagpur, and Kolkata, represent different climate types: arid, semi-arid, sub-humid and humid, respectively. For trend analysis, we have used over forty years (1957/60-2003) global solar radiation (R_G) and thirty-one years (1973-2003) diffuse solar radiation (R_D) observational data. Based on the Mann-Kendall test (MKT) a significant declining trend in R_G (solar dimming) has been observed at the all the stations at p<0.05 during 1957/60-2003. The decresaing trend in R_G in past decades indicates the increasing atmospheric load over the four cities of India. Maximum solar dimming has been observed over Jodhpur (-1.54 Wm^-2y^-1) for the period 1960-2003, followed by New Delhi (-0.98 Wm^-2y^-1, 1957-2003), Nagpur (-0.79 Wm^-2y^-1, 1960-2003), and Kolkata (-0.35 Wm^-2y^-1, 1957-2003). R_D has shown the increasing trend in all the station-Jodhpur (0.08 Wm-2y-1), New Delhi (0.05 Wm^-2y^-1), Nagpur (0.21 Wm^-2y^-1) and Kolkata (0.04 Wm^-2y^-1) for the period 1973-2003.The increase in R_D indirectly hinted the increase in scattering of insolation by aerosols and clouds. However, in the later decades (after 1990s) R_G has shown an increasing trend (solar brightening) in Jodhpur (1.14 Wm^-2y^-1) and New Delhi (2.24 Wm^-2y^-1) for the period 1991-2003. The present results show that solar dimming is the global phenomena, but solar brightening is the local phenomena.