Recent Trends in Solar Dimming and Brightening over India: Role of Aerosols and Clouds

A persistent dimming trend has been observed over the Indian region in recent decades in contrast with the global transition from dimming to brightening. We, using the data from NASAs CERES sensor, investigate the role of aerosols and clouds in the observed trends in incoming all-sky radiation reaching to the surface over India. The region of interest (638 N, 6598 E) is exposed with an annual average irradiance of 21017 W/m2 for the period of 2001-18. Our findings suggest that, 91% of the landmass experiences the dimming effect with an average rate of -0.290.19 W/m2 per year. The combined trends in clear-sky and no-aerosol sky radiations show that 39% of the Indian region suffers dimming from both the effect of aerosols and clouds where 98% and 41% are impacted individually by aerosols and clouds respectively. The annual average cloud-impact (-4719 W/m2) is almost twice as compared to the direct aerosol-impact (-248 W/m2) in diminishing the radiation. But, the observed dimming trends in clear-sky and hence in all-sky radiation over the eastern Indo-Gangetic Plain (IGP) and eastern coastal regions may be attributed to the increasing aerosols loading. The role of aerosols outweighs the effect of clouds over these regions while the clouds attribution has increased over the Thar desert of north-western part. Further, in north-eastern regions, a brightening effect has been noted in-line with the no-aerosol sky radiation but contrasted to the dimming in clear sky radiation. This confirms more sensitivity in clouds as compared to the aerosols over the region. Additionally, the seasonal analysis shows that the maximum decline trends in all-sky radiation occur during the winter (DJF) followed by the monsoon (JJA) seasons. The spatial gradients in aerosol loading in winter and clouds during the monsoon drive this variability. Studying the spatio-temporal variations in surface fluxes at a regional scale has prominent implications in solar resource assessments. India is abandoned with 60% of its regions suitable for Solar Photo-Voltaic (SPV) generations but it is depriving at an annual rate of ~0.89% for the last 18-year. Therefore, by alleviating the aerosol-induced dimming over this region will accelerate its growth in solar renewable expansions exercises in coming years.