Long-Term Trends and Variability in Global and Diffuse Solar Radiation Over India

Abstract

Surface solar radiation (SSR) changes are of significance to the understanding of the atmospheric processes and regional climate variability. This study aims to provide a detailed analysis of the variations of both global and diffuse solar radiation in India over the past 54 years, based on the observations obtained from 12 stations of India Meteorological Department (IMD) across various parts of the country using quality-controlled pyranometers. To make sure the long-term data set is reliable and consistent, a systematic multi-stage data screening framework was implemented, such as removing outlier values by using IQR, excluding stations with more than 30% missing data, and performing extra data consistency checks for abrupt variability. Results show that there is a statistically significant reduction in solar radiation at all stations with the range of trends between − 0.42 and − 1.07 W m^–2 yr^–1 and maximum reduction in solar radiation is seen over New Delhi. On the contrary, the diffuse components of solar radiation show generally very small and spatially scattered positive trends, for which statistically significant tendencies can be found at a few stations. Seasonal analyses show that in monsoon season, diffuse radiation increases, indicating that the scattering effect of clouds may modulate the radiation components on the surface. Longer time series available until 2023 allows better characterization of the variability in the recent period and also shows high regional gradient in the radiation trends over India. The reductions were greater at stations from the north and west where the aerosol loading and urban emissions over the Indo Gangetic Plain are higher, with moderate changes at stations at coastal/southern stations, due to maritime and monsoonal effects. ERA5 reanalysis data supports this analysis, showing a statistically significant but relatively weak negative correlation between cloud cover and SSR in the recent period from 2001 to 2023 (r =  − 0.22, p < 0.001), suggesting some of the observed variability in the recent period is due to the cloud variability. These findings offer a valuable observational baseline for monitoring long-term SSR variations across India, along with recent research, and reveal significant spatial variability and dynamic radiation patterns. The results of this study highlight the need for future multi-variate analysis using aerosols, cloud properties and atmospheric moisture to better parameterize the drivers of long-term radiation variability.