Estimating the effects of aerosol, cloud, and water vapor on the recent brightening in India during the monsoon season

Abstract. India is experiencing a leveling-off trend of solar radiation, even a transition from dimming to brightening. This process is significantly complicated because of the active atmospheric action during monsoon season. In this study, we use observations from multiple sensors in the A-Train satellite constellation to evaluate the effects of aerosol, cloud, and water vapor variations on recent changes of solar radiation during monsoon season (June–September) in India from 2006 to 2015. The results show that the increase in aerosol optical depth is paradoxical with the variation of surface shortwave radiation in India. Instead, the decreases in water vapor amount and clouds significantly contribute to the brightening, further affecting the surface warming in India. In general, clouds are reduced and thinned by approximately 9.4 % and 182 m when cloud water path (by 53.4 g m⁻²) and particle number concentration in cloud-sky condition decrease. The corresponding change of clouds weakens the shortwave cloud radiative effect in surface by approximately 45.5 W m⁻². Moreover, the precipitable water in clear-sky decreases by approximately 3.0 mm over the brightening period. Consequently, solar brightening increases by roughly 2.8 W m⁻² owing to its weakened absorption. Overall, the decreases in water vapor and clouds result in the increased absorption of direct solar radiation in surface and subsequent surface brightening.