Dependence of water vapor radiative forcing on aerosol loading: Analyses over the densely populated and highly polluted Indo-gangetic plain region

Abstract

This study analyzes the aerosol and precipitable water vapor (PWV) properties at six sites in the Indo-Gangetic Plains (IGP), a densely populated and highly polluted region. The main objective is to explore how the columnar PWV is related to the attenuation of shortwave solar radiation (SWR), as well as the combined role of aerosol properties and PWV on radiative forcing based on AERONET data and model (SBDART) simulations. The analysis revealed high aerosol optical depth (AOD) values (0.4–0.6) throughout the year in all the sites, associated with increased PWV (4–5 cm) during the summer monsoon. Comprehensive investigation shows that changes in PWV levels also affect aerosols’ size distribution, optical properties and radiation balance in a similar way - but in different magnitudes - between the examined sites. The water vapor radiative effect (WVRE) is highly dependent on aerosol presence, with its magnitude for both surface (− 130 to − 140 Wm− 2) and atmospheric forcing becoming higher under clean atmospheres (without aerosols). Aerosol presence is also considered in the computations of the WVRE. In that case, the WVRE becomes more pronounced at the top of the atmosphere (TOA) (30 to 35 Wm− 2) but exhibits a lower forcing impact on the surface (about − 45 Wm− 2) and within the atmosphere (70–80 Wm− 2), suggesting important aerosol-PWV interrelations. The atmospheric heating rate due to PWV is more than double (3.5–4.5 K Day− 1) that of aerosols (1–1.9 K Day− 1), highlighting its essential role in radiative effects and climate implications over the IGP region. The radiative impacts of PWV and aerosols are further examined as a function of the single scattering albedo, solar zenith angle, and absorbing AOD at the different sites, revealing dependence on both astronomical and atmospheric variables related to aerosol absorption, thus unravelling the combined role of aerosols and PWV in climate implications.