Impact of Aerosol Optical Properties on Surface reaching Shortwave Radiation over Delhi in WRF-Chem

The Earth's radiation budget is a fundamental determinant of climate dynamics, serving as the primary energy source for the planet and influencing its climate system's evolution. Aerosols, as a climate forcer, modify the distribution of solar radiation in the atmosphere and reduce the radiation reaching the Earth's surface. 

The Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) is actively used in operational air quality forecasting systems across the globe. Previous studies have shown that the model has limited success in predicting the air quality over the Indian national capital, New Delhi, especially when the Air Quality Index is in ‘Severe’ conditions during the winter months. It has been reported that the model depicts a mean bias of ~ 34 Wm^-2 in downward shortwave radiation (SWDOWN) flux reaching the surface which may have led to overestimated near-surface temperature (~3.18 ⁰C). This warm bias in temperature might lead to a greater vertical dispersion of the near-surface pollutants, leading to an underestimation of air quality close to the surface. Such biases in the SWDOWN can be due to inadequate information of optical properties of aerosols in the model.  

This study aims to address this gap by incorporating realistic complex refractive indices of aerosol species into WRF-Chem simulations over the ambient environment of Delhi. Five sensitivity experiments (EXP) were conducted, focusing on the impact of aerosol optical properties on the radiative fluxes during the winter season of 2023-24. The results demonstrate that altering a single aerosol optical parameter leads to a reduction in surface shortwave radiation flux by 28–30 Wm⁻² during October and November, and 25 Wm⁻² during December and January, relative to control simulations. Model outputs, validated against observational data, indicate a reduction in the mean bias of SWDOWN by 12.99 Wm⁻² and 17.24 Wm⁻² in December and January, respectively. These results underscore the significant role of aerosol optical properties in modulating radiative fluxes and their implications for the surface energy budget. 

The study also examines the impacts of modified radiation parameterization on the model-simulated aerosol fields like the near-surface PM_2.5 concentration using ground-based measurements and the Aerosol Optical Depth (AOD) over the region as space-based measurements from instruments like MODerate resolution and Imaging Spectroradiometer (MODIS) onboard the TERRA and AQUA satellites. Preliminary findings, revealing the impact of radiation bias on the simulation of meteorological variables and subsequent weather events, will be presented during the session of EGU 2025.