Influences of regional and trans-regional anthropogenic emissions on meteorology and cloud properties over western India assessed using WRF-Chem model

Abstract

Aerosols significantly influence meteorology and cloud properties over Indian region, nevertheless, high-resolution modeling studies required for addressing mitigation strategies have remained few. This is particularly the case for western India, a landmass adjoining global hotspot of enhanced aerosol loading - the Indo-Gangetic Plain (IGP) and strong dust source - the Thar desert. In this study, with the aid of high-resolution regional modeling we quantify and differentiate the impact of regional versus trans-regional anthropogenic aerosols on the meteorology and cloud properties over western India. The numerical simulations are performed for four months (January, April, August and November) representing contrasting seasons (winter, pre-monsoon, monsoon and post-monsoon) for the year 2019. Model results are seen to be in good agreement with the IMDAA and ERA5 reanalysis, satellite (CERES) data and radiosonde observations across different seasons. Interestingly, the sensitivity simulations reveal equal (or higher) trans-regional anthropogenic influence on the meteorology and cloud properties over western India when compared to the regional influence. Regional and trans-regional emissions combined reduce net surface-reaching shortwave radiation by up to 42 W/m² across all seasons. Individually, regional and trans-regional emissions reduce net surface-reaching shortwave radiation by up to 23 and 30 W/m² respectively across seasons. Regional and trans-regional emissions combined typically reduce near-surface (2m) temperature by up to ~ 0.6 K across all seasons. Regional emissions reduce near-surface (2m) temperature across all seasons by up to ~ 0.2 K. Trans-regional emissions also tend to lower 2m temperature by up to ~ 0.4 K except during the monsoon when some areas experience a temperature enhancement of up to 0.1 K. Strikingly, regional as well as trans-regional emissions enhance the near-surface temperature over coastal areas and the Arabian Sea during winter and post-monsoon. In line with change in temperature, aerosols tend to suppress the boundary layer height in winter and post-monsoon, which enhance near surface aerosol concentration. The trans-regional emissions lead to increase in cloud droplet number concentration and thereby the cloud optical thickness in the monsoon. In addition, a reduction in accumulated monthly precipitation by up to 37 mm, mainly due to trans-regional emissions, could exacerbate water resource challenges in the already water-stressed region of western India. Our study highlights a large role of trans-regional emissions in influencing the regional climate over western India and emphasizes the need for a coordinated mitigation approach.

Keywords: Aerosols; Black carbon; Clouds; Meteorology; Precipitation; Radiation; WRF-Chem.