Delhi Model with Chemistry and aerosol framework ( DM‐Chem ) for high‐resolution fog forecasting

Abstract

We introduce here the Delhi Model with Chemistry and aerosol framework (DM-Chem), a high-resolution (330 m) visibility and fog forecasting model with the UK Chemistry and Aerosol scheme. DM-Chem is developed at the National Centre for Medium-Range Weather Forecasting (NCMRWF) in collaboration with UK Met Office and Carnegie Mellon University. We present a revised visibility scheme here which uses prognostic aerosols (mass and number concentration) from the two-moment prognostic Global Model of Aerosol Processes (GLOMAP) -mode aerosol scheme. This scheme represents aerosol microphysical processes and the internal mixing of chemical components within five log-normal size modes. The cloud microphysics parameterization also makes use of the GLOMAP scheme to predict cloud droplet number concentration ( N d ). Therefore we are able to evaluate the performance of the scheme indirectly by comparing the predicted N d with that derived from Moderate-Resolution Imaging Spectroradiometer (MODIS) retrievals over the Indo-Gangetic Plain. Aerosol emissions for the inner (330 m) nest are prescribed with a high-resolution inventory for Delhi, from the System of Air Quality and Weather Forecasting project (SAFAR). Winter-time stubble burning is represented using the MODIS-based Fire Energetics and Emissions Research inventory, updated daily. In this article we consider case studies spanning both foggy and haze days of winter 2019–2020. The model compares well with the observed visibility and its spatial and diurnal patterns. The SAFAR emissions help improve the near-surface model fields. For the “haze” case, the single scattering albedo (SSA) is high along the regions of high AOD, and the surface radiative cooling is enhanced. During the “foggy” case, the SSA is not clearly related to AOD, but aerosol absorption does lead to a minor surface warming via the semi-direct effect at the time of onset of fog. The DM-Chem entered operational mode at NCMRWF from winter 2020 and provides high-resolution visibility forecast for Delhi and the adjoining region.