Abstract
Abstract. This paper presents a three-dimensional variational (3DVAR) data assimilation (DA) system for aerosol optical properties, including aerosol optical thickness (AOT) retrievals and lidar-based aerosol profiles, developed for the Model for Simulating Aerosol Interactions and Chemistry (MOSAIC) within the Weather Research and Forecasting model coupled to Chemistry (WRF-Chem) model. For computational efficiency, 32 model variables in the MOSAIC_4bin scheme are lumped into 20 aerosol state variables that are representative of mass concentrations in the DA system. To directly assimilate aerosol optical properties, an observation operator based on the Mie scattering theory was employed, which was obtained by simplifying the optical module in WRF-Chem. The tangent linear (TL) and adjoint (AD) operators were then established and passed the TL/AD sensitivity test. The Himawari-8 derived AOT data were assimilated to validate the system and investigate the effects of assimilation on both AOT and PM2.5 simulations. Two comparative experiments were performed with a cycle of 24 h from 23 to 29 November 2018, during which a heavy air pollution event occurred in northern China. The DA performances of the model simulation were evaluated against independent aerosol observations, including the Aerosol Robotic Network (AERONET) AOT and surface PM2.5 measurements. The results show that Himawari-8 AOT assimilation can significantly improve model AOT analyses and forecasts. Generally, the control experiments without assimilation seriously underestimated AOTs compared with observed values and were therefore unable to describe real aerosol pollution. The analysis fields closer to observations improved AOT simulations, indicating that the system successfully assimilated AOT observations into the model. In terms of statistical metrics, assimilating Himawari-8 AOTs only limitedly improved PM2.5 analyses in the inner simulation domain (D02); however, the positive effect can last for over 24 h. Assimilation effectively enlarged the underestimated PM2.5 concentrations to be closer to the real distribution in northern China, which is of great value for studying heavy air pollution events.
Highlights
Atmospheric aerosols have considerable impacts on weather, climate, and human health (Menon et al, 2002; Qian et al, 2009; Gao et al, 2015)
Using an observation operator based on the Mie scattering theory, a comprehensive 3DVAR data assimilation (DA) system aiming for aerosol optical properties, including aerosol optical thickness (AOT) retrievals and aerosol profiles, is developed for the Model for Simulating Aerosol Interactions and Chemistry (MOSAIC) aerosol scheme within the Weather Research and Forecasting model coupled to Chemistry (WRF-Chem) model for the first time
A 3DVAR DA system was independently developed to directly assimilate aerosol optical properties. This system was built based on the framework of Li et al (2013) and developed for the MOSAIC scheme within WRF-Chem, a sophisticated aerosol model, rather than the Goddard Chemistry Aerosol Radiation and Transport (GOCART) scheme employed by Community Radiative Transfer Model (CRTM)
Summary
Atmospheric aerosols have considerable impacts on weather, climate, and human health (Menon et al, 2002; Qian et al, 2009; Gao et al, 2015). With an understanding of the aerosol chemical mechanism, as well as the improvement of computing performance, multi-variable aerosol DA studies were conducted, which mainly focused on the development of the 3DVAR technique and Coupled Chemistry Meteorology Model (CCMM). Liang et al (2020) employed the Interagency Monitoring of Protected Visual Environments (IMPROVE) equation, which is the linear link between the extinction coefficient and aerosol chemical species mass, as the forward operator to construct a 3DVAR DA system and assimilated ground-based lidar aerosol profiles and PM2.5 mass concentrations simultaneously. Using an observation operator based on the Mie scattering theory, a comprehensive 3DVAR DA system aiming for aerosol optical properties, including AOT retrievals and aerosol profiles, is developed for the MOSAIC aerosol scheme within the Weather Research and Forecasting model coupled to Chemistry (WRF-Chem) model for the first time.
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