Development of an Aerosol Assimilation System Using a Global Non-Hydrostatic Model, a 2-Dimensional Variational Method, and Multiple Satellite-Based Aerosol Products

Abstract

The computational balance between the model grid resolution and the complexity of the data assimilation technique is essential for accurate aerosol forecasting and obtaining aerosol reanalysis data sets. This study aimed to develop a high-resolution aerosol assimilation system. A 2-dimensional variational method (2DVar) was implemented in a non-hydrostatic icosahedral atmospheric model (NICAM). This new model (NICAM/2DVar), with a global grid size of 56 km, assimilated the observed aerosol optical depth (AOD) that is estimated by combining multiple products of geostationary and polar-orbital satellites. The model results were evaluated against ground-based AOD observations on a global scale. They exhibited higher correlations, lower uncertainties, and lower biases than those obtained without the 2DVar. The model also reproduced the observed surface aerosols (PM_2.5) mass concentrations, especially in Kyushu, Japan. This occurred because the satellite-estimated AODs over ocean close to air pollution sources were obtained for many occasions. The correlation coefficient values against the PM_2.5 observations increased from 0.44 to 0.65 compared to the results without the 2DVar. The impact of the 2DVar on the forecast results was investigated, and the forecast values for 2–3 days were improved. Because satellite-retrieved AODs are often lacking over land owing to retrieval difficulties, the use of ground-based AODs in assimilations is essential for precise processing the of aerosol reanalysis data sets. The computational cost with the use of the 2DVar was only 0.6% more than that without its use. Thus, aerosol assimilation using the NICAM/2DVar can be realistically extended to finer grid sizes.