Assimilating visible-band MODIS radiance for regional air-quality analysis and forecast

Assimilation of the Moderate Resolution Imaging Spectroradiometer (MODIS) radiances has been developed within the NCEP Gridpoint Statistical Interpolation (GSI) 3DVAR data assimilation system using the Community Radiative Transfer Model (CRTM) as the observation operator. The MODIS Bidirectional Reflectance Distribution Function (BRDF) product, MCD43, was used to better estimate the land surface reflectance for MODIS bands 1 through 7 to improve the CRTM forward radiance calculations. Weather Research and Forecasting/Chemistry (WRF/Chem) model with the Goddard Chemistry Aerosol Radiation and Transport (GOCART) module was used to provide the initial aerosol concentrations. The assimilation of MODIS aerosol-sensitive radiances was tested over a domain encompassing the continental United States. The impact on aerosol analyses and subsequent forecasts was compared to that of assimilating retrieved total aerosol optical depth (AOD). We found that MODIS radiance assimilation produced better analyses and forecasts over the western and central US than assimilating MODIS AOD retrieval when verified against ground-based AERONET AOD observations. Over the eastern US, assimilating MODIS AOD outperforms assimilating MODIS radiances, likely due to inaccurate BRDF input for radiance calculation.