Simulating aerosols using a chemical transport model with assimilation of satellite aerosol retrievals: Methodology for INDOEX

A system for simulating aerosols has been developed using a chemical transp ort model together with an assimilation of satellite aerosol retrievals. Th e methodology and model components are described in this paper, and the mod eled distribution of aerosols for the Indian Ocean Experiment (INDOEX) is p resented by Rasch et al. [this issue]. The system generated aerosol forecas ts to guide deployment of ships and aircraft during INDOEX. The system cons ists of the Model of Atmospheric Transport and Chemistry (MATCH) combined w ith an assimilation package developed for applications in atmospheric chemi stry. MATCH predicts the evolution of sulfate, carbonaceous, and mineral du st aerosols, and it diagnoses the distribution of sea salt aerosols. The mo del includes a detailed treatment of the sources, chemical transformation, transport, and deposition of the aerosol, species. The aerosol forecasts in volve a two-stage process. During the assimilation phase the total column a erosol optical depth (AOD) is estimated from the model aerosol fields. The model state is then adjusted to improve the agreement between the simulated AOD and satellite retrievals of AOD. During the subsequent integration pha se the aerosol fields are evolved using meteorological fields from an exter nal model. Comparison of the modeled AOD against estimates of the AOD from INDOEX Sun photometer data show that the differences in daily means are -0. 03 +/- 0.06. Although the initial application is limited to the Indian Ocea n, the methodology could be extended to derive global aerosol analyses comb ining in situ and remotely sensed aerosol observations.