Climate and Global Dynamics Division
1999
Climate Modeling Section
A system for forecasting aerosols was developed by members of the CGD Climate Modeling Section (Collins, Rasch, and Eaton), and collaborators in the Atmospheric Chemistry Division (Khattatov, Lamarque, and Zender). The system used a chemical transport model together with an assimilation of satellite aerosol retrievals to construct an estimate of the atmospheric aerosol distribution. The forecast system was used to plan aircraft missions during the recent Indian Ocean Experiment (INDOEX), with the goals of determining the direct and indirect radiative forcing of anthropogenic and natural aerosols from the Indian and South-east Asian subcontinents. The forecasting system consists of the Model of Atmospheric Transport and Chemistry (MATCH) combined with an assimilation package developed for atmospheric chemistry. The aerosols forecast by MATCH include sulfate, mineral dust, carbonaceous, and sea-salt aerosols. The model includes a detailed treatment of the sources, chemical transformation, and wet and dry deposition of the aerosol species. The aerosol forecasts involve a two-stage process. During the assimilation phase, the total column aerosol optical depth (AOD) is estimated from the model aerosol fields driven from a meterological specification using National Center for Environmental Prediction (NCEP) weather analyses and forecasts. The model state is then adjusted to minimize differences between the simulated AOD and satellite retrievals of aerosol optical depth. During the subsequent forecast phase, the aerosol fields evolve using meteorological forecasts. Although the initial application was limited to the Indian Ocean, the methodology could be extended to derive global aerosol analyses combining in situ and remotely-sensed aerosol observations. Figure 1 below shows a comparison of the aerosol prediction of total aerosol optical depth from the assimilation, and a measured optical depth (c/o Satheesh and Ramanathan, ms. in preparation) from the Maldive islands during INDOEX. Figure 2 shows the total aerosol for the 27 February 1999 at 6:00 GMT during INDOEX. Note the strong aerosol plume extending south and west from the Calcutta region out over the Indian subcontinent and Arabian Sea on this day.
Comparison of AOD from Model and Microtops Sun Photometer
Figure 1
Comparison of modeled aerosol optical depth (green line) and measured optical depth (red
crosses) at the Kaashidhoo Climate Observatory (4.97 N, 73.47E) during the INDOEX
Intensive Field Phase in 1999. The measured optical depths are courtesy of Ramanathan and
Satheesh (1999, ms in preparation).
Figure 2
Estimated total aerosol optical depth on 27 February 1999 from the assimilation procedure.