NUMERICAL MODELING OF CLOUD AND PRECIPITATION CHEMISTRY ASSOCIATED WITH 2 RAINBANDS AND SOME COMPARISONS WITH OBSERVATIONS

A diagnostic modeling study is described of the chemistry of a pair of rainbands, consisting of a narrow and a wide cold-frontal rainband, which were associated with a cold front on the mid-Atlantic coast of the United States. Precipitation growth in the narrow cold-frontal rainband (NCFR) was dominated by collection processes. The wide cold-frontal rainband (WCFR), which followed the NCFR, had most of its precipitation associated with frontal lifting. Surface sulfate and nitrate deposition rates predicted by the model for the WCFR were of the same order as those obtained from measurements on the ground. When the two rainbands were allowed to interact in the model, allowing the NCFR to process the air that entered the WCFR, sulfate deposition from the WCFR decreased, but nitrate deposition remained unchanged. The total sulfate deposition from the two bands, with and without interaction, differed by only approximately 2%. The effects on the sulfate and nitrate deposition of the initial concentrations of their gaseous precursors in the clear air ahead of each rainband were examined. For most cases modeled, the sulfate and nitrate depositions responded approximately linearly to variations in the concentrations of the precursors. Examination of the pathways for oxidizing aqueous sulfur dioxide shows that iron-catalyzed aerobic oxidation may be an important mechanism for converting sulfur dioxide to sulfate.