Numerical Modeling of Sulfur and Nitrogen Chemistry in a Narrow Cold-Frontal Rainband: The Impact of Meteorological and Chemical Parameters

Abstract

To better understand the impact of various meteorological and chemical parameters on chemical deposition from winter storms, the chemistry and microphysics of a narrow cold-frontal rainband and its associated stratiform region were examined with a two-dimensional numerical cloud model. The peak precipitation was associated with the lifting at the leading edge of the cold front. However, the peak sulfate deposition occurred behind the primary updraft, where melting graupel was the dominant source of precipitation, and in the leading rainband. The peak nitrate deposition occurred behind the main updraft and at the leading edge of the main updraft. Sulfur dioxide, aerosol nitrate, and peroxyacetylnitrate were transported to higher altitudes, while aerosol sulfate, nitric acid, and hydrogen peroxide were depleted by the storm. Examination of the pathways for oxidizing aqueous sulfur dioxide showed that iron-catalyzed aerobic oxidation was an important mechanism for converting sulfur dioxide to sulfate. Sensitivity studies of the chemical parameters indicated that this was a sulfur-limited storm rather than an oxidant-limited one, and that nitric acid contributed significantly to the deposition of nitrate. The presence of graupel in the storm controlled the pattern of sulfate and nitrate deposition. Because this model has a sound dynamical framework, the influence of meteorological parameters on the chemical deposition can be studied in detail. When the depth of the storm was increased, the accumulated sulfate deposition decreased, while the accumulated precipitation increased. When the initial shear for the storm was decreased, the accumulated sulfate deposition increased, while the accumulated precipitation decreased. This inverse correlation between sulfate deposition and accumulated precipitation or peak vertical velocities in the storm's updraft should be considered when parameterizing sulfur transport and deposition with a large-scale model.