Clouds and precipitation play an important role in cycles of various tropospheric chemical species, especially sulfur and nitrogen. This is very complex problem that has to include many processes, such as cloud dynamics, microphysics and tropospheric chemistry.
In this paper, an aqueous chemistry module was incorporated into complex 3D cloudresolving mesoscale ARPS (Advanced Regional Prediction System) model developed in the Center for analysis and prediction of storms (CAPS) at the University of Oklahoma. The goal of this paper was to examine the sensitivities of vertical redistribution of sulfate to the physical processes that take place in cloud. Six water categories were considered: water vapor, cloud water, rainwater, cloud ice, hail and snow, and five chemical species: gases H2O2, SO2, O3, and aerosols SO42- and NH4+. Each chemical constituent in each microphysical category was represented by differential equation for mass continuity, so there are 30 new prognostic equations. The absorption of a gas phase chemical species in the cloud water and rainwater is calculated either by the equilibrium according to Henry’s law and by real kinetic calculation of gas uptake. The source and sink terms in equations of continuity represent either transfer of a chemical species from one microphysical category to another (e. g. transfer of cloud ice sulfate to cloud water sulfate by melting, transfer of cloud water sulfate to rainwater sulfate by autoconversion etc.) or a chemical reaction (e. g. oxidation of cloud water SO2 by H2O2 and O3 to cloud water sulfate). Comprised microphysical processes in source/sink terms are: autoconversion, accretion, Bergeron processes, freezing, depositional growth, melting, sublimation and evaporation. It is assumed that initial concentrations of chemical fields fall off exponentially, from the given values of mixing ratios at the lowest model level. Two environments were simulated: continental background and moderately polluted. The cloud model is initiated by a single sounding giving the values of temperature, humidity, pressure, wind direction and velocity. The initial meteorological fields are horizontally homogeneous. The experiments were made with the real orography.
The resulting cloud model coupled with chemistry module provides a powerful diagnostic and prognostic tool for studying the relative importance of physical and chemical processes in determining the distributions of sulfate and nitrate species in convective clouds and precipitation, as well as the transport of trace chemical species within convective systems. A special emphasis was dedicated on sulfate redistribution in different water categories during the convective cloud life. Vertical profiles of sulfate following the cumulonimbus trajectory can give us a lot of information about sulfate redistribution also. The maximum
values of SO42- in the cloud water is located in the lower part of the cloud, at the place of maximum vertical wind or just a slightly behind. The same situation is with maximum values of SO42- in cloud ice: maximum of SO42- correspond to maximum of vertical wind.