Modeling Studies on the Formation, Growth, Dissipation, and Acidification of Cumulus Clouds.

A parcel model, with nineteen aqueous-phase reactions, was developed to preliminarily study cumulus acidification and to compare with experimental results from the Summer 1983, 1984, and 1985 field experiments conducted around Lexington, KY, through the VENTEX program. Cumulus parameterizations in the model included uniform updraft velocity, constant cloudwater content, and monodisperse drop size distribution. The model predicted first-order rate constants for the aqueous -phase conversion of S(IV) to S(VI) that matched well with the experimental results. A one-dimensional kinematic model, based on the thermodynamics of PLUVIUS, was formulated to simulate the cumulus life cycle and to study the acidification processes during that cycle. A temporally constant parabolic vertical wind velocity profile was used to initiate cloud formation and growth. Detailed gas- and aqueous-phase chemistries were also incorporated into the model. At low updraft velocities, the model-predicted rate constants agreed well with the experimental results. Under the modeled updraft conditions, the cloudwater contents increased with time. This finally resulted in precipitation formation in the cloud. To control the precipitation formation and to form a cloud of finite lifetime, a time-dependent velocity profile was adopted. Downdraft at cloud top was introduced to cause cloud dissipation. The resulting cloud properties, including vertical profiles of entrainment/detrainment, liquid water content, and temperature, matched well with recent observations and theories on cumuli. Even though rain and snow were allowed to form, the amount of precipitation was negligible. Detailed chemistries were incorporated into the model. The predicted rate constants, with parameterized background conditions, were in the range of experimentally determined values (5-36%/min). Hydrogen peroxide was the most important oxidant of S(IV) in the aqueous-phase. HO _2, OH, and Cl_sp {2}{-} also contributed significantly to the oxidation. Oxidation by O_3 was pH-dependent and contributed less than 5 per cent to the overall oxidation. Faster aqueous-phase oxidation of S(IV) resulted in greater cloudwater acidity. Under high NO_{rm x} conditions, significant H_2O_2 formed in the gas phase. These results were compared with those using the parcel model.