Atmospheric sulfur and deep convective clouds in tropical Pacific: A model study

A high-resolution limited area nonhydrostatic model was used to simulate sulfate-cloud interactions during the convective activity in a case study from the Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment, December 20-25, 1992. The model includes a new detailed sulfate-cloud microphysics scheme designed to estimate the effects of sulfate on cloud microphysics and radiative properties and the effects of deep convection on the transport and redistribution of aerosol. The data for SO₂ and SO₄^(2-) species were taken from the Pacific Exploratory Mission West B observations during February-March 1994. Results show that a change in sulfate loading from the minimum to the maximum observed value scenarios (i.e., from about 0.01 to 1 μg m^-3) causes a significant decrease of the effective radius of cloud droplets (changes up to 2 μm on average) and an increase of the diagnostic number concentration of cloud droplets (typical changes about 5-20 cm^-3). The change in the average net shortwave (SW) radiation flux above the clouds was estimated to be on average -1.5 W m^-2, with significant spatial and temporal variations. The horizontal average of the changes in the net SW radiation fluxes above clouds has a diurnal cycle, reaching typical values approximately -3 W m^-2. The changes in the average net longwave radiation flux above the clouds were negligible, but they showed significant variations, typically between -10 W m^-2 and 10 W m^-2 near the surface. These variations were associated mainly with the changes in the distribution of cloud water, which showed typical relative changes of cloud water path of about 10-20%. Other notable changes induced by the increase of aerosol were the variations in air temperature of the order of 1°C. The case study presented here suggests that characteristics of convective clouds in tropical areas are sensitive to atmospheric sulfate loading, particularly during enhanced sulfate episodes.