| Abstract: 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. |