Friday, 7 June 2002
Solar Influences on Vapor Depositional Growth
The effects of solar heating and infrared cooling on the vapor
depositional growth of cloud droplets is investigated. In this study, a
marine stratocumulus cloud was simulated using a large eddy simulation
(LES) model and a detailed microphysical bin model. The LES model, which
simulates microphysics-radiative feedbacks, calculated thermodynamic
variables and diabatic heating/cooling rates that were then used as input to
the detailed microphysical bin model.
Including infrared cooling as well as solar heating had primary two
effects. The first is that the combined radiative influence on
vapor-depositional growth is radius dependent: Solar heating effects
dominated at larger drop sizes (> 50 - 200 microns) and infrared cooling
effects dominated at smaller drop sizes (10 - 50 microns). This
size-dependence of the radiative influences was also found to be dependent
upon the location of the drops with respect to cloud top.
The second effect is strongly tied into the fact that cloud base is
stabilized by the solar heating. Due to the stabilization of the cloud
base, drops tend to spend longer periods of time in the cloud, thus
increasing the time that drops are affected by radiation. Although the
longer in-cloud times might be expected to enhance drop growth, it turns
out that the strong solar heating effect on larger drops narrows the drop
size distribution. This narrowing of the drop size distribution means
that the onset of drizzle may be retarded by solar heating.
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