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.