Though less studied than high-level cirrus or low-level stratocumulus clouds, mid-level altocumulus clouds are estimated to cover 22% of the earth's surface and to be important for understanding the planetary radiation balance. They are characterized by distinctive physical processes, which include (1) entrainment into both cloud base and cloud top; (2) maintenance by large-scale vertical motions; (3) the frequent presence of both ice and liquid phases. This project is aimed at studying two aspects of altocumulus clouds: the mechanisms responsible for their dissipation and the factors affecting the horizontal variability of their structure. Modes of inquiry include large-eddy simulation (LES), theory, and data analysis. Observations used for validating the simulations are from the Complex Layered-Cloud Experiment (CLEX) of 1999. The aim of the simulations is to evaluate the relative importance of four processes that lead to cloud dissipation: large-scale subsidence, radiative heating, entrainment drying, and precipitation fallout. The LES is carried out using a Beowulf cluster of 22 personal computers running the Regional Atmospheric Modeling System (RAMS) model with an advanced treatment of microphysics. Findings of the research bear on both climate modeling and remote sensing of cloud properties.
