GEOTHERMAL FORCING OF MICROMETEOROLOGICAL CONDITIONS IN CAVES

Caves micrometeorology is important to cave atmospheric composition, speleogenesis, growth of cave decorations, speleothem based paleoclimate studies, and cave biology. It has three major forcings. Earth’s geothermal gradient heats cave air from below, creating convection cells and forcing circulation within the cave. Cave connections to Earth’s lower atmosphere bring pressure and wind forcings, and together with water percolating from above bring in latent heat. In this paper we focus on geothermal forcing of both isolated caves and caves connected to a quiet atmosphere. Heat conduction in the surrounding rock mass, and laminar/turbulent air flow and heat conduction/convection within the cave, are simulated using COMSOL Multiphysics. Caves with various heights, depths, aspect ratios, shapes, and slopes are considered. Results are presented in terms of patterns and rates of air circulation (e.g., convection cells), and new versions of classical dimensionless numbers from heat transfer theory. The Rayleigh number, which characterizes the geothermal forcing, is sensitive to the 4^th power of the cave height, leading to turbulent flow in even modest sized isolated caves. Convection in and heat transfer through caves is most sensitive to cave height, slope, shape, and connection to the atmosphere.