Modeling and Field Study of Cave Micrometeorology: Role of Natural Convection

Cave micrometeorology is important to cave atmosphere composition, speleogenesis, growth of cave decorations, speleothem-based paleoclimate, and cave biology studies. This research project was undertaken to better define micrometeorological patterns and their interactions in subterranean cavities using the latest air temperature sensing instruments and by applying mathematical computer modeling of convective heat transport in a cave environment. Micrometeorology field experiments were conducted in the Left hand Tunnel (LHT) in Carlsbad Caverns National Park. In this field experiment, cave air temperature and its fluctuation intensity were recorded. Measurements of cave air temperature provide a better understanding of how the geothermal forcing affects heat and air-flow due to the existence of positive feedback between cave weather properties such as cave air temperature, relative humidity, wind velocity, and CO2. Three major determining factors, or forcings, influence cave meteorology: 1) geothermal heat flux, 2) surface weather, and 3) water inflow into a cave system. In the numerical simulations only the geothermal forcing was changed while the other forcings were set as quiescent surface conditions in a dry cave (no latent-heat effects). Field experiments done at the LHT tested the applicability of the latest air temperature measurement instruments. An innovative “turbulence tower” was designed and tested during this project and judged suitable for micrometeorological applications, as was a Distributed Temperature Sensing (DTS) unit. These two sensors record cave air temperature fluctuation and evolution profiles, respectively. These sensitive instruments record very minute temperature variations and were used to record high-resolution cave air temperature data at both small and large spatial and temporal scales. In addition to the field tests, the COMSOL R © Multiphysics software package was used to model laminar and turbulent natural convection in simplified cavities with various aspect ratios, slopes, shapes, surface connections, and slopes. The turbulent convection results are emphasized because that is the most common heat transport process in cavities of any significant size. Numerical modeling results have shown that heat flux passing through a cavity is low, as compared to the surrounding rock mass, when air is stagnant, even in cavities with an entrance system. On the contrary, turbulent airflow in a cavity causes more through-flowing heat flux. The total heat flux passing through a cavity depends on the cavity aspect ratio, the slope, shape, size, and width of any entrances, along with the buoyancy forcing. Modeling showed that the total heat flux passing through a cavity increases as these factors are increased. Modeling also showed the number and pattern of convection cells within a cavity are sensitive to the same factors. For a cavity with two surface connections at different elevations, the pattern of convection cells is mostly insensitive to buoyancy forcing. The modeling results confirmed that air always enters a cavity from the lower entrance even if that is the smaller entrance. The use of both field experiments and modeling give the conclusions from this project more significance. The turbulence tower allowed accurate determination of air temperature fluctuation intensities and yielded very fineresolution data. The Distributed Temperature Sensing units produced very fine-scale air temperature patterns. The Computational Fluid Dynamics software COMSOL R © Multiphysics explains and predicts patterns of major cave weather properties very well if the numerical models are first calibrated.