Air circulation and its impact on microclimatic variables in the Císařská Cave (Moravian Karst, Czech Republic)

Abstract

Airflows and their impact on microclimatic parameters were studied in the two-entranced Císařská Cave (Moravian Karst, Czech Republic). Basing on the airflow direction with respect to the upper entrance (UE) and lower entrance (LE), three ventilation modes were distinguished: upward airflow one (UAF mode), downward airflow one (DAF mode), and transitional mode. A switching between the UAF and DAF happens in the transitional mode, in the range of external temperatures from 9.5 to 11.1°C. Basing on the extent of cave ventilation, a period of active ventilation and period of stagnant ventilation were recognised. The cave air is intensively exchanged with external airs during the active ventilation period when the cave persists in either the DAF or UAF mode. During the stagnant ventilation period, the single airflow modes switch during diurnal cycle and only cave entering passages are vented (impacted). The airflows were found to depend (i) linearly on the differences in cave-outdoor air densities, (ii) nonlinearly on the external temperature, and (iii) both linearly (LE) and non-linearly (UE) on the temperature gradient. Majority of the airflows showed high-frequency oscillations with the period from tens of seconds to minutes and with the amplitude up to 0.1 m 3 s −1 . Different airflow spectra were found at the individual entrances. The highest spectral densities were registered at low driving forces. The air circulation changed the pattern of temperature distribution depending on the ventilation mode and ventilation period. The cave relaxation length varied in the extreme range from 1 to 69.6 m. The asymptotic temperature and asymptotic specific humidity varied from 7.5 to 11.8°C, and from 5.47 to 7.15 kg kg −1 , respectively. The impact of airflows on the cave CO 2 levels is discussed based on a dynamic model.

Abiotic and seasonal control of soil-produced CO2 efflux in karstic ecosystems located in Oceanic and Mediterranean climates

“…Rainfall and the relative humidity of air regulate the water content in soil and host rock porous media that play important roles in regulating the gas exchange between the underground environment and the surface Fernandez-Cortes et al, 2013). Air density contrasts (controlled by temperature gradient between the cave air and the atmosphere) and cave geometry determine the advective movement of air that produces the cave ventilation (Fernandez-Cortes et al, 2009;Faimon et al, 2012;SanchezCañete et al, 2013;James et al, 2015). Interior temperature of caves is in general very stable compared to the exterior air due to the characteristic low thermal conductivity of rocks.…”

Evidence of daily and seasonal inversions of airflow in Petites Dales cave, Normandy, France

“…INVERSION A large consensus exists stating that driving forces of airflow are proportional to the differences in air densities (Cigna 2002;De Freitas et al 1982;Spötl et al 2005;Faimon et al 2012;Faimon & Lang 2013;Borsato et al 2015).…”

“…Many others authors introduce the virtual temperature (Sánchez-Cañete et al 2013;Faimon et al 2012;Vieten et al 2016) instead of the Rayleigh number to estimate the driving force. The virtual temperature calculation included CO 2 effects which are not measured systematically in the present study.…”

“…According to their ventilation mode, caves were historically classified into two groups: (i) static caves (single-entrance caves), and (ii) dynamic caves (multientrance caves) (Geiger 1961;Bögli 1980;Faimon et al 2012). This classification is obsolete for many authors (Lismonde 2002;Perry 2013).…”

“…The study of the microclimate of caves is a topic that is regaining relevance due to the study of cave flora and fauna, thermodynamic conditions for karst processes underground and hydro-geologic features of speleothems (Luetscher et al 2008;Ravbar & Kosutnik 2014;Faimon et al 2012). Emphasis is given to the study of caves to estimate human impact when caves are visited.…”