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

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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 m3 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 CO2 levels is discussed based on a dynamic model. Copyright © 2011 Royal Meteorological Society

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