Properties of turbulent air avalanches in a vertical pit

Temperature and humidity measurements have been performed since 2000 in the atmosphere of the 20-m deep vertical access pit of an underground quarry. Air avalanches take place in the pit when the outside air density is larger than the equilibrium air density of the quarry. These avalanches are associated with a mean linear vertical temperature profile, and with broad-band, spatially organized and coherent temperature fluctuations. Temperature distributions are nearly Gaussian, with a skewness smoothly varying with position. The power spectra of the fluctuations show, in the frequency range from 3 ×10 -4 Hz to 4 ×10 -3 Hz, an exponent varying from -0.15 at the bottom of the pit, to -0.6 at the top. Heat exchange with the walls accounts for the mean temperature profile, and contributes to the power spectra for periods smaller than about 20 minutes. Vertical cross-correlations provide estimates of velocities and indicate quasi-stable polarization of the direction of the vertical mean flow. A horizontal vorticity index, constructed from a horizontal cross-correlation and the sign of the temperature gradient, exhibits properties of a scale-invariant process, similar to the structure of wind reversals observed in Rayleigh-Bénard laboratory convection experiments or to the reversals of the geomagnetic fields. This experiment may offer a genuine realization of bulk turbulence forced by a linear temperature gradient. More generally, it provides a description of turbulent fluid dynamics in a medium-scale natural system in the presence of non-adiabatic boundaries. Copyright EDP Sciences/Società Italiana di Fisica/Springer-Verlag 2005