Air Convection Reversal in a Hot Tunnel above the Elbrus Volcano Chamber

A unique geophysical phenomenon, i.e., an inverted vertical air temperature profile (3–5°C at the 1.5 m altitude) in a tunnel above the Elbrus volcano chamber, is discovered for the first time to our knowledge. The observed deviation from the normal thermo-gravitational convection is explained by an influx of hot heavy magmatic gases (radon, carbon dioxide) originating from the volcano chamber into the tunnel through pores and cracks in the rock. These gases “drown” in air due to higher density and form a near-bottom layer that flows over the inclined floor down to the tunnel entrance. The heavy gas output to the tunnel induces rarefaction near the dead-end wall which creates an influx of external “cold” air (~33°С) under the tunnel ceiling from the main tunnel, thus establishing an inverted air vertical temperature profile. The revealed air flow structure triggers the formation of fog at the contact interface between “warm” and “cold” air flows (volcanic gases and air from the main tunnel, respectively). The fog induces large fluctuations to the aerosol scattering coefficient measured by a lidar installed in the tunnel for monitoring volcanic activity. To avoid these undesirable water aerosol fluctuations, we change the lidar sensing path by submersing it into the heavy gas layer near the tunnel floor. Unwanted fog signal filtering by introducing lidar depolarization measurements is discussed.