Effects of wind on entrainment efficiency in volcanic plumes

Abstract

The entrainment of air by turbulent mixing plays a central role in the dynamics of volcanic eruption clouds, as the amount of entrained air controls the height of the plume. In one-dimensional models of volcanic plumes, the efficiency of entrainment under a wind field is parameterized using two empirical constants. The first is the coefficient associated with the entrainment caused by the shear between the volcanic plume and the ambient air (i.e., the radial entrainment coefficient), and the other is associated with the entrainment caused by wind (i.e., the wind entrainment coefficient). In this study, we used three-dimensional numerical simulations of volcanic plumes to determine the effective values of these empirical constants from the relationship between eruption conditions and plume heights. These simulations suggest that the value of the radial entrainment coefficient is 0.05-0.06, which is slightly smaller than that of pure jets or plumes seen in laboratory experiments (0.07-0.15). The value of the wind entrainment coefficient was estimated to be 0.1-0.3, which is significantly smaller than those estimated from laboratory experiments (0.3-1.0). The entrainment coefficients derived from these simulations successfully explain the observations made during the 2011 Shinmoe-dake eruptions in which the volcanic plumes were significantly distorted by the wind.