3.1 Plume characteristics considerations (Morton 1956) show that the rise height of the plume depends upon et al. H the heat flux at the vent and should scale as . As can be seen in 1, the modelled results Q suggest a dependency of the form , although there is some scatter in the plot. In H~ Q0.29 Fig. 1 the height is plotted as a function of the vent thermal forcing can be related to the Q vent temperature perturbation Table 1. The plume height in Fig. 1 is determined either by ΔT examining the return of the density profile to its background state, or by the examining the location of a suitably judged surface of concentration. no appreciable difference between these two methods. The thermal perturbations used here are much lower than those that would be . However the model here is required to lift air (rather than an air-particle mixture) and so lower values of heat flux are required to reachtypical heights. In this and successive plots, the plume is examined at after eruption, at which time the plume has reached t=1200s its final height and is spreading radially (in the absence ofbackground wind). A visualisation of a typical plume extent is shown in Fig. 2(a) where the characteristic shape of a volcanic plume is seen, he overshoot region approximately 14 km above ground level (AGL) and the level of neutral buoyancy at approximately 10 km AGL (corresponding to a surface temperature perturbation of 285K). The of the ejection column (2b) agrees with the theoretical value (e.g. Turner 1973) of approximately 0.1; gradient of ash concentration with height reflects the underlying entrainment of unpolluted air and will be considered in more detail below.
3.1 Plume characteristics considerations (Morton et al. 1956) show that the rise height H of the plume depends upon the heat flux Q at the vent and should scale as . As can be seen in 1, the modelled results suggest a dependency of the form H~ Q0.29 , although there is some scatter in the plot. In Fig. 1 the height is plotted as a function of the vent thermal forcing Q can be related to the vent temperature perturbation ΔT Table 1. The plume height in Fig. 1 is determined either by examining the return of the density profile to its background state, or by the examining the location of a suitably judged surface of concentration. no appreciable difference between these two methods. The thermal perturbations used here are much lower than those that would be . However the model here is required to lift air (rather than an air-particle mixture) and so lower values of heat flux are required to reachtypical heights. In this and successive plots, the plume is examined at t=1200s after eruption, at which time the plume has reached its final height and is spreading radially (in the absence ofbackground wind). A visualisation of a typical plume extent is shown in Fig. 2(a) where the characteristic shape of a volcanic plume is seen, he overshoot region approximately 14 km above ground level (AGL) and the level of neutral buoyancy at approximately 10 km AGL (corresponding to a surface temperature perturbation of 285K). The of the ejection column (2b) agrees with the theoretical value (e.g. Turner 1973) of approximately 0.1; gradient of ash concentration with height reflects the underlying entrainment of unpolluted air and will be considered in more detail below.