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The footprint of column collapse regimes on pyroclastic flow temperatures and plume heights

PUBBLICATO: 28 Giugno 2019
Fig. 1
Comparison between the internal structures of the two column collapse end-members. Input conditions are reported in Table 1. a, c Cross-sectional images showing the instantaneous distribution of the temperature T difference relative to the stratified atmospheric temperature Ta at the same vertical position normalized to the initial temperature T0, at 100 and 315 s respectively, for the partial (a) and total (c) collapse. Isolines correspond to T − Ta/T0 − Ta = 0.25, 0.50, 0.75, 0.99. b, d Same as a, c, but in a time-averaged domain. Black and red lines represent the time-averaged velocity (vertical component) and temperature profiles, respectively. Profiles along the vertical axis are dashed, whereas horizontally integrated quantities are solid. All the values are normalized by using the initial conditions. The averaging window is from 500 to 1000 s. Symbols represent the uneroded (Hun; green square), collapse (Hc; red star), and jet (Hjet; blue circle) heights

Trolese M., M. Cerminara, T. Esposti Ongaro, and G. Giordano (2019).
Nature Communications, 10/2476, doi: 10.1038/s41467-019-10337-3.

Abstract

The gravitational collapse of eruption columns generates ground-hugging pyroclastic density currents (PDCs) with highly variable temperatures, high enough to be a threat for communities surrounding volcanoes. The reasons for such great temperature variability are debated in terms of eruptive versus transport and emplacement processes. Here, using a three-dimensional multiphase model, we show that the initial temperature of PDCs linearly correlates to the percentage of collapsing mass, with a maximum temperature decrease of 45% in the case of low percentages of collapse (10%), owing to an efficient entrainment of air into the jet structure. Analyses also demonstrate that column collapse limits the dispersal capabilities of volcanic plumes, reducing their maximum height by up to 45%. Our findings provide quantitative insights into the mechanism of turbulent mixing, and suggest that temperatures of PDC deposits may serve as a marker for determining column collapse conditions, which are of primarily importance in hazard studies.

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