TITLE:
New model for dispersion of volcanic ash and dust in the troposphere
AUTHORS:
Jonas Eliasson
KEYWORDS:
Dust Plume, Gravitational Dispersion, Load Capacity, Streak Fallout
JOURNAL NAME:
International Journal of Geosciences,
Vol.11 No.8,
August
25,
2020
ABSTRACT: Dispersion of volcanic ash and dust is traditionally modeled as advection and Gaussian diffusion. This is the tradition
in treating smoke stack plumes. About 100 meters above earth the velocity profile may disintegrate, diffusion coefficients
become rather unpredictable and stratified flow occur. It is suggested that gravitational flattening may be the main cause of
dispersion in dust plumes above the turbulent boundary layer. A dust plume in between two layers of small temperature
difference has a certain carrying capacity of dust. The corresponding mass loading can be estimated from the temperature
difference between the layers above and beneath the plume. Such dust plumes will be forced to jettison a load they may have
in excess of this carrying capacity; this may be seen as streak fallout from the plume. In the same time, the plume will be
subjected to gravitational flattening to the sides, in addition to any diffusion if there is any. The plume width resulting from
the flattening may be estimated from the temperature difference. This can explain the behavior of plumes like the plume from
the Eyjafjallaj?kull 2010 in absence of diffusion. In the long run diffusion and gravitational flattening will cause different
developments of the plume width. Gravitational flattening and streak fallouts are important elements from plume physics not
included in most plume models. It is concluded that modelling dust plumes with diffusion and ordinary fallout only; can cause
serious errors in the model, the simulated plumes will become too big. To avoid them, the new model should be included in dust
models in the same manner as the turbulent diffusion, i.e. as a sub grid model. Then, the plume model only needs to
include horizontal turbulent diffusion of the same order of magnitude as the
vertical one.