Date:         Fri, 23 Sep 1994 12:35:03 MST
Reply-To: VOLCANO 
Sender: VOLCANO 
From: G G J Ernst 
Subject:      RABAUL: Shape of main plume (19sept.94)
To: Multiple recipients of list VOLCANO 
In-Reply-To:  <> from "David Schneider" at
              Sep 19, 94 03:35:05 pm
Status: OR

Those who have looked at the satellite images kindly provided by the
Remote Sensing Team at Michigan Tech. will have seen that the shape
of the main plume (about first 20 hours of eruption on 19Sept.) was
not at all elliptical. The plume could be seen dispersing mostly to
the NW, W and SW and showed two lobes at the plume front for several
hours thereby developping what is called "plume bifurcation" in the
engineering literature. Plume bifurcation was visible here despite
the complicating effect of changing wind directions during the first
24hours of explosive activity (the plume was made more asymmetrical
as winds were reported to shift to the SW).

A paper reviewing Plume Bifurcation and applying it to Volcanic
Plumes in Crosswinds has just appeared and may be of interest to some
of you, with the following reference:

GGJ Ernst, JP Davis and RSJ Sparks (1994), Bifurcation of Volcanic
Plumes in a Crosswind, Bulletin of Volcanology 56: 159-169

Even the main plume of Rabaul is typically a bent-over plume, even if
it is a strong one and it probably just about reached the height of
the tropopause. As explained in the Bull Volc paper, it is believed
that the tropopause may enhance plume bifurcation, especially of
strong bent-over plumes in moderate crosswinds.

If anyone has further information on the wind conditions around
Rabaul (18-23 Sept; and at different heights) as well as about plume
altitude, this would be most appreciated.

Thank you,

Gerald ERNST, Department of Geology, University of Bristol, Wills
Memorial Building, Bristol BS8 1RJ, United Kingdom
email for reply:
Fax: 44-(0)272.253.385
Tel: 44-(0) ext.4142
PS: Reprints of Bull.Volc. paper available on request