"DUST SPOUTS"
A clue to cloud formation along the dryline?
While not being very successful chasing tornadoes this weekend (May 26-27,
1996), my chase partner and I did observe a very interested phenomena in
Vernon, Texas on Sunday afternoon around 3 pm. At this time, all of the
convective activity was located in western Oklahoma about 50 miles north
of our location. We were waiting (hoping) for the dryline to fire down into
Texas. At our position in Vernon, we had the dryline immediately to our
southwest. Along the dryline was a set of cumulus congestus towers, which
were growing fairly slowly. However, each "cell" maintained its
identity for as long as we watched. South of Vernon the fields were very
dry, and a lot of dust was being picked up and blown about. About 10 minutes
before the series of pictures shown below was taken, I noticed a plume of
dust extending from the ground upward into the base of one of the congestus
towers. This feature persisted for several minutes and reminded me of the
initial development of a landspout, even though the conditions were very
unfavorable for that type of phenomena.
We moved further southwest to observe what was going on, and we noticed
another, shorter lived plume of dust develop. About 3:15, Adrian Burd (my
chase partner) observed another plume forming about 1000 m to our south.
At this time, the congestus tower which was associated with all this activity
was located directly over our heads, and was moving to the north. This new
plume of dust was clearly rotating, rapidly increased in diameter during
the next few minutes. It appeared that multiple vortices developed as the
vortex expanded. We estimated the base of the rotation to be at least 200-300
m wide, and several thick bands of dust plumes extended upwards 500 m or
more. Dust was clearly being carried upward toward the base of the congestus
cloud which was now about 1 km to our north. These vortical plumes persisted
with the congestus for at least another 15 minutes as it moved north of
Vernon toward the Red River.
After the congestus/vortex moved northward, the dryline moved through Vernon.
While several dust devils did occasionally develop in the fields southwest
of Vernon, they were very shallow features and very transient. They were
of totally different character from these early circulations. My hypothesis
regarding what was happening is that small-scale horizontal shearing
instabilities were present along the dryline just south of our position.
Several of these instabilities became coupled to boundary layer updrafts
which were feeding the congestus towers. The stretching associated with
these updrafts was enough to organize and intensify the instabilities into
a rather large "dust devil". The flow around this vortex was strong
enough to change the winds at our located to the east for about 30 seconds
as the vortex approached our position. We estimated maximum winds around
60 mph in the circulation. At this point, I am calling this phenomena a
"dust spout", because it appears to lie in the vortex spectrum
somewhere between a dust devil and a landspout. Similar dust spouts were
observed during VORTEX operations on 3 June 1995 near Dimitt, Texas. During
the early afternoon a series of congestus towers were located along the
dryline west and southwest of Dimitt. Dust devils were observed underneath
several of these towers, and it appeared that these vortices were associated
with boundary layer updrafts which were feeding into the cumulus congestus
located above them. Those circulations were also fairly large, and existed
in an environment with strong winds and large horizontal and vertical shear.
These dust spouts persisted for 5-10 minutes. Unfortuately, most of them
dissipated before Doppler On Wheels (DOW) could be set up to take radar
observations.
I find all of this interesting because it may yield some insights as to
how the boundary layer is organized to produce convectiion along the dryline.
I believe these processes are yet to be clearly understood, and I wonder
whether this type of phenomena can give us clues as to what is really happening
out there. I welcome any comments on these ideas.
Lou Wicker
Louis.Wicker@nssl.noaa.gov
These images were captured from a video of this dust spout. Look later
this week for a QuickTime movie made from the video.
Photo #1: Initial formation of the "dust spout". We are looking
south-southwest of Vernon.
Photo #2: About 1 minute later than the previous photograph, the dust
spout begins to widen
Photo #3: About 1 minute later than the previous photograph, the dust
spout is continuing to widen, and multiple vortex features are indicated
by the 3-4 plumes which extend well above dust bowl.
Photo #4: About 1 minute later than the previous photograph, the rotation
is at its most intense now.
Photo #5: About 3-4 minutes after previous photograph, looking northwest.
The dust spout has moved north of our position, and the congestus cloud
associated with the dust spout can be seen at the top center of the picture.