Transcript Meteorology 1014 - University of Oklahoma

Supercell Thunderstorms
Part II
Adapted from Materials by Dr. Frank Gallagher III
and Dr. Kelvin Droegemeier
School of Meteorology
University of Oklahoma
1
Supercell Updraft Rotation
In order for supercells to rotate, there
must be some type of rotation already
available in the environment.
 We shall consider several different ways
of creating vertical vorticity or rotation
about a vertical axis:

2
Convergence

Consider your sink. Initially there is
some weak rotation, but as the water
converges toward the drain, the speed
of the rotation increases.
This is similar to the way an ice skater speeds up when their arms are pulled in.
3
Horizontal Temperature
Differences

Air travelling along a frontal zone will develop
a horizontal rotation.
4
Recall the Cold-Air Outflow
5
Vertical Wind Shear

Another method of creating horizontal
rotation is by vertical wind shear.
Fast Wind
Slower Wind
6
Vertical Wind Shear
Up
Westerly Winds
Increase in Speed
with height
North
East
7
Development of Rotation
Up
North
East
8
Tilting

In order to create vertical rotation from
horizontal rotation, we must tilt the
horizontal rotation into the vertical.
9
Development of Rotation
Thunderstorm
Up
North
East
10
Development of Rotation
Updraft - Stretch
Up
North
East
11
Tilting

In thunderstorms, this tilting is achieved
by the updraft.
Updraft
12
Tilting

Viewed from above, we see a pair of
counter-rotating vortices:
“Positive Rotation”
“Negative Rotation”
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Tilting
Vortex Tube
Updraft
Play Movie
© 1990 *Aster Press -- From: Cotton, Storms
14
Supercell Rotation
In supercells, the updraft usually spins
only in one direction (usually CCW).
 Because of the environmental shear,
the updraft is enhanced on the southern
flank of the storm.
 The CCW rotation is typically found on
the southern flank and is favored if the
storm moves toward the south (inflow
along vortex lines)
 The northern flank rotation is not
favored and usually is weak.

15
Importance of Storm-Relative Winds
Want to intensify
the cyclonic vortex
on the south side
Vortex Tube
Updraft
Play Movie
© 1990 *Aster Press -- From: Cotton, Storms
16
Importance of Storm-Relative Winds
Want to intensify
the cyclonic vortex
on the south side
Vortex Tube
Updraft
Storm-Relative
Winds
Play Movie
© 1990 *Aster Press -- From: Cotton, Storms
17
Importance of Storm-Relative Winds
Storm-Relative
Winds
Vortex Tube
Updraft
Play Movie
© 1990 *Aster Press -- From: Cotton, Storms
18
Importance of Storm-Relative Winds
Vortex Tube
Storm-Relative
Winds
Updraft
Play Movie
© 1990 *Aster Press -- From: Cotton, Storms
19
Importance of Storm-Relative Winds
Vortex Tube
Storm-Relative
Winds
Updraft
Play Movie
© 1990 *Aster Press -- From: Cotton, Storms
20
Importance of Storm-Relative Winds
We obtain strong updraft rotation if the storm-relative
winds are parallel to the horizontal vorticity – or perpendicular
to the environmental shear vector – this is easily determined
via a wind hodograph
Vortex Tube
Storm-Relative
Winds
Updraft
Play Movie
© 1990 *Aster Press -- From: Cotton, Storms
21
Vertical Wind Shear
Up
Westerly Winds
Increase in Speed
with height
North
East
22
Vertical Wind Shear
Up
Shear = V(upper) – V(lower)
North
East
23
Vertical Wind Shear
Up
Shear = V(upper) – V(lower)
North
East
24
Vertical Wind Shear
Up
Shear = V(upper) – V(lower)
Shear Vector
East
25
Development of Rotation
Up
Note that the vorticity
vector points 90 deg to
the left of the shear vector
North
Shear Vector
East
26