Lecture #14: 
Atmospheric Stability (Continued) and its Applications;
Thunderstorm Development
Monday, 19 February 2001

Text Reading for Lecture #14
Cloud Development (Read pages 167-173)
Severe Thunderstorms (385-395)

CLOUD AND THUNDERSTORM
DEVELOPMENT

Mechanisms of cloud formation:

  Note how the clouds form from discrete
"parcels" or thermals of air rising.  This is why cumulus
clouds grow in "spurts".

For an atlas of clouds, click HERE.

Orographic uplift and rain shadowing.

Consider the early stages of thunderstorm development
and how the environment changes with time:

SEVERE THUNDERSTORMS

The National Weather Service defines a severe
thunderstorm as containing the following:

a.  3/4-inch hail and/or
b.  Surface wind gusts of 50 kts (58 mph) or
c.  A tornado

Recall the three stages of an airmass storm and
the built-in self-destruct mechanism:

Severe storms tend to be much longer-lived, highly
organized, and dependent upon specific
environmental conditions, especially winds that
increase in speed with height and change from
southeasterly and southerly direction at low levels
to southwesterly and westerly aloft, as shown below:

Also, as noted in our previous discussions, the
intensity of convective storms depends upon the
Convective Available Potential Energy (CAPE),
which represents the integrated temperature
excess of a rising air parcel relative to the
environment.

Types of organized convective storms:

Multicells
Squall lines
Bow echoes
Supercells
Mesoscale convective complexes and systems
Hurricanes

We'll look at each separately and compare and
contrast them.  Then, we'll examine precipitation
processes as well as hail, lightning, and other
intense weather produced by severe convective
storms.

MULTICELL STORMS

- Consist of many ordinary cells in various
  stages of their life cycle
- Can be highly organized
- Can last for a few hours
- Typically produce hail, strong winds, and lightning

Multicell storms are highly organized and

Snapshot of a multicell storm

Time sequence of a multicell storm

Schematic and photo of a multicell storm

Photograph of a multicell storm

Multicell storm on radar

Thunderstorm motion is governed by several
factors, including the environmental winds.
The motion of multicell storms is very complicated
because of the combined effects of cell growth,
cell movement, and movement of the area
overall, as illustrated below.

Motion of multicell storms

Multicell storms differ from airmass storms
because of the vertical wind profile, especially
in relation to the gust front (see Figure 15.9 in
the book for a photo of a gust front shelf
cloud).

Airmass storms -- winds uniform with height

Multicell storms - low-level winds oppose the gust front
and trigger new cells along the cold outflow boundary.

Flow structure along the gust front

Animation of cell generation along gust front
(numerical simulation by Prof. Robert Fovell,
UCLA, using the OU ARPS model)

SQUALL LINES

- Highly organized, long-lived lines of convective
   storms
- The most intense region of convection tends to
  be rather narrow, and the line can extend for
  hundreds of miles

- Typically form along or ahead of cold fronts or
   drylines

 

- Gravity waves generated by a front
  can trigger squall lines tens of miles ahead of
  the front, as shown below.  Such lines are called
  pre-frontal squall lines (page 390 on text).

- Typically contain numerous individual cells, but
  can contain a few supercell storms

- Rarely produce tornadoes, except at the southern
   end, where mutual interference of cells is
   minimized. 
- Often exhibit a region of trailing stratiform
  precipitation that can extend backward, behind
  the main convective area, for 100+ miles
- Leading edge exhibits strong gradients in radar
  reflectivity (precipitation rate)

- Long-lived squall lines require sufficient CAPE as
  well as specific wind profiles, as shown below (include
  Ming Xue plots, animations, old spotters series images,
  gustnadoes)