ATMO 171 Introduction to Meteorology and Climatology

11/8/99

THUNDERSTORMS (cont'd)

Consider an ordinary or "air-mass" thunderstorm in the mature stage (as pictured in Figure 12.1 in Danielson). This is the stage where all the action occurs - heavy rain, lightning, thunder, strong winds. How long this stage lasts will depend on several factors, including how much moisture is in the air at the surface, how unstable the environment is, and how the winds at the upper levels interact with the storm. These factors will determine how strong the updrafts and downdrafts become. The presence of an updraft and a downdraft withing the mature thunderstorm together is called a "cell".
The amount of water vapor in the air is measured in terms of the dew point temperature. The stability of the environment is determined from measuring the environmental lapse rate (i.e. change in air temperature with increasing height). When the surface dew point temperatures are high, and the environmental lapse rate is conditionally unstable, heating of the surface can set off a thunderstorm. The winds aloft can aid in thunderstorm formation as follows: If the horizontal wind speed increases with increasing height, we call this vertical shear. When shear is present, the cloud tops will be carried downwind by the faster winds aloft, and the whole thunderstorm cloud will tilt in the downwind direction. When this occurs, the updraft within the mature stage cloud becomes tilted as well (i.e. not oriented straight up-and-down).
A tilted updraft in a mature stage thunderstorm, caused by vertical shear in the winds, allows precipitation particles to escape the updraft region in which they formed and start to fall. The tilt also helps prolong the storm by moving the cold downdraft and surface gust front out ahead of the storm. This prevents the downdraft from "shutting off" the warm, buoyant air mass that feeds the thunderstorm. The gust front can also be important for setting off additional thunderstorm cells, as we will see later. [Note: while vertical shear aids in thunderstorm formation, too much shear (i.e. really strong winds at upper levels) can actually blow off the tops of the thunderstorms before they reach the mature stage. Therefore, a little bit of wind shear is good, but too much can actually hinder thunderstorm development.)
When and where are thunderstorms most likely to occur?

As discussed above, you need a specific set of meteorological conditions to occur in order for thunderstorms to form. These conditions (warm, moist air masses, strong daytime heating of the surface, conditionally unstable lapse rate) are often found over the southeastern portion of the United States in summer. It is no surprise that thunderstorms are most common over this part of the country (see Figure 12.5) Thunderstorms are also prevalent over portions of the Rocky Mountains in Colorado, New Mexico, and southeastern Arizona. These thunderstorms are triggered by strong daytime heating in the summer, and are aided by orographic lifting. The Great Plains also see a large number of thunderstorms every summer, and many actually form at night. These nighttime thunderstorms are triggered by convergence of air near the surface (another lifting mechanism, in addition to buoyant, orographic, and frontal lifting).
Worldwide, thunderstorms are common anywhere that large land masses are subject to strong daytime heating in the presence of moist, unstable air masses. A prime example of this is the Indian monsoon. The massive Indian subcontinent extends far south into tropical regions where, for most of the years, Typical wind patterns direct air flow from the land offshore towards the ocean. In summer, the sun is directly overhead. This intense daytime heating causes large scale rising motions; these motions lead to low pressure over the land (a "thermal low"). The resulting pressure gradient draws in warm, moist air from the oceans, and wind flow pattern becomes reversed (this reversal is what many use to definr "monsoon"). When heated, this moist air mass sets off thunderstorms on a continental scale and brings much needed rain to India and parts of Asia.
Arizona has its own, much smaller, version of the monsoon. It is so small compared to the Indian monsoon that some people object to even calling it a monsoon. In our case, the hot summer days establish a thermal low over the desert. Moist air is directed toward Arizona from the Gulf of Mexico and the Gulf of California by high pressure centers over the Atlantic and Pacific (the "Bermuda" and "Pacific" highs). Combining the hot daytime temperature, moist air masses, and orographic lifting gives us our summer rainy season in July and August.
Finally, another global feature of thunderstorm activity is the InterTropical Convergence Zone. As the name implies, it is an area of stong converging motions near the surface of the earth. This region is located somewhere between the Tropic of Cancer and the Tropic of Capricorn, and it moves north and south following the sun's path throughout the year. This region is sometimes visible on global satellite images as a nearly continuous belt of thunderstorms circling the planet. More about this when we discuss the global atmospheric circulation.

MULTI-CELL THUNDERSTORMS, FRONTAL SQUALL LINES, AND MESOSCALE CONVECTIVE COMPLEXES

11/10/99
Even under the most favorable conditions, a single ordinary or "air-mass" thunderstorms only last about one hour or so. Often, however, we can observe thunderstorm activity to last all afternoon in the summer around Tucson. In the Midwestern states, it is common for thunderstorms to cover an entire state at once. How does this come about? To explain this, we examine how thunderstorms can form in clusters.
When thunderstorms are observed to form in groups, they can be categorized as follows:

Multicell or "daughter cells": Lines of propagating thunderstorms can be set off by one or more ordinary thunderstorms in the mature stage. The gust front can be strong enough to displace the surface air ahead of the storm, forcing the surface air to rise. This rising motion can trigger cumulus growth and start the thunderstorm life cycle over again. The newly formed storm is referred to as a "daughter cell". Daughter cells can also be triggered when the downdrafts of two adjacent mture thunderstorms meet at the surface.

Frontal squall lines: A mass of cold, dense air advancing on warm, less dense air can trigger lifting of air parcels at the frontal boundary. If enough moisture is present, this lifting can set off a frontal thunderstorm. This storm will move along with the advancing frontal boundary. The temperature contrasts associated with the front maintain the storms updrafts and downdrafts, allowing it to remain in the mature stage for a long time. Strong winds can push the storms far ahead of the front; in this case the line of thunderstorms triggered by the front is referred to as a pre-frontal squall line. Wave motions set off by air flowing over the cold, dense air can also trigger rising air motions far ahead of the front, which is another way pre-frontal squall lines can form. These processes are summarized in Figure 12.18.

Mesoscale convective complexes: This term refers to clusters of thunderstorms that become organized and move as a group growing to sizes of up to 400 kilometers (~250 miles) across. These features are most pronounced at nighttime when low level convergence and upper level divergence are present. MCC's deliver much needed rain to the Midwest during the growing season, but they are also capable of producing strong winds, hail, and flooding which cause considerable damage. About the the only thing that produces more damage are tornadoes, which we will discuss next

11/12/99
SUPERCELLS, MESOCYCLONES, AND TORNADOES

The type of thunderstorms that are responsible for producing most of the severe weather (i.e. strong winds, flooding, hail, tornadoes, etc.) in the United States are referred to as supercells. A supercell thunderstorm consists of one single storm that reaches immense size and intensity. What sets off supercells from other types of thunderstorms is that their great size and intense updraft region allows the storm to acquire its own rotation. This rotation within a supercell is referred to as a mesocyclone. The mesocyclone within a supercell thunderstorm is key to the formation of most tornadoes.
Figure 12.22 in Danielson illustrates the structure inside a typical supercell. Note that there is a strong updraft region that feeds into the msocyclone, and two downdraft regions. The storm as a whole moves according to the wind flow. The downdraft at the leading edge of the storm produces much of the precipitation from the supercell. Most tornadoes tend to form near the rear downdraft region.
Where does the mesocyclones's rotation come from? As Figure 12.24 illustrates, it can come from the overall rotation of the environment if the storm is located near the center of low pressure. [Remember that in the Northern Hemisphere winds flow counter-clockwise aorund a surface low pressure center]. Another source of rotation arise when there is strong vertical shear in winds near the surface. Converginf winds fron different directions can form a "tube" of spinning air; when this tube comes into contact with a strong updraft, it can become twisted into a vertical column of rotating air. Generally speaking, the mesocyclone rotates conunterclockwise beacuse that is the direction of the large scale rotation of the air in which the storm is moving. However, it is possible (although very rare) to see mesocylclones and tornadoes roate clockwise.

Funnel Clouds and Tornadoes

Vortex: An area of intense rotation within a column of fluid (fluid = liquid or gas); a vortex is 3-dimensional Example: water going down a drain; when a column of fluid is STRETCHED, it acquires ROTATION
Funnel Cloud: An example of a vortex in the atmosphere. It is associated with STRONG UPWARD MOTION which STRETCHES a column of air and gives it ROTATION.
In theory this rotation can be clockwise or counterclockwise; clouds are observed to rotate the counterclockwise direction because they form in updraft regions of a mesocyclone. "Funnel" shape comes from fact that speed of rotation (angular velocity) increases as you go higher up Air pressure at center of funnel cloud is EXTREMELY LOW; very strong surface inflow replaces rapidly rising air Funnel cloud's dark appearance is due to condensation of water vapor in rapidly rising (expanding, cooling) air parcels; also, dirt and debris from surface swept up into funnel.
Tornado: A funnel cloud that comes into contact with the ground is called a TORNADO
Steps in tornado formation:

Mesocyclone forms within supercell – rotation
Wall cloud - dark, slowly rotating cloud descending from base of supercell directly below mesocyclone
Funnel cloud appears from wall cloud and grows downward towards the surface
Tornado - funnel cloud touches down at the surface. Suction vortices (small-scale circulation rotating around main funnel cloud) cause most of the damage.

Fujita Scale: Ranks strength of tornado by amount of damage on the ground. Goes from F0 (light; very little structural damage) up to F5 (total devastation; nothing left standing)
Tornado (or Severe Storm) Watch: No tornadoes or severe weather has been reported, but conditions are favorable for their formation (unstable sounding; warm moist air at surface, cold dry air aloft; passage of front, etc.) Be prepared to take shelter if necessary.
Tornado (or Severe Storm) Warning: Tornadoes or severe weather have been observed in the area. Take shelter immediately!