Lectures on Thunderstorms and Tornadoes Chanh Q. Kieu Department of Atmospheric and Oceanic Science University of Maryland AOSC400, Fall 2008

Definition of thunderstorms Thunderstorms are rain showers with accompanied thunder. As thunder is a result of lighting, you can also define thunderstorms as rain showers with lighting (NOAA/NWS)

Thunderstorm lifecycle The typical lifecycle of a thunderstorm consists of three stages: towering, mature cumulus, and dissipating stage Towering cumulus cloud indicates lifting air. Such lifting may be due to the converging air associated with anomaly heating of the ground, or due to the upstream terrain Usually little if any rain during this stage. Lasts about 10 minutes. Occasional lightning

Thunderstorm lifecycle (cont’d) The air continues to rise up to the tropopause. This upper lid forces the air to be spread out, resulting in a “devil” shape. Most likely time for hail, heavy rain, frequent lightning, strong winds, and tornadoes. Storm occasionally has a black or dark green appearance. Lasts an average of 10 to 20 minutes but may last much longer in some storms in the absence of wind shear.

Thunderstorm lifecycle (cont’d) This stage signifies the dissipation of thunderstorms, and it dominates by the downdraft. Rainfall decreases in intensity. Can still produce a burst of strong downdraft near the surface, thus cutting off the inflows at lower levels. Lightning remains a danger.

Thunderstorm formation Three important ingredients for thunderstorm formation are: Moisture, Instability, and a lifting mechanism. Moisture sources: In North America, mostly transported by large scale flows from the Atlantic, Pacific, and Gulf of Mexico. SST plays an important role in controlling the moisture source Southeast US can access both sources of moisture. This partly explains for the large number of thunderstorms here

Thunderstorm formation (cont’d) Instability: Occurs when the air near the surface is heated, or when cold air aloft is migrating to some area with warm surface. Cumulus cloud is generated as the warm moist air rises, condensed under such instability Note that “cold” and “warm” here are in the context of potential temperature. The naive use of temperature will tell nothing as the air above will be always colder than below (why?)

Thunderstorm formation (cont’d) Lifting: Fronts: the boundary between cold air mass and warm air mass is an ideal place for the lifting to occur, as this is place where instability is met (CAPE?) Dry lines: boundaries between two air masses of different moisture. Differential heating. Heating efficiency over grass is not the same as that over paved streets. Terrains: wind blows over terrain will be forced to rise upstream.

Thunderstorm statistics

Thunderstorm classification Thunderstorms can be classified based on the numbers of principle updraft cores. The formation of different types of thunderstorm is caused by various combinations of above three conditions Single-cell: There is one updraft core, so-called “cell”. Such single cell is usually formed in isolation with the other cells. Very often, there are more than just one cell if the environment is favorable for the development singlecell

Thunderstorm classification Multi-cell cluster: Has more than one cell, formed by mergers of several storms. The multi-cell clusters are often associated with convective updrafts near mountain ranges or strong cold fronts. Sometimes, there is a “back building” storm upstream. multicell

Thunderstorm classification Multi-cell line (squall-line): multiple cells merge and form a line instead of a cluster. May be as long as several hundred miles, move very fast. squalline

Thunderstorm classification Supercell: a special kind of singlecell thunderstorm but stay as long as few hours. Have very organized and strong updrafts (100 mph) and downdraft cores. Extremely dangerous and responsible for nearly all of tornadoes supercell

Thunderstorm hazards Thunderstorms are particular dangerous because of their products during their developments including hails, strong wind, heavy rain, tornadoes. Hails are produced by strong updrafts, which carry the raindrops continuously back into the cloud.

Definition of tornadoes The most severe products of thunderstorms are tornadoes. By definition, a tornado is a violently rotating column of air extending from a thunderstorm to the ground.

Tornado characteristics Wind speeds between 40 mph and 110 mph Radius varies from few to hundreds feet. Some stretch more than a mile across, and stay on the ground for dozens of miles.[[ Some attain wind speeds of more than 300 mph (480 km/h), Often travel a few miles before dissipating. Rotate counterclockwise in NH. Opposite in SN (Is Coriolis a good explanation?)

Tornado scales The Fujita (F) Scale is the most commonly used scale for rating the damage of tornadoes, which is based on damage left behind by a tornado. There are some disadvantages of the F scale, and a recently revised scale, so-called Enhanced Fujita (EF) Scale seem to make improvements to the original F scale. This EF Scale has replaced the original F scale, which has been used to assign tornado ratings since 1971.

Tornado formation Before thunderstorms develop, vertical wind shear creates an invisible, horizontal spinning effect in the lower atmosphere Updraft within the thunderstorm tilts the rotating tube from horizontal to vertical. An area of rotation of few miles wide (mesocyclone) is formed within the storm. Most tornadoes form within this area of strong rotation

Tornado classification There are four basic types of tornadoes: Multiple vortex tornado: a type of tornado in which two or more columns of spinning air rotate around a common center, very often observed in intense tornadoes. These vortices often create small areas of heavier damage along the main tornado path.

Tornado classification Satellite tornado: a weaker tornado which forms very near a large, strong tornado. The satellite tornado may appear to rotate around a larger tornado.

Tornado classification Waterspout: simply as a tornado over water. Can be divided into two smaller types: fair weather waterspout and tornado waterspout

Tornado classification Landspout: a tornado over the ground that is not associated with a mesocyclone. Share many defining characteristics, including relative weakness, short lifespan, and a small, smooth condensation funnel with fair weather waterspout

Tornado statistics

References