Chapter 11

• Thunderstorm- cloud, or cluster of clouds, that produces thunder, lightning, heavy rain, and sometimes hail and tornadoes. Associated with cumulonimbus clouds and unstable air.
• Overshooting Tops- can identify a thunderstorm where the top of a cloud may overshoot the tropopause and penetrate the stratosphere.
• Thunderstorms require warm, moist air, so they are most prevalent in regions with maritime Tropical (mT) air masses and in regions where mountains and frontal cyclones help lift air vertically.
• Areas with a position around the ITCZ and Africa have the most thunderstorm days. Within the US, Florida has the most thunderstorm days.
• Thunderstorms are triggered by various lifting mechanisms: Surface heating and along boundaries such as fronts.
• The Lifted Index is a stability index that is used to determine a numerical estimate of the severity of thunderstorm activity based on observations near and above the ground. It is computed by comparing the parcel and environmental temperatures at 500 mb. The more negative the index, the more likely thunderstorms will develop, with a –6 or less being when the chance of a severe thunderstorm is highest.
• Vertical Wind Shear, or the change of environmental wind speed and direction from the ground up, is another factor affecting thunderstorm type. Large amounts of shear cause thunderstorm clouds to tilt and become more severe storms.
• Nocturnal Low Level Jet and Capping Inversion are two other factors affecting thunderstorm growth and development.
• Thunderstorms can be classified by either their severity or their structure.
• The NWS (National Weather Service) defined a Severe Thunderstorm as a thunderstorm that produces one or more of the following: wind gusts of 50 knots or greater, hail 0.75 inches in diameter or larger, or a tornado.

Thunderstorm Structure

· Thunderstorms are composed of basic building block referred to as Cells. A cell is a compact region of a cloud that has a strong vertical updraft.

· There are two categories of cells: Ordinary Cells (few kilometers or miles in diameter and exist for an hour or less) and Supercells (larger and can last for several hours).

· Multicell storms are composed of lines or clusters of thunderstorm cells, either ordinary or supercells or both.

· Ordinary Single Cell Thunderstorms are the most common, but the others are responsible for a vast majority of severe weather.

· Ordinary Single Cell Thunderstorms are short-lived and localized single cell thunderstorms. An Air Mass Thunderstorm is an example.

· An individual thunderstorm has a life cycle with three distinct stages: the cumulus, the mature, and the dissipating stages.

· The Cumulus Stage is the initial stage of a thunderstorm where warm air near the ground rises and cools approaching saturation at the Lifting Condensation Level (LCL). Once saturated, Entrainment occurs which leads to different particle sizes and eventual collision and coalescence to form more droplets of both ice and water.

· The Mature Stage begins when precipitation starts to fall from the cloud. This is when the storm produces the most lightning, hail, and rain. Downdrafts begin to form here to counter the updrafts.

· The Dissipating Stage occurs when the updraft, which provides the required moisture for cloud development, begins to weaken and collapse. The downdrafts now dominate the updrafts and cumulonimbus clouds begin to disappear. With no updraft the precipitation ends.

· Multicell Storms are composed of several individual single cell storms, each one at a different stage of development: cumulus, mature, and dissipating, so they can last for several hours.

· Mesoscale Convective Systems (MCSs) are groups of thunderstorms that often join into larger systems. Two examples of MCSs are squall lines and Mesoscale Convective Complexes (MCCs). These are also examples of multicell storms.

· A Squall Line is composed of individual intense thunderstorm cells arranged in a line, or band. They occur along a boundary of unstable air, and have typical life spans of 6-12 hours, often occurring ahead of a cold front.

· A Gust Front is the boundary between the outflow of the cold downdraft of a thunderstorm and the warmer, more humid air around it.

· The Mesoscale Convective Complex (MCC) is another severe storm composed of multiple single cell storms, covering a large area (100,000 square kilometers). They are long lived and last for longer than 6 hours. They appear as a cluster of thunderstorms that look like a large circular storm. They often occur underneath a ridge of high pressure.

· For MCCs to exist and grow, the individual thunderstorms that comprise the system must support the formation of other convective cells

· Supercell Thunderstorms are single cell storms that almost always produce dangerous weather by initiating strong wind gusts, large hail, dangerous lightning, or tornadoes.

· The development of these supercell storms requires a very unstable atmosphere and strong vertical wind shear. Updrafts and downdrafts wrap around one another, and they last for several hours because updrafts and downdrafts do not interfere with one another.

Tornadoes

· A microburst is a strong, localized downdraft less than 4 km in diameter that sometimes develops underneath a thunderstorm as a result of evaporative cooling. They can be very dangerous around airports.

· Tornadoes are rapidly rotating columns or funnels of high wind that spiral around very narrow regions of low pressure beneath a thunderstorm. They nearly always rotate cyclonically and often move toward the northeast along with the parent thunderstorm.

· A Funnel Cloud is a tornado whose circulation does not extend to the ground.

· Tornadoes are usually very short lived and are less than 1 mile wide.

· No one knows exactly why tornadoes form, but most drop down out of supercell thunderstorms.

· Inside a tornado, the air initially rotates around a horizontal axis, however a vertical updraft within the storm then tilts this spinning air so that it spins in the vertical. This then turns into two columns of vertically spinning and rising air, one rotating clockwise, the other counterclockwise. The cyclonic part is called a Right-Mover and is usually the more intense part.

· Tornadoes exhibit a four-stage life cycle: Organizing Stage (funnel cloud picks up debris as it reaches the surface and widens), Mature Stage (tornado often at peak intensity and width), Shrinking Stage (funnel narrows), and Decaying or “Rope” Stage (funnel thins our to a very narrow, ropelike column and dissipates).

· Doppler Radar can reveal a Tornado Vortex Signature, which is a reliable indicator that a tornado is forming.

· The Fujita Scale estimates the winds of a tornado after the fact based on the damage caused by them on a scale of 0 to 5 (5 being most severe).

· Only about 1% of tornadoes reach the F4 or F5 categories, yet they account for two-thirds (67%) of all deaths by tornadoes.

· A Multiple Vortex Tornado is an especially damaging tornado that contains smaller spiraling whirlwinds inside the main funnel.

· Tornadoes occur in regions of the atmosphere that have extremely unstable air, large amounts of vertical wind shear, and weather systems such as fronts that force air upwards. Tornado Alley in the US has the highest frequency (Great Plains centered to TX, OK, and KS).

· Tornado season is based on when the ingredients for severe weather come together in a particular place, with tornado season moving north and south during the year with the polar jet.

· The most likely time for tornadoes is late afternoon or early evening, b/c they thrive on solar heating.

· A waterspout is a type of weak whirlwind that forms underneath cumulus clouds over a large body of water, often associated with sea breeze showers and not nearly as strong as a tornado.

Thunderstorm Severe Weather

· Lightning is a huge electrical discharge that results from the rising and sinking air motions that occur in mature thunderstorms.

· Lightning can travel from cloud-to-cloud, within the same cloud, or cloud-to-ground.

· Lightning is actually a series of flashes through a sequence of events. Cloud-to-ground lightning has the following sequence:

o Charges collect in the base of the cloud

o As negative charges build up near the base of the cloud, the ground repels negative charges and changes from its usual negative to a positive charge, Lightning formation has begun with the Pilot Leader (the initial discharge of negative charges near the cloud base).

o The Stepped Leader (attempt to establish a conductive channel from the cloud to the ground for the electrons to flow through and neutralize the charge difference between the cloud and ground) connects the cloud to ground.

o The return stroke (a brilliant flash of electrical current) surges upward from the ground.

· After the initial return stroke, negative charges from higher in the cloud move toward the ground and are called Dart Leaders.

· A Flood is a substantial rise in water that covers areas not usually submerged. A flood occurs when water flows into a region faster than it can be absorbed.

· A Flash Flood is a sudden, local flood that has a great volume of water and a short duration. Rainfall intensity and duration are two key elements of a flash flood, while topography, soil conditions, and ground cover also play key roles.

· Flash floods occur within about 6 hours of rain, while a flood is a longer term event and can last weeks or months.

· Hail is precipitation in the form of large balls or lumps of ice. They begin as small ice particles and grow primarily by accretion.

· Hail occurs when the strong updrafts and downdrafts in a thunderstorm cause repeated freezing of supercooled water on small ice particles.

· The Front Range of the Rocky Mountains is the most frequent area for hailstorms.

· The Hailshaft is the curtain of hailstones that falls below the cloud base.

· A Hailswath is a section of ground covered with hail.