Thunderstorms

Thunderstorms are convection storms comprising cumulonimbus clouds that are accompanied by lightning and thunder and moderate to heavy precipitation. Additional attributes may include strong winds, hail, and/or tornadoes. In terms of the hierarchy of atmospheric scales of motion, thunderstorms are classified as mesoscale systems, the second smallest after microscale systems. Mesoscale systems have a typical life span of 1 hr. (hour) or more and range in size from a few kilometers to 100 km (kilometers) in diameter. Based on system duration, structure, and formation, thunderstorms are classified as ordinary cell, multicell, or supercell and may or may not be considered severe.

Also known as air mass thunderstorms, ordinary cell thunderstorms have the simplest structure and formation conditions of the three major thunderstorm categories. These storms materialize within warm, humid, and unstable air mass environments amid often weak vertical wind shear (i.e., wind speed and direction do not change greatly with height). Ordinary cell thunderstorms are usually compact (<1 km in width), nonsevere, and not associated with mechanical lifting mechanisms (e.g., frontal uplift). Ordinary cell thunderstorms typically undergo a three-stage sequence of development: cumulus, mature, and dissipating. The cumulus stage is characterized by rising air parcels or thermals that result from differential surface heating. Updrafts dominate this growth stage, and lightning and thunder are absent. As the cloud particles enlarge and become heavier, the updrafts may not be sufficiently strong enough to sustain their suspension, and the cloud particles begin to fall as precipitation. The manifestation of a precipitation downdraft ushers in the mature stage, the most intense phase typified by strong updrafts and downdrafts, heavy precipitation, lightning and thunder, and gusty surface winds. The air mass thunderstorm begins to dissipate as the warm, humid air source inflow supplying the thunderstorm's energy is suppressed by the precipitation downdraft. The final stage is marked by light precipitation, weak downdrafts, and cloud evaporation. Since the entire development cycle may be completed in less than 1 hr., these scattered storms may be referred to as “popup showers,” for their ability to mature and disperse quickly. Of the approximately 50,000 thunderstorms that occur each day across the world, most are of this ordinary cell variety.

Multicell thunderstorms contain several ordinary thunderstorm cells under various development stages, appearing as a continuous, long-lasting storm. A single thunderstorm cell may initiate new cell development or “daughter cells” along the precipitation outflow boundaries (known as gust fronts) and form a larger cluster of thunderstorms. If the thunderstorms align linearly, this configuration is known as a squall line. Not all squall lines, however, are formed along gust fronts. Frontal and prefrontal squall lines are longer lasting than their ordinary squall line counterparts and are frequently associated with severe weather. Fast-moving squall lines may also produce especially enduring and widespread wind storms called derechos, which can produce wind speeds in excess of 100 mph (miles per hour).

Instead of a linear configuration, multicells may also cluster and expand over an extensive [Page 2831]region and intensify. Mesoscale Convective Complexes (MCCs) are long-lasting (>12 hrs. or more), nocturnal multicell storm systems that develop in high-moisture environments with weak vertical wind shear, a situation common during the midlatitude summer on the tropics. These slow-moving storms are thousands of times larger than ordinary cell thunderstorms and can produce copious amounts of precipitation. In the maritime tropics, a concentrated thunderstorm cluster around a rotation center may signify the potential for hurricane development.

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