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Thunderstorm and Tornado Lecture Thunderstorms Thunderstorms occur where you have strong lifting of air parcels: • Near fronts or in advance of large scale upper air troughs • In an unstable environment – Summer time surface heating destabilizes the atmosphere More frequently in moist environments Thunderstorms are a special case of the cumulonimbus cloud – only becomes a thunderstorm in the presence of lightning Generally require instability in the atmosphere Instability re-visited Three stages of Thunderstorm Development • Cumulus Stage • Mature Stage • Dissipating Stage Cumulus Stage Starts with a warm plume of rising air. Condensational heating causes air to remain warmer than the surrounding environment. Water droplets are carried above freezing level. Mature Stage The top of the cloud approaches the tropopause and forms an anvil top. A downdraft is initiated by frictional drag of increasingly large raindrops. Entrainment brings in dry air: leads to evaporation of falling particles. Evaporative cooling leads to negative buoyancy, stronger downdraft. 1 Hail Dissipating Stage • Hail can form in clouds with – High supercooled liquid water content – Very strong updrafts The downdraft takes over the entire cloud and cuts off the supply of warm, humid air from below. • Hailstones associated with deep and intense cumulonimbus Precipitation decreases/the cloud evaporates. – Typically make 2-3 trips up through cloud – Layering tells about hailstone history Such single cell storms are short lived, and cause relatively little damage. The Coffeyville hailstone (5.5”, 1.7 lbs) Note the density layering What leads to more severe thunderstorms? Low-level winds bring warm, moist air from the southeast Downdrafts often lead to development of more storms (multicell storms). Severe thunderstorm schematic - above Severe Thunderstorm/Tornado frequency Why do we have so many tornadoes in the US? Upper-level winds bring colder, dryer air from the polar regions Net effect – warm moist air underneath colder, dryer air. This is a good start. Even more important – wind profile as you go up with height is ‘pushing’ precipitation away from the warm moist inflow – this helps maintain the storm 2 Thunderstorm motion What causes the storm to rotate? • Varying theories, but the predominate theory is one of vertical stretching of a surface vortex tube • A vortex tube is simply a ‘tube’ of fluid that is being rotated horizontally by the low-level wind shear • The convection this picks this tube up, and stretches it into the vertical – thus causing rotation around the vertical axis A source of the spin As the vortex tube is stretched, it becomes vertical, creating rotation around a vertical axis The precipitation may at some point bisect the vortex tube, creating two oppositely rotating updrafts This is seen as the thunderstorm splitting, creating two new thunderstorms; one we call the right-mover, the other, the leftmover Vertical stretching and conservation of angular momentum will intensify the rotation of the dominant storm, which can create a mesocyclone and perhaps even a tornado Tornado Conditions • Strong winds aloft (> 25,000 ft) – Often associated with leading edge of trough • Cool, dry air at mid levels (~500-mb) – Wind typically out of the south west • Warm humid air at low levels (~700-mb) – Strong gusty winds typically out of the south to south, south-west • Rotation of the parent storm due to wind shear for supercell tornadoes 3 When do Tornadoes occur? • Tornadoes can occur anytime when you have severe thunderstorms (including hurricanes). • Most large-scale outbreaks of tornadoes occur in the spring , but can also occur throughout the summer thunderstorm season Simulation of a tornadic supercell thunderstorm • Storm occurred on the 20th of May, 1977 near Del City, Oklahoma • What you’re about to see is a computer-generated numerical model of this thunderstorm, generated by the environmental conditions that existed on the 20th of May • Names have been changed to protect the innocent Gratuitous Tornado Photos • Sequence of tornado formation from mesocyclone, taken on 30 May 1996 by Greg Thompson (former CSU grad student, now scientist at NCAR) • Storm occurred near Elba, CO • Don’t try this at home, kids Fujita Tornado Intensity Scale F-0 (40-72 mph): Breaks branches off trees; damages sign boards. F-1 (73-112 mph): Peels surface off roofs; mobile homes pushed off foundations or overturned; moving autos pushed off the roads. F-2 (113-157 mph): Roofs torn off frame houses; mobile homes demolished; large trees snapped or uprooted. F-3 (158-206 mph): Roof and some walls torn off well-constructed houses; trains overturned; most trees in forest uprooted. F-4 (207-260 mph): Well-constructed houses leveled; structures with weak foundations blown off some distance; cars thrown and large missiles generated. F-5 (261-318 mph): Strong frame houses lifted off foundations and carried considerable distances to disintegrate; automobile sized missiles fly through the air in excess of 100 meters; steel-reinforced concrete structures badly damaged. 1 of 2 4