Thunderstorms and Severe Weather. (Chapt 15)

Transcription

1 Thunderstorms and Severe Weather (Chapt 15)

2 The Big Picture We ve emphasized horizontal transport of energy to balance the planetary energy budget: Hadley Cell Subtropical divergence Midlatitude cyclones and conveyor belts What about vertical motion? Up-warm, down cold Up moist, down-dry Severe weather is all about vertical motion, and represents local release of energy that contributes to planetary energy balance

3 Atmospheric Thermal Structure Heated from below by latent and sensible heat fluxes, in stratosphere by ozone absorption Thunderstorms (esp in tropics) provide the energy to mix the troposphere, determine height of tropopause

4 How is Energy Transported to its escape zones? Both atmospheric and ocean transport are crucial Buoyancy-driven convection drives vertical transport Latent heat is at least as important as sensible heat

5 Thunderstorms Occur: where moist air is lifted in an unstable environment Near fronts In advance of large scale upper air troughs In warm air sector of mid latitude cyclones Sometimes isolated without any synoptic scale support In association with surface convergence In an unstable environment Where surface heating or cooling aloft destabilizes the atmosphere More frequently in moist environments

6 Summary of Cyclone Weather Roles of convergence and divergence aloft Pattern of clouds, precipitation, and temperatures on the ground

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8 Three Stages of Thunderstorm Development Cumulus Stage Mature Stage Dissipating Stage

9 Cumulus Stage Starts with a warm plume of rising air. The updraft velocity increases with height. Entrainment pulls outside air into the cloud. Supercooled water droplets are carried far above freezing level.

10 Mature Stage The heaviest rains occur. The downdraft is initiated by frictional drag of the raindrops. Evaporative cooling leads to negative buoyancy. The top of the cloud approaches tropopause and forms anvil top.

11 Dissipating Stage The downdraft takes over entire cloud. The storm deprives itself of supersaturated updraft air. Precipitation decreases. The cloud evaporates.

12 Thunderstorm Sounding Temperature and dew point have typical vertical profile in the warm sector before a thunderstorm occurs, including the shallow inversion at 800 mb that acts like a cap on the moist air below. The cold dry air above warm humid air produces convective instability

13 Thunderstorm Climatology

14 Hail Climatology

15 Severe Thunderstorms Figure 15.5

16 Severe Thunderstorm Structure

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18 Gust Front Roll Clouds Turbulence in the fast moving gust front will spawn eddies and possibly roll clouds beneath the shelf cloud. These clouds spin about a horizontal axis near the ground.

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24 Flying into a Microburst Figure A pilot flying into a microburst must anticipate sudden and strong changes in wind direction and speed. Initially a headwind is encountered that lifts the plane, followed by a strong downdraft, and when leaving the storm a tailwind causes a loss of altitude.

25 Multicell Storms Cool downdrafts leaving a mature or dissipating storm may offer relief from summer heat, but they may also force surrounding, low-level moist air upward. Hence, dying storms often trigger new storms, and a line of storms may propagate downwind

26 Squall line Multicell Thunderstorm

27 Vorticity from Horizontal to Vertical Figure Figure Spinning horizontal vortex tubes created by surface wind shear may be tilted and forced in a vertical path by updrafts. This rising, spinning, and often stretching rotating air may then turn into a tornado.

28 Supercell Storm Mesocyclone of rotating winds formed by tilting of horizontal vorticity upwards

29 A Supercell Storm

30 Cluster of Thunderstorm Supercell

31 Tornado A rapidly rotating column of air often evolve through a series of stages, from dust-whirl, to organizing and mature stages, and ending with the shrinking and decay stages. Winds in this southern Illinois twister exceeded 150 knots.

32 Tornado Conditions Strong winds aloft (> 25,000 ft) Often associated with leading edge of trough Cool, dry air at mid levels (10,000 ft) Wind typically out of the south west Warm humid air at low levels (5,000 ft) Strong gusty winds typically out of the south to south, south-west

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34 Actual vertical structure Upper level low is tilted westward with height with respect to the surface. UPPER LEVEL DIVERGENCE INITIATES AND MAINTAINS A SURFACE LOW.

35 Atmospheric Conditions for Tornadoes Open-wave mid-latitude cyclone juxtaposes cold dry air and warm moist air at the surface Warm moist air around 850 mb is overrunning the cold air (warm frontal lifting), forming a capping inversion At 700 mb, cold dry air flows north and north east, providing a cold environment for growing cumulus clouds An approaching upper-level trough to the west of the surface low provides upperlevel divergence, enhancing lifting

36 Tornado Occurrence Tornadoes from all 50 states of the U.S. add up to more than 1000 tornadoes annually, but the highest frequency is observed in tornado alley of the Central Plains. Nearly 75% of tornadoes form from March to July, and are more likely when warm humid air is overlain by cooler dryer air to cause strong vertical lift.

37 Tornado Wind Speed Figure As the tornado moves along a path, the circular tornado winds blowing opposite the path of movement will have less speed. For example, if the storm rotational speed is 100 knots, and its path is 50 knots, it will have a maximum wind of 150 knots on its forward rotation side.

38 Suction Vortices & Damage A system of tornadoes with smaller whirls, or suction vortices, contained within the tornado is called a multi-vortex tornado. Damage from tornadoes may include its low pressure centers causing buildings to explode out and the lifting of structures. Human protection may be greatest in internal and basement rooms of a house. Figure 15.32

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46 Fujita Tornado Intensity Scale F-0 Gale tornado (40-72 mph): Some damage to chimneys; breaks branches off trees; pushes over shallow-rooted trees; damages sign boards. F-1 Moderate tornado ( mph): The lower limit is the beginning of hurricane wind speed; peels surface off roofs; mobile homes pushed off foundations or overturned; moving autos pushed off the roads; attached garages may be destroyed. F-2 Significant tornado ( mph): Considerable damage. Roofs torn off frame houses; mobile homes demolished; boxcars pushed over; large trees snapped or uprooted; light object missiles generated. 1 of 2

47 Fujita Tornado Intensity Scale F-3 Severe tornado ( mph): Roof and some walls torn off well-constructed houses; trains overturned; most trees in forest uprooted. F-4 Devastating tornado ( mph): Well-constructed houses leveled; structures with weak foundations blown off some distance; cars thrown and large missiles generated. F-5 Incredible tornado ( 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; trees debarked; steel-reinforced concrete structures badly damaged. F-6 Inconceivable tornado ( mph): These winds are very unlikely. The small area of damage they might produce would probably not be recognizable along with the mess produced by F-4 and F-5 wind that would surround the F-6 winds. Missiles, such as cars and refrigerators would do serious secondary damage that could not be directly identified as F-6 damage. 2 of 2