Thunderstorms and Tornadoes Chapter 14
Thunderstorms A storm containing lightning and thunder convective storms Severe thunderstorms (NWS def) one of following: large hail - ¾ in dia Surface wind gusts greater than or equal to 50kts(58mph) or produces a tornado Ordinary Cell Thunderstorms, usually simple, pop up Air-mass thunderstorms: limited wind sheer May form at sea breeze fronts, topographic irregularities, outflow boundaries Stages: cumulus, mature, dissipating
Stages of Development Simplified model depicting the life cycle of an ordinary cell thunderstorm that is nearly stationary as it forms in a region of low wind shear. Mature Stage - As cloud droplets get heavier, they fall trough drier air, causing it to cool, and become denser downdraft process is called entriainment, and is enhanced by falling precipitation. Dissipating Stage updrafts have weakened and downdrafts dominate no longer fueled
Multi-cell Thunderstorms Thunderstorms that contain a number of convection cells, each in a different stage of development, moderate to strong wind shear Form when there is moderate to severe vertical wind shear Causes the convection cell to tilt, with updraft riding over downdraft Precip does not fall into updraft, so, fuel not cut off, and it lasts much longer Gust Front forms out ahead, with possible a shelf cloud
Figure 14.7 A dramatic example of a shelf cloud (or arcus cloud) associated with an intense thunderstorm. The photograph was taken in the Philippines as the thunderstorm approached from the northwest.
Figure 14.8 A roll cloud forming behind a gust front.
Figure 14.9 Radar image of an outflow boundary. As cool (more-dense) air from inside the severe thunderstorms (red and orange colors) spreads outward, away from the storms, it comes in contact with the surrounding warm, humid (less-dense) air, forming a density boundary (blue line) called an outflow boundary between cool air and warm air. Along the outflow boundary, new thunderstorms often form.
Micro-bursts: localized downdraft(downburst) that hits the ground and spreads horizontally in a radial burst of wind 4km or less outward generate wind shear - rapid change in wind speed and direction Figure 14.11 Flying into a microburst. At position (a), the pilot encounters a headwind; at position (b), a strong downdraft; and at position (c), a tailwind that reduces lift and causes the aircraft to lose altitude.
Squall-line thunderstorms line of multi-cell thunderstorms pre-frontal squall-line, form out ahead of advancing cold front
Pre-frontal squall-line thunderstorms may form ahead of an advancing cold front as the upper-air flow develops gravity waves downwind from the cold front.
Figure 14.14 A model describing air motions and precipitation associated with a squall line that has a trailing stratiform cloud layer.
Figure 14.15 A side view of the lower half of a squall-line thunderstorm with the rear-inflow jet carrying strong winds from high altitudes down to the surface. These strong winds push forward along the surface, causing damaging straight-line winds that may reach 100 knots. If the high winds extend horizontally for a considerable distance, the wind storm is called a derecho.
Figure 14.16 The red and orange on this Doppler radar image show an intense squall line moving south southeastward into Kentucky. The thunderstorms are producing strong straightline winds called a derecho. Notice that the line of storms is in the shape of a bow. Such bow echos are an indicator of strong, damaging surface winds near the center of the bow. Sometimes the left (usually northern) side of the bow will develop cyclonic rotation and produce a tornado.
Meso-scale Convective Complex( MCC) a number of individual multi-cell thunderstorms grow in size and organize into a large circular convective weather system Tend to form in summertime, with weak upper level winds Low level jet brings in moisture, and is at max late at night, early morning May cover entire states - 100,000km 2 Figure 14.17 An enhanced infrared satellite image showing the cold cloud tops (dark red and orange colors) of a Mesoscale Convective Complex extending from central Kansas across western Missouri. This organized mass of multicell thunderstorms brought hail, heavy rain, and flooding to this area.
Supercell thunderstorms Large, long-lasting thunderstorm with a single violent rotating updraft Strong vertical wind shear Outflow never undercuts updraft, updrafts may exceed 90kts and can cause large sized hail 3 types: Classic, high precipitation low precipitation
FIGURE 14.19 Some of the features associated with a classic tornado-breeding supercell thunderstorm as viewed from the southeast. The storm is moving to the northeast.
A wall cloud photographed southwest of Norman, Oklahoma.
Conditions leading to the formation of severe thunderstorms, and especially supercells. The area in yellow shows where supercell thunderstorms are likely to form. Why? - position of cold air above warm creates conditionally unstable atmosphere - Strong vertical wind shear induces rotation Creates a rotating updraft and sets stage for a tornado
Figure 14.22 A typical sounding of air temperature and dew point that frequently precedes the development of supercell thunderstorms. cap on instability at 800mb cold dry air above means convective instability
Thunderstorms and the Dryline Sharp, horizontal change in moisture Thunderstorms form just east of dryline cp ct mt
Intense thunderstorms often can create flash flood conditions especially if storms are training
Big Thompson Canyon July 31, 1976, 12 inches of rain in 4 hours created a flash flood associated with $35.5million in damage and 135 deaths
Distribution of Thunderstorms Most frequent Florida, Gulf Coast, Central Plains Fewest Pacific coast and Interior valleys Most frequent hail Central Plains
Figure 14.26 The average number of days each year on which thunderstorms are observed throughout the United States. (Due to the scarcity of data, the number of thunderstorms is underestimated in the mountainous far west.)
Figure 14.27 The average number of days each year on which hail is observed throughout the United States.
Lightning and Thunder Causes of electrification of clouds graupel and hail fail into region of supercooled water, water freezes, releasing latent heat and keeping the hailstone warmer than surrounding ice crystal nuclei Net transfer.+ ions from warmer to colder, this leaves larger hail stones negatively charged and smaller ice crystals positively charged
Updrafts carry the tiny positively charged ice crystal into the upper reaches of the cloud, while the heavier hailstone falls through the updraft toward the lower region of the cloud.
Figure 14.28 The lightning stroke can travel in a number of directions. It can occur within a cloud, from one cloud to another cloud, from a cloud to the air, or from a cloud to the ground. Notice that the cloud-to-ground lightning can travel out away from the cloud, then turn downward, striking the ground many miles from the thunderstorm. When lightning behaves in this manner, it is often described as a bolt from the blue.
Figure 14.30 The generalized charge distribution in a mature thunderstorm.
The Lightning Stroke A discharge of static electricity Positive charge on ground, cloud to ground lightning Thunder Lightning heats air to 54,000deg F hotter than Sun s surface Explosive expansion of air - shock wave Sound travels at 330m/s or 1100 ft/s, so delay about 5 sec per mile Sound is refracted upward in unstable atm and we do not hear lightning at approximately 15km away Heat Lightning
The development of a lightning stroke. (a) When the negative charge near the bottom of the cloud becomes large enough to overcome the air s resistance, a flow of electrons the stepped leader rushes toward the earth. (b) As the electrons approach the ground, a region of positive charge moves up into the air through any conducting object, such as trees, buildings, and even humans. (c) When the downward flow of electrons meets the upward surge of positive charge, a strong electric current a bright return stroke carries positive charge upward into the cloud.
Figure 14.33 The lightning rod extends above the building, increasing the likelihood that lightning will strike the rod rather than some other part of the structure. After lightning strikes the metal rod, it follows an insulated conducting wire harmlessly into the ground.
Figure 14.35 The four marks on the road surface represent areas where lightning, after striking a car traveling along south Florida s Sunshine State Parkway, entered the roadway through the tires. Lightning flattened three of the car s tires and slightly damaged the radio antenna. The driver and a six-year-old passenger were taken to a nearby hospital, treated for shock, and released.
Figure 14.36 Cloud-to-ground lightning strikes in the vicinity of Chicago, Illinois, as detected by the National Lightning Detection Network
Tornadoes Rapidly rotating column of air that blows around a small area of intense low pressure with a circulation that reaches the ground. Funnel cloud tornado not on ground Tornado life cycle Dust whirl, organizing, mature, shrinking, decay stage Tornado outbreaks families of T. usually due to a long lived supercell outbreak, usually 6 or more
Tornado Occurrence US experiences most tornadoes all 50 states Tornado Alley (warm, humid surface; cold dry air aloft) Highest spring, lowest winter Tornado winds Measurement based upon damage after storm or Doppler radar For southwest approaching storms, winds strongest in the northeast of the storm, 220 kts maximum - most less than 125 kts
Figure 14.38 Tornado incidence by state. The upper figure shows the average annual number of tornadoes observed in each state from 1953 2004. The lower figure is the average annual number of tornadoes per 10,000 square miles in each state during the same period. The darker the shading, the greater the frequency of tornadoes. (NOAA)
Figure 14.40 The total wind speed of a tornado is greater on one side than on the other. When facing an onrushing tornado, the strongest winds will be on your left side. Example forward motion 50kts Rotational speed 100 kts
Figure 14.41 A powerful multi-vortex tornado with three suction vortices.
Dr. T. Theodore Fujita The Fujita scale was revised in 2007 as the EF-scale (Enhanced F-Scale) The EF-scale is based on rotational wind speeds estimated from property damage Ranges from EF0 to EF5 EF5 tornadoes are rare About 77% of tornadoes in the U.S. are considered weak (EF0 to EF1) and 95% are below EF3
Seeking shelter Basement or small, interior room on ground floor Indoor vs. outdoor pressure
Tornadic Formation Basic requirements are an intense thunderstorm, conditional instability, and strong vertical wind shear
Supercell Tornadoes Wind sheer causes spinning vortex tube that is pulled into thunderstorm by the updraft Figure 14.44 (a) A spinning vortex tube created by wind shear. (b) The strong updraft in the developing thunderstorm carries the vortex tube into the thunderstorm producing a rotating air column that is oriented in the vertical plane.
Figure 14.45 A tornado-spawning supercell thunderstorm over Oklahoma City on May 3, 1999, shows a hook echo in its rainfall pattern on a Doppler radar screen. The colors red and orange represent the heaviest precipitation.
Stepped Art Fig. 14-46, p. 402
Nonsupercell Tornadoes Gustnadoes form along a gust front Land spout - form out of cumulus congestus clouds and similar look to water spout (a) Along the boundary of converging winds, the air rises and condenses into a cumulus congestus cloud. At the surface the converging winds along the boundary create a region of counterclockwise spin. (b) As the cloud moves over the area of rotation, the updraft draws the spinning air up into the cloud producing a nonsupercell tornado, or landspout.
Figure 14.47 A well-developed landspout moves over eastern Colorado.
Waterspouts Rotating column of air that is connected to a cumuliform cloud over a large body of water Tornadic waterspout one that formed over land fair weather waterspouts from under cumulus congestus clouds,45kts Does not pull up water more than a few meters what you see is a cloud of condensation
Severe Weather and Doppler Radar Doppler radar measures the speed of precipitation toward and away radar unit Two Doppler radars can provide a 3D view Figure 14.49 Doppler radar display of winds associated with the supercell storm that moved through parts of Oklahoma City during the afternoon of May 3, 1999. The close packing of the horizontal winds blowing toward the radar (green and blue shades), and those blowing away from the radar (yellow and red shades), indicate strong cyclonic rotation and the presence of a tornado.
Doppler Lidar 150 NEXRAD units - WSR-88D and computer system Also portable Figure 14.50 Graduate students from the University of Oklahoma use a portable Doppler radar to probe a tornado near Hodges, Oklahoma.
Monitoring Tornadic Thunderstorms The annual number of reports of tornadoes in the U.S. 1950-2008