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Chapter 10 Thunderstorms and Tornadoes Thunderstorms Characterized by strong up- and down- movements, producing lightning and thunder, and generating gusty winds, heavy rain, and hail. Annual number of lighting Average Number of Thunderstorm Days ~2000 thunderstorms at any given time; ~45,000 per day; ~16 million per year Air-mass thunderstorms mT air mass moving northward from the Gulf of Mexico. Occurring most of the time in the afternoon. Stages of development of a thunderstorm Cumulus stage entrainment Mature stage Dissipating stage Severe Thunderstorm Definition (NWS), wind: 58 miles/per hour or hailstone: D=1.9 cm or tornado. ~100,000 thunderstorms in US, ~10% reach severe status Supercell Thunderstorm Diameter: 20 - 50 km; depth: 20 km Rotated updraft ~2000 -3000 per year in US Cluster of Supercell Thunderstorms Temperature Inversion Enhances Formation of Severe Thunderstorms Squall lines A squall line is a line of thunderstorms that usually have a common lifting mechanism. photo satellite radar Mammatus sky preceding squall line Formation mechanisms a. Lifting associated with cold front b. Lifting induced by upper level jet Jet divergence Lifting Surface strong persistent warm, humidity air from south c. Lifting created by dryline Mesoscale Covective Complex (MCC) Many individual thunderstorms organized into a large oval to circular cluster A typical MCC covers an area of at least 100,000 square km Mostly found in the Great Plains MCC develops from a group of afternoon air-mass thunderstorms. In the evening as the local storms decay, MCC starts developing. It requires a strong low-level flow of very warm and moist air. MCC can remain self-propagating as the gust fronts lead to the formation of new cells. Microbursts Beneath thunderstorms, strong localized downdrafts, less than 4 km Formation: downdrafts are accelerated by a great deal of evaporative cooling. Can cause a significant atmospheric hazard. Lightning The Earth's Electrical Structure Earth's electrical charge The Earth is electrically charged. The Earth surface has a net negative charge, while an equal and positive charge resides in the upper atmosphere. + + ++ + + + Averaged fair-weather electric field near surface is about 130 V/m Thunderstorm - - - - The atmosphere is not a perfect insulator. Negative charge leaks from the Earth and rises to the electrosphere. So, the Earth's charge would dissipate in less than an hour if there were no recharge! What charges the Earth? Lightning recharges the Earth by delivering negative charges to the surface. The system can be viewed as an electric circuit in which electrified clouds are the generator (batteries). - - -- -- -- -- How do clouds get charged? 1. Convective theory: charged particles are moved by the convective currents to produce the charged regions. 2. Gravitational theory: charged particles are separated by gravitational settling. Where do charged particles come from? Inductive and non-inductive processes Ice-Ice process (non-inductive): thermo-electric properties of ice The mobility of the (OH3)+ defect in ice is greater than the (OH)- defect and the number of defects increase with temperature. When warm and cold ice particles come in contact, the positive defect flows faster giving the colder particles a net positive charge. Therefore, a warm hailstone or snow pellet will acquire a net negative charge as it falls through a region of cold ice crystals. Collision process (inductive): Droplet breakup: E + + - - - + + + + + Stages of a Cloud-Ground (CG) Lightning Strike STEP 1 A CG lightning strike typically initiates inside the thundercloud. When enough electrons collect in the bottom of the cloud, a very faint, negatively charged channel, called the step-leader, emerges from the base of the cloud. The leader propagates towards the ground in a series of luminous steps (~50 meters in length and 1 microsecond in duration). Between steps there is a pause of about 50 microseconds, during which the step-leader "looks" around for an object to strike. If none is "seen", it takes another step, and repeats the process until it "finds" a target. (a single leader can be comprised of more than 10,000 steps before reaching ground!) As the step-leader's channel approaches the ground, it has a VERY strong electric potential of about 100 million to billion volts with respect to the ground. STEP 2: When the step-leader approaches the ground, its strong, negative electric field repels negative charge in the surrounding ground, while attracting positive charge. This induces an upward moving positive charge from the ground. When this positive charge collects into a high enough concentration, they form bolts of ground-to-air lightning known as streamers. When one of these positively charged streamers contacts the tip of a negatively charged leader, (~ 30 to 100 meters above the surface), The leader channel's electric potential is connected to the ground. STEP 3: All other branches of the leader channel cease further propagation toward the ground, and all negative charge within these branches starts flowing to the ground through the newly established ground/cloud connection. An electric current wave then propagates up the channel as a bright pulse, known as return stroke, a discharge process taking < 100 microseconds. It is so intense that illuminates the Conductive path. The lightning is traveling FROM the ground INTO the cloud. STEP 4: After the current ceased, there is a pause (~20 to 50 milliseconds). After that, if additional charge is available at the top of the leader channel, another leader can propagate down the established channel. This leader is called a dart leader because it is continuous instead of stepped. Dart leaders are what give lightning its flickering appearance. Not every lightning flash will produce a dart leader though. STEP 5: The negatively charged dart leader then will induce a new, positively charged return stroke. The peak amplitude of the current usually decreases as additional dart leaders are produced till all the charges are used up. The combination of each leader (stepped and dart) and the subsequent return stroke is known collectively as a stroke. All strokes that use the same cloud-to-ground channel constitute a single cloud-to-ground flash. A flash can be made up of a single stroke, or as many as tens of strokes. THUNDER Thunder is the acoustic shock wave caused by the extreme heat generated by a lightning flash. When a lightning bolt occurs, the air surrounding its channel is instantaneously heated to as much as ~33,000 C, a temperature that is five times the surface of the sun! So air molecules will expand. The faster they are heated, the faster their rate of expansion. Since air is heated to 33,000 C in a fraction of a second, its expansion rate exceeds the speed of sound, and a sonic boom (thunder) results. This pressure wall is called shock wave Tornadoes Tornado development: The central vortex of a tornado is typically about 100-600 m in diameter. The averaged wind speed is about 96 mph, but can be as strong as or greater than 220 mph. The central pressure in a tornado can be lower than normal atmospheric pressure by over 100 hPa. The air around the vortex is pulled into the low pressure zone where it expands and cools rapidly to form clouds to make it visible. The exact mechanism of tornado formation is still a matter of dispute but it appears that a tornado grows in a similar fashion to the small vortices that form in draining bathtubs. When the plug is pulled in a bathtub, water from other parts of the tub rushes in to replace that going down the drain. If the water has any swirl in it, the drain soon has a little vortex. Tornadoes appear to be upside down versions of this phenomenon. A precondition closely linked to tornado formation is the development of a mesocyclone with a typical scale of 3 -10 km. A wide vortex is created, called the mesocyclone. The mesocyclone begins to build vertically, extending itself upward throughout the entire height of the cloud. The rapid upward motion causes the air pressure to drop, pulling more air into growing vortex. The upward motion also causes the incoming air to cool quickly and form clouds before they rise to the cloud base. This forms the wall cloud, a curtain-shaped cloud often seen before a tornado forms. As the rotating wall clouds contract due to strong updrafts, it dips to form the so-called funnel clouds, indicating this storm is on its way to producing a tornado. Tornado Descends from Rotating Wall-cloud Tornado climate In US, tornadoes are mostly seen in the southern Great Plains, where cold and dry polar air mass meets warm and moist air mass from the Gulf of Mexico. The greater the contrast, the more intense the storm tends to be, and thus, the more tornadoes. More than 40% tornadoes take place during the spring because of the large contrast of air masses. During May and June the maximum frequency moves through the southern Great Plains and then to northern Plains and Great Lakes due to increasing penetration of warm and moist air. Tornado Intensity and Destruction Fujita Intensity Scale: rating intensity based on the damage produced by a storm Destruction: 1. sudden drop of pressure; 2. violent winds; 3. debris. Tornado Forecasting Tornado watch: issued when there is organized severe weather events where the tornado threat will affect at 26,000 square km and/or persist for at least three hours. Tornado warning: issued when a tornado has actually been signed in an area or is indicated by weather radar. Warnings are issued for a much smaller area than watches, usually covering portions of a county or counties for typically 30 – 60 min. Predicting Tornadoes Doppler Radar on wheels Doppler Radar Doppler Radar Network Doppler Effect