Lightning AOSC 200 Tim Canty Class Web Site: http://www.atmos.umd.edu/~tcanty/aosc200 Topics for today: Thunderstorms Lightning Initiation Lightning Detection Upward lightning Lecture 22 Nov 14, 2017 1 Multicell Thunderstorms Wind shear keeps the downdraft from interfering with updraft Strong down draft aids the updraft (gust front) Increased updraft aids in formation of new cells Fig 10-5 Essentials of Meteorology 2
Multicell Thunderstorms http://www.srh.noaa.gov/jetstream/tstorms/tstrmtypes.htm 3 Multicell Thunderstorms Wind shear keeps the downdraft from interfering with updraft Strong down draft aids the updraft (gust front) Increased updraft aids in formation of new cells Fig 10-4 Essentials of Meteorology 4
Gust Front Cold air (downdraft) hits the grounds and spread out. Acts like a cold front and forces up warmer air Warm air rises, condenses and can form a shelf cloud Fig 10-6 Essentials of Meteorology 5 Multicell Thunderstorms: Squall Lines Line of intense, individual storms May last 6-12 hours Occur along or ahead of frontal boundary (i.e. cold front) Strong wind shear tilts updraft and separates it from downdraft Fig 10-12 Essentials of Meteorology 6
Squall Line: Side View Downdraft can re-direct upper level, fast moving wind (rear inflow jet) toward the surface. This wind will push up the warm air in front of the squall line Fig 10-13 Essentials of Meteorology 7 Multicell Thunderstorms: Squall Lines Pre-frontal squall lines may form ahead of an advancing cold front as the air aloft forms waves downwind from the cold front 8
Derechos Line of storms (squall line) forms along cold front Updraft brings in warm, humid air Downdraft spreads cool air along the surface http://earthsky.org/earth/what-you-need-to-know-about-derechos 9 Derechos Cold air spreads out along the front, creating bow shape This causes more storms to develop and spread out along the front These storms strengthen updraft, lead to upshear tilting Draws in upper level winds from behind the cloud: rear inflow jet http://earthsky.org/earth/what-you-need-to-know-about-derechos 10
Derechos Fast, upper level winds drawn down and reach surface Increase speed of gust front moves storms forward, spread out along front http://earthsky.org/earth/what-you-need-to-know-about-derechos 11 Multicell Thunderstorms: Mesoscale Convective Complex Composed of multiple single-cell storms in different stages of development Individual thunderstorms supports formation of other convective cells To last a long time, good supply of moist air near surface is needed Fig 11-14 Meteorology: Understanding the Atmosphere 12
Lightning Some lightning statistics: 1. Travels very quickly ~130,000 mph 2. Is very hot!! ~54,000 F 3. Carries a lot of energy ~1 billion volts ~20,000 amps or more 4. Average strike is 6 miles long http://www.weatherscapes.com/index.php 13 Lightning Some lightning myths: 1. Rubber car tires or shoes protect you from a lightning strike 2. Lightning always strikes the highest point 3. Lightning never strikes twice in the same spot 4. Lightning rods attract lightning 5. Lightning always travels from cloud to the ground http://www.weatherscapes.com/index.php 14
Lightning 15 Lightning Charge separation: occurs due to collisions between ice crystals and droplets and/or Fig 10-27 Essentials of Meteorology 16
Lightning Charge separation occurs due to collisions between ice crystals and water droplets and/or graupel Updrafts carry positive charges up and the negtively charged graupel sink to cloud base Fig 10-27 Essentials of Meteorology 17 Lightning When the charge builds up enough: Induced charge forms at surface Cloud sends out pilot leader followed by a stepped leader Basically, the negative charges are trying to find a way to get to the ground Fig 10-28 Essentials of Meteorology 18
Lightning Ground sends out positive streamers that wait for the stepped leaders to make contact Once contact is made, an ionized channel is created between cloud and ground Fig 10-28 Essentials of Meteorology 19 Lightning Ground sends out positive streamers that wait for the stepped leaders to make contact Once contact is made, an ionized channel is created between cloud and ground Positive charges rush up through the channel making the return stroke This is the brightest part of the lightning Process can happen several times, very rapidly Fig 10-28 Essentials of Meteorology 20
How can we study lightning? 21 Upward Lightning Fig 11-39 Meteorology: Understanding the Atmosphere 22