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CU-Boulder ATOC 1050 - ATOCch18-24

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Chapter 18Thunderstorms are- Critical for redistribution of heat and moisture in the atmosphere- Occur continuously- Provide essential water and agricultural resources- $2.4 billion property/crop loss in US over past decade NWS requirements for a sever thunderstorm- Hail with diameter of 1 inch or larger (any thing smaller does not inflict damage) - Wind damage or gusts of 50knots (58mph) or greater- And or tornado - Lightning occurs in all thunderstorms, it is not used to differentiate between sever and non-severFour elements required for thunderstorm formation1. Source of moisture2. Conditionally unstable atmosphere3. Mechanism to trigger updraft4. Vertical wind shearDestructive thunderstorms develop most often in an environment characterized by large conditional instability and strong vertical wind shear Mechanisms for updraft of lifting include - Frontal boundaries- Daytime heating- Differential heating of air over surfaces with different properties- Leading edge of cool air outflow from a past thunderstorm *The primary mechanism that triggers the updraft of an airmass thunderstorms is: solar heating of the surfaceNon-sever thunderstorms- Often called “airmass” thunderstorms- Typically form within an airmass (not along a front)- Triggered by weak boundaries: cool air outflow, surface heating, lifting along slopes- Form during hot afternoons- Winds do not change with height, lifetime time is short about an hour Airmass thunderstorms grow vertically without significant tilt- No vertical shear- Symmetric anvil- In mature stages, downdraft falls into updraft cutting off the moisture source - In the dissipation stage downdrafts dominate often producing a gust front outflow boundary 1Anvil the flat top of a thunderstorm, formed when updraft hits tropopasue Mammatus clouds often form at base of thunderstorm anvils*Airmass thunderstorm would most likely form: far away from any frontal boundaries around 4pm*Airmass thunderstorms; tend to occur in environments where the winds do not increase substantially with height Meso Scale Convective Systems (MCS) when thunderstorms breed other thunderstorms- Progress over a large area- Produce damaging winds and occasional weak tornadoes - Mesoscale-atmospheric processes (a few km to a few hundred km) or one thunderstorm to a group of thunderstormsLifecycle of MCS- Lifting air along weak boundaries triggers a thunderstorm or two (often in late afternoon)- Storms organize, often aligning into an arc-shaped squall line where precip falls in a continuous line- Squall = violent burst of wind- Squall lines often have strong straight lines winds- Squall line may blow outward, producing bow echo on radar- Behind the squall (typically on W or NW) widespread less-intense precipitation develops in the stratiform regionSquall line – a long line of thunderstorms in which adjacent thunderstorm cells are so close together that the heavy precipitation falls in a long continuous lineBow Echo – portion of the squall line that bow outward from MCS Trailing stratiform region – area of less intense precipitation on rear side of squall line*Why is wind shear a key environmental condition for sever thunderstorms? Strong winds cause the updraft to tilt and separate from the downdraft, allowing the inflow of warm moist air to continue to feed the storm Gust front – leading edge of evaporating cooled air 2Evolution of a Mesoscale Convective System - Initially updrafts are upright- Rain evaporates as it falls, cooling the air into a cold pool- Cold pool spreads outward toward the warm moist air feedingthe thunderstorm and spreads rearward- New updrafts preferentially form along advancing cold poolforming a squall line, now tilted- Cold pool can become deep and cold enough to rush outwardas a gust front which can have a shelf cloud or roll clouddetectable on a radar as a fine-line return- Air in the evaporation region flows forward toward line ofstorms creating rear inflow jet Key Features of a MCS squall line:*Cold pool is the accumulation of rain cooled air near the surface under a thunderstorm*A thunderstorm’s anvil cloud is: composed of ice crystals 3*What develops when cool air generated by evaporation of rain within the downdrafts spreads outward away from the thunderstorm after reaching the surface? Gust front Frontal squall lines can persist for thelifetime of a mid-latitude cycloneNote the tail of comma cloud associatedwith the mid-latitude cyclone Squall line is normally along leading coldfront or upper-level front within theextratropical cyclone Many features of frontal squall lines are similar to MCS squall lines:Overshooting top – the portion of the updraft that penetrates the tropopause Super Cell Thunderstorms - Most intense thunderstorms on Earth - Always rotate - Responsible for majority of most dangerous sever thunderstorm weather andstrongest tornadoes in US- Hail as large as grapefruit- Extreme winds- Flash flooding- Updrafts typically 45-90 mph, some >100mph- Same 4 requirements as otherthunderstormCAPE (convective available potentialenergy) quantifies a parcel’s buoyancy Moist layer at the surface feeds theupdraft providing fuel necessary to4drive the rapidly growing supercellSupercells typically first form where airmass boundaries intersectKey features of a Supercell- Mesocyclone (5-10km)- Tilted updraft- Overshooting top- Anvil - Rain free base- Bounded weak echo region - Wall cloud- Mammatus- Virga- Rear flanking line5*You are outdoors in spring in the Central Plains. A supercell thunderstorm is traveling from SW to NW and passes directly over you. In what order would you observe the different precipitation types? Virga, light rain, moderate rain, heavy rain,hail SUMMARY CH 18The National Weather Service determines a thunderstorm is severeif it meets at least one of the following criteria:– hail with diameter . inch or larger,– wind damage or gusts over 50 kts, or– a tornado.• Boundaries are important in thunderstorm development because lifting occurs along the boundaries.• Wind shear is important to a severe thunderstorm because vertical wind shearseparates the updraft and downdraft, preventing precipitation from falling into the updraft. It can also induce rotation in a thunderstorm.• Airmass thunderstorms can be expected to form during afternoon, far from frontal boundaries.• MCSs are important to the


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CU-Boulder ATOC 1050 - ATOCch18-24

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