DownburstsA downburst is a strong downdraft that comes in contact with the groundSinking air must be colder than the air around itMicrobursts are smaller (and often more intense) downburstsDownburst formation:Rain drags air downward with itRain falls into unsaturated air and evaporatesThis cools the airCold air sinksDownbursts can sink as fast as 40-50 mph!Straight-line winds often result after downburst hits the groundStraight-Line WindsStraight-line winds are intense winds blowing in the same direction over an areaDifferent from a tornado because winds in tornadoes rotateStraight-line winds often result from downbursts or micorburstsStraight-line winds can reach 100 mphAll of the debris falls in the same directionMesoscale Convection SystemsAbbreviated MCSAn MCS is a large complex of thunderstormsCan cover an entire stateProduce a lot of rainfallImportant for the agricultural MidwestHazards include flooding, straight-line winds, and the occasional weak tornadoMCS lifecycle:An MCS generally begins as a cluster of scattered thunderstormsNew thunderstorms initiate along the gust fronts of the existing thunderstormsConvergenceThe cold pools of the thunderstorms eventually mergeThe resultant gust front forms a lineLinear liftingResults in line of thunderstormsStratiform Precipitation DevelopsRegion of stratiform precipitation develops behind line of thunderstormsMCS Dissipation:Gust front outruns thunderstorms (no lift; storms become surrounded by cold air)MCS moves into stable air mass (no instabilityMCS StructureConvective zoneLine of thunderstorms just behind gust frontHeavy rain, strong gusty windsMain squall line on imageStratiform regionLarge area of light to moderate rain found behind convective zoneVertical Wind ShearWe don’t want this!Vertical wind shear is important to the development and maintenance of thunderstorm complexesPrevents gust front from moving too far in front of updraftsShear also generates more lift at gust frontBetter chance of developing new thunderstormsThe role of vertical wind shear:Gust front stays close to thunderstorms and provides more lift.Deeper/slower cold poolmore liftIf gust front outruns thunderstorms, thunderstorms dissipate.Shallower/faster cold poolless liftMCS Structure: UpdraftWarm moist air ascends and moves from front to rear of systemPrecipitation forms within updraftAlso called front-to-rear flowMCS Structure: Rear Inflow JetRain falls into cool dry air near rear of systemEvaporation cools this airAir descends, eventually reaching surfaceCalled rear inflow jetGust FrontAlso called outflow boundaryGust fronts are the boundaries between warm and cold air at front of systemMay appear as fine line on radarWarm air ascends over gust front and may form shelf cloudWarm air lifted at gust front, rises, forms shelf cloudBow EchoesBow echoes are known for producing prolific wind damageThunderstorms form shape of bowThink of bow and arrowEspecially intense bow echoes that affect a large area are known as derechoesThunderstorm Complexes 11/28/2012Downbursts-A downburst is a strong downdraft that comes in contact with the groundoSinking air must be colder than the air around it-Microbursts are smaller (and often more intense) downbursts-Downburst formation:oRain drags air downward with itoRain falls into unsaturated air and evaporatesThis cools the airCold air sinksoDownbursts can sink as fast as 40-50 mph!oStraight-line winds often result after downburst hits the groundStraight-Line Winds•Straight-line winds are intense winds blowing in the same direction over anarea–Different from a tornado because winds in tornadoes rotate–Straight-line winds often result from downbursts or micorbursts•Straight-line winds can reach 100 mph•All of the debris falls in the same directionMesoscale Convection Systems-Abbreviated MCS-An MCS is a large complex of thunderstormsoCan cover an entire state-Produce a lot of rainfalloImportant for the agricultural Midwest-Hazards include flooding, straight-line winds, and the occasional weak tornado-MCS lifecycle:oAn MCS generally begins as a cluster of scattered thunderstormsoNew thunderstorms initiate along the gust fronts of the existing thunderstormsConvergenceoThe cold pools of the thunderstorms eventually mergeoThe resultant gust front forms a lineLinear liftingResults in line of thunderstormsoStratiform Precipitation DevelopsRegion of stratiform precipitation develops behind line of thunderstorms-MCS Dissipation:oGust front outruns thunderstorms (no lift; storms become surrounded by cold air)oMCS moves into stable air mass (no instability-MCS StructureoConvective zoneLine of thunderstorms just behind gust frontHeavy rain, strong gusty windsMain squall line on imageoStratiform regionLarge area of light to moderate rain found behind convective zone-Vertical Wind Shear-We don’t want this!-Vertical wind shear is important to the development and maintenance of thunderstorm complexes-Prevents gust front from moving too far in front of updrafts-Shear also generates more lift at gust frontoBetter chance of developing new thunderstorms-The role of vertical wind shear: oGust front stays close to thunderstorms and provides more lift.Deeper/slower cold poolmore liftoIf gust front outruns thunderstorms, thunderstorms dissipate.Shallower/faster cold poolless liftMCS Structure: UpdraftWarm moist air ascends and moves from front to rear of system-Precipitation forms within updraft-Also called front-to-rear flowMCS Structure: Rear Inflow JetRain falls into cool dry air near rear of system-Evaporation cools this air-Air descends, eventually reaching surface-Called rear inflow jetGust Front-Also called outflow boundary-Gust fronts are the boundaries between warm and cold air at front of system-May appear as fine line on radar-Warm air ascends over gust front and may form shelf cloudoWarm air lifted at gust front, rises, forms shelf cloudBow Echoes-Bow echoes are known for producing prolific wind damage-Thunderstorms form shape of bowoThink of bow and arrow-Especially intense bow echoes that affect a large area are known as