Lecture 21Outline of Last Lecture I. ReviewII. Rising AirIII. Forced vs. Buoyant AscentIV. Stability and Thunderstorm DevelopmentV. Envrionmental Lapse RateVI. Dry Adiabatic Lapse RateVII. Moist Adiabatic Lapse RateVIII. Lapse RatesIX. Lapes Rates and SoundingsX. Lifting Condensation Level (LCL)XI. Level of Free Convection (LFC)XII. Equilibrium Level (EL)XIII. CAPE and CINXIV. Lifted IndexXV.Atmospheric StabilityXVI. Make Atmosphere more StableXVII. Make the Atmosphere More UnstableXVIII. Stability and ThunderstormsOutline of Current Lecture XIX. ThunderstormsATMS 100 1st EditionXX.Thunderstorm Ingredients: InstabilityXXI. Thunderstorm Ingredients: TriggerXXII. Trigger: Front or DrylineXXIII. Trigger: Sea Breeze (Lake Breeze)XXIV. Trigger: MountainsXXV. Thunderstorm IngredientsXXVI. Severe ThunderstormsXXVII. Types of ThunderstormsXXVIII. Ordinary ThunderstormsXXIX. Ordinary Thunderstorms: Stage DevelopmentXXX. Cumulus StageXXXI. Mature StageXXXII. OutflowXXXIII. Dissipation StageCurrent LectureXXXIV. Thunderstormsa. tall, vertically developed cloud that produces lightning and thunderi. can’t have thunder without lightningii. usually produce heavy precipitationb. thunderstorm clouds are called cumulonimbus cloudsXXXV. Thunderstorm Ingredients: Instabilitya. all thunderstorms require instability:i. warm and humid air near the surface and/or cold air aloftii. air can rise on its own due to buoyancyXXXVI. Thunderstorm Ingredients: Triggera. all thunderstorms require a triggeri. something to make the air riseb. What can make air rise?i. front or drylinesii. sea breezes (or lake breezes)iii. outflow boundaires (gust fronts)XXXVII. Trigger: Front or DrylineXXXVIII. Trigger: Sea Breeze (Lake Breeze)a. cool air over water moves inland, converges with warm air over land b. warm air rises, often forming thunderstormsXXXIX. Trigger: Mountainsa. atmosphere heats from ground up b. air over mountain warmer than air around it and risesXL. Thunderstorm Ingredientsa. severe thunderstorms also require vertical wind sheari. recall that wind shear is a change in wind speed and direction with heightii. help storms organizeXLI. Severe Thunderstormsa. a severe thunderstorm contains any of the following:i. wind gusts at the surface greater than 50 knots (57.5 mph)ii. hail with diameter greater than 1 inch 1. roughly quarter sizediii. tornadoXLII. Types of Thunderstormsa. oridnary (single-cell) thunderstormsii. usually not severeiii. form in envrionments with weak vertical wind shearb. multicell thunderstorms (MCSs)ii. thunderstorm complexesiii. form in environments with moderate vertical wind shearc. supercell thunderstormsii. almost always severeiii. produce nearly all intense tornadoes and hail larger than golf-ball sizediv. form in envrionments with strong vertical wind sheard. vertical wind shear most important factor in determining thun-derstorm typeXLIII. Ordinary Thunderstormsa. most common type of thunderstormb. usually not severec. last about an hour from formation to dissipationd. generally develop in regions of weak wind shearii. often form in summer not near frontsXLIV. Ordinary Thunderstorms: Stage Developmenta. cumulus stageb. mature stagec. dissipation stageXLV. Cumulus Stagea. warm air rises, expands and coolsb. water vapor condenses and forms a cloudc. the rising air is called updraftii. air rises due to buoyancy iii. cannot have a thunderstorm without an updraftiv. latent heat release (condensation) in updraft powers thunder-stormd. as cloud gets deeper; precipitation particles begin to forme. fair weather cumulus cloudsii. most amount to nothing but some can grow into thunderstormsXLVI. Mature Stagea. updraft may eventually reach tropopauseii. tropopause is very stable (inversion layer) acts as lid on stormb. air diverges outward when it reaches tropopause, forms anvil cloudc. top of updraft may penetrate into stratosphereii. called overshooting topd. precipitation particles grow, become heavy, begin to fall into updraftii. falling precipiation drags air downward with ite. some rain evaporates, cooling air even moref. when air is cooler than its surroundings, it sinksii. this sinking air is called a downdraftiii. precipitation is required to form a downdraftXLVII. Outflowa. when cold downdraft reaches ground, it spreads outii. forms a pool of cold air beneath stormiii. known as a cold pool or outflowb. boundary between cold outflow and warm inflow is called gust front or outflow boundaryc. this is why temperature drops and wind increase just before a thunder-storm beings ii. also why is it cooler just after a thunderstormd. convergence at outflow boundary may trigger new stormsXLVIII. Dissipation Stagea. rain falls into updraftii. cools updraft, drags rising air downwardb. updraft weakens, replaced by downdraftc. surging outflow cuts off supply of warm, moist, unstable air needed to feed updraftd. updraft dissipatesii. thunderstorm dominated by downdrafts and precipitation and quickly