5Thunderstorm Life Cycle Towering Cumulus StageCumulus clouds build vertically and laterally, and surge upward to altitudes of 8,000-10,000 m (26,000-33,000 ft) over a period of minutesProduced by convection within the atmosphereFree convection – triggered by intense solar heating of Earth’s surfaceGenerally not powerful enough to produce thunderstormsForced convection – orographic uplift or converging winds strengthen convectionThis is generally the cause of thunderstormsLatent heat released during condensation adds to buoyancyDuring the cumulus stage, the updraft is strong enough to keep water droplets and ice crystals suspendedAs a result, precipitation does not occur in the cumulus stageAmerican Meteorological Society Education Program
6Thunderstorm Life Cycle Mature Stage – maximum intensityStage typically lasts about minutesBegins when precipitation reaches Earth’s surfaceFeatures heaviest rain, frequent lightning, strong surface winds, and possible tornadoesWeight of droplets and ice crystals overcome the updraftDowndraft created when precipitation descending through the cloud drags the adjacent air downwardEntrained dry air at the edge of the cloud leads to evaporative cooling, which weakens the buoyant uplift and strengthens the downdraftAt the surface, the leading edge of downdraft air resembles a miniature cold front and is called a gust frontOminous-appearing low clouds associated with a gust front include a roll cloud and a shelf cloudAmerican Meteorological Society Education Program
7Thunderstorm Life Cycle When the upward billowing cumulonimbus cloud reaches the tropopause, it spreads out forming a flat anvil top. Cloud tops during the mature stage can build to altitudes in excess of 18,000 m (about 60,000 ft).Thunderstorm cells can develop along the gust front ahead of the main thunderstorm. The radar image shows up as an arc-shaped band.American Meteorological Society Education Program
8Thunderstorm Life Cycle In this visible satellite image, clusters of intense thunderstorm cells appear as bright white blotches over portions of Texas, Oklahoma, and MissouriAmerican Meteorological Society Education Program
9Thunderstorm Life Cycle Dissipating StagePrecipitation and the downdraft spread throughout the thunderstorm cell, heralding the cell’s demiseSubsiding air replaces the updraft and cuts off the supply of moistureAdiabatic compression warms the subsiding air and the clouds gradually vaporizeAmerican Meteorological Society Education Program
10Thunderstorm Classification NOAA classification of thunderstorms, and the likelihood of severe weather.American Meteorological Society Education Program
11Thunderstorm Classification Thunderstorms are meso-scale convective systems (MCS) and are classified based on the number, organization, and intensity of their constituent cellsSingle-cell thunderstormsUsually a relatively a weak system forming along a boundary within an air mass (i.e., gust front)Typically completes its life cycle in 30 minutes or lessMulticellular thunderstormsCharacterizes most thunderstorms. Each cell may be at a different stage in its life cycle, and a succession of cells is responsible for a prolonged period of thunderstorm weather.Two types:Squall lineMesoscale convective complexEither can produce severe weatherAmerican Meteorological Society Education Program
12Thunderstorm Classification A thunderstorm may track at some angle to the path of its constituent cells, complicating the weather system motion. In the above idealized situation, the component cells of a multicellular thunderstorm travel at about 20 degrees to the eastward moving thunderstorm. As they travel toward the northeast, the individual cells progress through their life cycle.
13Thunderstorm Classification Multicellular thunderstorm typesSquall line – elongated cluster of thunderstorm cells that is accompanied by a continuous gust front at the line’s leading edgeMost likely to develop in the warm southeast sector of a mature extra-tropical cyclone, ahead of and parallel to the cold frontMesocyclone convective complex (MCC)A nearly circular cluster of many interacting thunderstorm cells with a lifetime of at least 6 hrs, and often hrsThousands of times larger than a single cellPrimarily warm season phenomena (March – September)Usually develop at night over the eastern 2/3 of the U.S.Is not associated with a frontUsually develops during weak synoptic-scale flow, often develops near an upper-level ridge of high pressure, and on the cool side of a stationary frontA low level jet feeds warm humid air into the systemSupercell thunderstorms are long-lived single cell stormsExceptionally strong updraft, with rotational circulation that may evolve into a tornado
14Thunderstorm Classification Radar image of a squall line stretching from Texas to IllinoisInfrared satellite image showing meso-scale convective complexes over western Kansas and most of Arkansas
15The Geography of Thunderstorms Frequency decreases with distance from equator. None above 60oMost occur during summer’s warm temperatures.
18Thunderstorm Frequency Probably 1500 to 2000 thunderstorms active around the world at any given time.
19Severe ThunderstormsA severe thunderstorm is accompanied by locally damaging surface winds, frequent lightning, or large hailSurface winds stronger than 50 kts (58 mph) and/or hailstones 0.75 in. (1.9 cm) or larger in diameterMay also produce flash floods or tornadoesWhat causes some thunderstorms to be severe?Key is vertical wind shear, the change in horizontal wind speed and direction with increasing altitudeWeak vertical wind shear favors short-lived updrafts, low cloud tops, and weak thunderstormsStrong vertical wind shear favors vigorous updrafts, great vertical cloud development, and severe thunderstormsWith increasing vertical wind shear, the inflow of warm humid air is sustained for a longer period because the gust front cannot advance as far from the cell. Also, most precipitation falls alongside the titled updraft, sustaining the updraft.American Meteorological Society Education Program
20Severe Thunderstorms American Meteorological Society Education Program A synoptic weather pattern that favors development of severe thunderstorms. A dryline is the western boundary of the mT air mass and brings about uplift in a manner similar to a cold front.American Meteorological Society Education Program
21Severe ThunderstormsThe polar front jet stream produces strong vertical wind shearThis maintains a vigorous updraftThis supports great vertical development of thunderstormsThe jet contributes to stratification of air that increases the potential instability of the troposphereA jet streak induces both horizontal divergence and convergence of air in the upper troposphereConvergence occurs in the right front quadrant of a jet streak, causing weak subsidence of airSinking air is compressionally warmed and forms an inversion (capping inversion) over the mT air massThe underlying air mass becomes more humidContrast between air layers mountsAll that is needed is a lifting mechanism for severe weather to occurAmerican Meteorological Society Education Program
22Severe Thunderstorms American Meteorological Society Education Program A temperature sounding that favors the development of severe thunderstorm cells. A capping inversion separates subsiding dry air aloft from warm, humid air near the surface.American Meteorological Society Education Program
23Severe Thunderstorms American Meteorological Society Education Program Mammatus clouds occur on the underside of a thunderstorm anvil and sometimes indicate a severe storm system. Their appearance is caused by blobs of cold, cloudy air that descend from the anvil into the clear air beneath the anvil.American Meteorological Society Education Program
24Thunderstorm Hazards Lightning A brilliant flash of light caused by an electrical discharge within a cumulonimbus cloud or between the cloud and Earth’s surfaceDirect hazard to human lifeIgnites forest and brush firesVery costly to electrical utilitiesLightning detection network provides real-time informationAmerican Meteorological Society Education Program
25Thunderstorm Hazards Lightning, continued What causes lightning? Large differences in electrical charge develop within a cloud, between clouds, or between a cloud and the groundUpper portion and much smaller region of the cumulonimbus cloud become positively charged, with a disk-shaped zone of negative charge in between. A positive charge is induced on the ground directly under the cloudLightning may forge a path between oppositely charged regionsCharge separation within a cloud may be due to collisions between descending graupel striking smaller ice crystals in their path. At temperatures < -15 °C (5 °F) graupel become negatively charged while ice crystals become positively charged. Vigorous updrafts carry ice crystals to upper portions of the cloud.Positive charge near cloud base also due to graupel-ice crystal collision, but temps > -15 °C (5 °F) induce positive charge to graupel and negative charge to ice crystalsAmerican Meteorological Society Education Program
27Thunderstorm Hazards Lightning, continued A cloud-to-ground lightning flash involves a regular sequence of eventsStepped ladders: streams of electrons surge from the cloud base to the ground in discrete stepsReturn stroke: forms as an ascending electric current when the positive and negative charges recombine; often emanates from tall, pointed structuresDart leaders, subsequent surges of electrons from the cloud, follow the same conducting pathSequence takes place in < two-tenths of a secondLightning causes intense heating of air so rapidly that air density cannot initially respondShock wave is generated and propagates outward, producing sound waves heard as thunderFlash-to-bang method: Thunder takes about 3 seconds to travel 1 km (or 5 seconds to travel 1 mi)If you must wait 9 seconds between lightning flash and thunderclap, the lightning is about 3 km (1.8 mi) awayAmerican Meteorological Society Education Program
28Thunderstorm Hazards - Lightning American Meteorological Society Education Program
29Thunderstorm Hazards American Meteorological Society Education Program DownburstsExceptionally strong downdrafts that occur with or without rainStarburst pattern causes ground destructionAlso very dangerous to aircraft because they trigger wind shearAircraft have warning systems that use the same principle as Doppler radarA macroburst cuts a swath of destruction > 4 km (2.5 mi) wide with surface winds that may top 210 km per hr (130 mph)A microburst is smaller and shorter livedDerecho: a family of straight-line downburst winds that may be hundreds of kilometers long; sustained winds in excess of 94 km per hr (58 mph)American Meteorological Society Education Program
30Thunderstorm Hazards Flash Floods Short-term, localized, often unexpected rise in stream level usually in response to torrential rain falling over a relatively small geographical areaCaused by excessive rainfall in slow moving or stationary thunderstorm cellsAtmospheric conditions that favor flash floods:More common at night and form in an atmosphere with weak vertical wind shear and abundant moisture through great depthsPrecipitation efficient atmosphere has high values of precipitable water and relative humidity and a thunderstorm cloud base with temperatures above freezingAmerican Meteorological Society Education Program
31HailFormationLargest? Coffeyville, KS, (1.75 lb, 14 cm diameter)
32Thunderstorm Hazards Hail Frozen precipitation in the form of balls or lumps of ice > 5 mm (0.2 in.) in diameter, called hailstonesAlmost always falls from cumulonimbus clouds that are characterized by a strong updraft, great vertical development, and an abundance of supercooled waterDevelops when an ice pellet is transported vertically through portions of the cloud containing varying concentrations of supercooled water dropletsComposed of alternating layers of glaze and rimeGrows by accretion (addition) of freezing water droplets and falls out of cloud base when it becomes to large and heavy to be supported by updraftsAmerican Meteorological Society Education Program
33Accretionary Lapilli are perhaps best explained by a process which involves vertically developed clouds and ice, especially hail and graupel.
34Likely microphysical processes and particles in volcanic cloud. C Textor et al., 2006, JVGR 150:
35Thunderstorm Hazards Hail, continued May accumulate on the ground in a long, narrow strip known as a hailstreak; typically 2 km (1.2 mi) wide and 10 km (6.2 mi) longThe figure below is a model of hailstreak developmentAmerican Meteorological Society Education Program
36TornadoesAbout 10% of the annual 10,000 U.S. severe thunderstorms produce tornadoesA tornado is a violently rotating column of air in contact with the groundMost are small and short-lived and often strike sparsely-populated regionsThe most prolific tornado outbreak on record occurred over the Great Plains and Midwest on May 2004More than 180 tornadoes were reported
37Lightning discharge of electricity that occurs in mature thunderstorms Cause: charge separation in cloud sets up electrical potentialRole of lightning is to equalize these differences in electrical potential.Important fixer of Nitrogen.
38Stepped leader Upward leader Return stroke Electrons down Protons up Circuit completeRepeats every few microseconds with new leader.
41Overshooting Top Overshooting top - characteristic of a strong updraft The updraft goes higher than the rest of the clouds near it (in the anvil)Overshoots the tropopause or equilibrium level btwn the troposphere & stratosphereUpdraft penetrates stratosphere and then is forced back down to equilibrium levelAmerican Meteorological Society Education Program
43Supercell Thunderstorms A supercell thunderstorm is a t.s. with a deep rotating updraft (mesocyclone)Updraft elements usually merge into the main rotating updraft and then accelerate rapidlyFlanking updrafts "feed" the supercell updraft, rather than compete with itSmall percentage of all t.s.’s are supercells but they cause the majority of damage
46Umbrella CloudThe volcanic cloud had a distinct morphology, with a column feeding an umbrella cloud. There was very high winds which drove the cloud eastward and it spread over eastern Washington in a few hours.High winds at km height made the volcanic cloud spread like a mushroom
47Recent work by Baines and Sparks calls attention to the fact that large ignimbrite eruptions will lead to giant volcanic clouds because of the large size of ignimbrite outflow sheets and the large amounts of bouyant fine ash. This probably helps to explain why the ashfall deposits are so extensive--note the comparison between MSH 1980 and the Bishop and Yellowstone deposits.GRL 32 no L24808GRL 32 L
48Features of Supercells Mesocyclone (p.125) organizes updraft and downdraft and keeps them separateUpdraft is slanted downwind (aloft) so hail/rain doesn’t fall through it and kill itSupercell can last for hours and travel a hundred plus milesOften moves to the right of the mean flow - has to do with rotation (vorticity) and propagationWhat does propagation mean?
49How Supercells Move Movement = Advection + Propagation This little formula applies to pretty much everything in weatheradvection = just the horizontal transport of the feature (like a supercell) along with the windspropagation = development of the feature (usually happens towards inflow or flanking line in the case of a supercell)American Meteorological Society Education Program
59Fully glaciated volcanic cloud with abundant CCN Above Ephrata on 18 MayPhoto by Douglas MillerDensity of cloud increases as a wholeLatent heat effects significant in rise and possibly in fallBright band effects during descent (thawing)Overall sublimation/evaporationQuite different from a thunderstorm
60Meteorological Cloud Volcanic Cloud Durant et al., 2008, JGR 113Few INBergeronLarge Ice HMPrecipitationMany INSmall ice HMLittle PrecipSublimationMeteorological Cloud Volcanic Cloud