Chapter 11 Meteorology

Super tornado outbreak

tornado record-breaking year with 1691 tornadoes confirmed nationwide, spring most active season with 1243, april with most tornadoes of any month, 25-28 april 2011 outbreak with 342 tornadoes in 21 states from TX to NY, 321 people died, over $10 billion in damages, 27 april with deep upper level low pressure system with warm humid air flowed ahead of cold front, vertical wind shear and humidity increases with greater instability, convective currents surged into upper troposphere and lower stratosphere, massive supercell thunderstorms developed spawned tornadoes, 200 tornadoes touched down with 4 EF5 and 8 EF4, EF5 tornado swept newly constructed brick houses from foundations removed sections of asphalt and picked up an anchored 800 Ib safe, similar tornado outbreaks in spring 2019 from 17-30 may with 301 tornadoes, 50 EF2, 18EF3, EF4, record set for most EF1, 190 tornadoes in Dayton, OH and Linwood KS, 8 fatalities, seamless forecasting system with nearly 30 min of lead time on average credited for preventing deaths and allowing ample preparation

Thunderstorm

mesoscale weather system accompanied by lightning and thunder, affects a relatively small area and is short-lived, product of vigorous convection that extends high into troposphere

Towering cumulus stage

cumulus clouds build vertically and laterally with surge upward to altitudes of 26,000-33,000 fr in 10 to 15 min, produced by convection within the atmosphere, free convection, forced convection, latent heat released during condensation adds to buoyancy, cumulus congestus cloud, cumulonimbus cloud, updraft

Free convection

triggered by intense solar heating of earth’s surface with generally not powerful enough to produce thunderstorms

Forced convection

frontal/orographic lift/converging winds strengthen convection, generally the cause of thunderstorms

Cumulus congestus cloud

cumuliform cloud with significant vertical growth, resembles a huge cauliflower

Cumulonimbus cloud

cumulus congestus cloud continues to grow vertically, characteristic anvil top, produces precipitation, lightning and thunder

Updraft

upward flowing stream of saturated air, strong in the initial stage of the thunderstorm life cycle, suspends water droplets and ice crystals, prevents precipitation in the cumulus stage

Mature stage

begins when precipitation first reaches surface lasting 10-20 min, updraft can no longer support weight of growing water droplets and ice crystals, downdraft, gust front, roll cloud and shelf cloud, thunderstorm attains maximum intensity with features of heavy rain, most frequency lightning, strong surface winds, and possible tornadoes, upward billowing cumulonimbus clouds reach the tropopause with spreading into a flat anvil top, build to altitudes above 60,000 ft

Downdraft

forms alongside updraft when precipitation descending through the cloud drags adjacent air downward, entrained dry air at edge of cloud leads to evaporative cooling (entrainment) and weakens updraft and strengthens downdraft

Gust front

arc-shaped leading edge of downdraft air at surface, ominous-appearing low clouds include roll and shelf clouds

Roll cloud

tube-shaped cloud that appears to rotate about horizontal axis

Shelf cloud

low, elongated cloud that’s wedge-shaped with flat base, appears at edge of a gust front

Dissipating stage

precipitation and downdraft spread throughout the thunderstorm cell, subsiding air replaces the updraft and cuts off the moisture supply, adiabatic compression warms the subsiding air with RH below cloud base decreases, precipitation ends, and convective clouds gradually vaporize

Thunderstorm classification

classified based on number, organization, and intensity of constituent cells, single, multi-cellular, supercells, MCS

Mesoscale convective system

large thunderstorm cluster/multicell line

Single-cell thunderstorms

relatively weak system forming along a boundary within an air mass

Example of single-cell thunderstorm

gust front

Multi-cellular clusters

characterizes most thunderstorms, each cell often at a different stage in its life cycle, succession of cells creates prolonged of thunderstorm

Supercell thunderstorm

most severe and long-lived “isolated” storm, mesocyclone with rotating updraft, speeds can reach 240-280 km/hr

Squall line

elongated cluster of thunderstorm cells with continuous gust front at leading edge, type of MCS, distinguished by large length to width ratio

MCS

large MCS can include several connected squall lines with vast cold cirrus cloud shield, most likely in the warm SE sector of a mature extratropical cyclone ahead of and parallel to cold front, usually more intense that single-cell thunderstorms, account for 80% of growing season rainfall across great plains and midwest, severe weather with weak tornadoes, moderate hail, and flash flooding

Conditions for thunderstorm development

humdi air in low to mid troposphere often mT air destabilized by uplift, atmospheric instability with mT air becomes unstable when lifted to the convective condensation level, source of uplift along fronts, up mountain ranges, or horizontal convergence of surface winds, more humid the air the less uplift needed to destabilize it, solar heating drives atmospheric convection with thunderstorms most frequent when solar radiation is most intense at warmest part of the day, thunderstorm frequency expressed in thunderstorm days/yr with count of number of days thunder is heard, underestimate actual number of thunderstorms if days with multiple lines of thunderstorms pass over a weather station

Exception of solar heating driving atmospheric convection

low-level jet stream up the missouri/mississippi river valleys at night contributes to nocturnal thunderstorm maximum

Where thunderstorms take place?

tropics and subtropics with intense solar heating combining with converging surface winds to trigger thunderstorm development with characteristics of ITCZ, N US with frequency increasing from N to S with highest frequency over C FL due to convergence of sea breezes, second highest over the Rocky Mountain Front Range due to topographically related differences in heating, areas with limited convection with middle and high latitudes in winter, coastal areas downwind from relatively cold ocean waters, Hawaii due to trade wind inversion

Severe thunderstorms

system with surface winds stronger than 58 mph and/or hailstones equal to/great than 1 in, produce flash floods/tornadoes, greater altitude of the thunderstorm top, more likely it will produce severe wind, vertical wind shear

Vertical wind shear

causes thunderstorms to be severe, change in horizontal wind speed/direction with increasing altitude, favors vigorous updrafts with great vertical cloud development and severe thunderstorms, increase in vertical wind shear with gust front cannot advance as far from cell to sustain influx of warm and humid air, most precipitation falls alongside the tilted updraft sustaining the updraft

Polar front jet stream

produces strong vertical wind shear that maintains a vigorous updraft, contributes to stratification of air and increases potential instability of the troposphere with the jet streak inducing both horizontal divergence and convergence of air in the upper troposphere, convergence occurs in the right front quadrant of a jet streak causing weak subsidence of air, subsiding air compressionally warmed and prevented from reaching Earth’s surface by mT air (low-level jet), subsiding air becomes warmer than underlying mT air, capping inversion between 2 layers, contrast between air layer mounts, only a trigger is needed to penetrate the capping inversion

Causes for upward trigger

intense solar midafternoon heating, lifting of air caused by approaching cold front/jet streak

What does capping inversion delaying onset of convection cause?

allows greater buildup of energy, causes more explosive convection when it occurs

Dryline

western boundary of mT air mass, bring about uplift similar to cold front

Lightning

brilliant flash of light, electrical discharge within a cumulonimbus cloud/between the cloud and Earth’s surface, direct hazard to human life, ignites forest and brush fires, very costly to electrical utilities

Lightning detection network (LDN)

provides real-time information on location and severity of lightning strikes

Geostationary lightning mapper (GLM)

helps increase severe storm warning lead time, reduces false alarms

Lightning charges

large differences in electrical charge develop within a cloud, between clouds, or between a cloud and the ground, positive in upper portion of cumulonimbus cloud and much smaller region at cloud base, negative charge between in disk-shaped zone, positive charge induced on ground directly under the cloud, lightning forges a path between oppositely charged regions, luminous events, believed to be caused by electromagnetic interaction between lightning in high thunderstorms and the ionosphere

Lightning charge separation

due to collisions between descending graupel and smaller ice crystals in their path with temps less than 5 with graupel become negatively charged and ice crystals become positively charged, vigorous updrafts carry ice crystals to upper portions of cloud, near cloud base positive charge due to graupel-ice crystal collision with temps greater than 5 for positive graupel and negative ice crystals

Cloud to ground lightning flash

stepped ladders, return stroke, dart leaders, sequence takes place in less than 0.2 sec, lightning causes intense heating of air so rapidly that air density cannot initially respond, shock wave propagates outward, producing sounds waves for thunder

Step ladders

streams of electrons surge from the cloud base to the ground in discrete steps

Return stroke

forms as an ascending electric current when the positive and negative charges recombine and often emanates from tall and pointed structures

Dart leaders

subsequent surges of electrons from the cloud, follow same path

Flash to band method

thunder takes about 3 sec to travel 1 km with 9 sec between lightning and thunderclap, lightning is about 3 km away

Types of upper atmospheric lightning

sprites, sprite halos, blue jets, gigantic jets, blue starters

Transient luminous events (TLE)

cloud lightning-like discharges collectively

Downbursts

exceptionally strong downdraft diverges horizontally when striking surface with or without rain, macroburst cuts a swath of destruction with topping 130 mph, microburst smaller and shorter lived with topping 170 mph, very dangerous to aircraft to aircraft because they trigger wind shear with aircraft have warning systems with same principle as doppler radar, success in coupling radar with light detection and ranging (LIDAR) system

Derecho

straight-line downburst winds that impact a swath at least 240 mi long with wind gusts over 58 mph, type of MCS, bow echo, track NW to SE, warm season event with 3 derechos 2012, 6 derecho between may and july 2019

Bow echo

rapidly moving band of showers/thunderstorms moving in a curved (bow) shape

Flash floods

short term, localized rise in stream level above bankfull in response to torrential rain falling over a small geographic area, occur from succession of thunderstorm cells part of an MCS that passes over same area/from slow-moving/stationary thunderstorms

Atmospheric conditions for flash floods

common at night in an atmosphere with weak vertical wind shear and abundant moisture through great depth, precipitation efficient atmosphere with high values of precipitable water and RH, thunderstorm cloud base with temperature above freezing

Flash flood events

hazardous in mountainous terrain with big thompson canyon flood in july of 1976 and fort collins in july 1997, urban areas prone to flash floods during intense downpours with concrete and asphalt surfaces impervious to water and storm sewer systems may be unable to handle excess runoff, caused by breaching of a dam/levee and sudden release of water during breakup of a river ice jam

Hail

frozen precipitation in the form of balls/lumps of rice with diameter of greater than 0.2 in with individuals being hailstones

Hail developing

ice pellet travels vertically through cumulonimbus clouds containing varying concentrations of supercooled water droplets, composed of alternating layers of glaze and rime, grow by accretion (add) of freezing water droplets, falls out of cloud base when too larger and heavy for updrafts

Hailshaft

column of hail falling of thunderstorm

Hailstreak

accumulate on the ground in a long, narrow strip, 1.2 mi wide, 6.2 mi long

Hail totals

US total annual damages to property and crops during 2014-2018 ranging from 719 million to 3.5 billion, large hailstones can smash windows and dent automobiles, damaging roofs, hurt livestock, and destroy crops, farmers purchase insurance, increase in hail and wind reports with greater public awareness, cell phones, easier report riling, improved weather radar, NWS aggressive verification, severe hail generally most frequent in central US including E CO

Tornadoes

violently rotating column of air in contact with the ground, usually produced by thunderstorm, 10% of annual 10,000 US severe thunderstorms produce tornadoes, small, short-lived with often striking sparsely-populated regions, occasional major outbreaks

Tornado characteristics

funnel cloud, vortex touching ground is the tornado, identified by funnel cloud and/or circulating dust/debris and actual tornadic circulation larger than suggest by funnel cloud, most spawned by and travel with severe thunderstorms, track from SW to NE and go any direction, great horizontal air pressure gradient responsible for tornado’s vigorous circulation with centripetal force maintains rotation of air column with NH tornadoes rotate counterclockwise due to parent thunderstorm, weak and violent tornado

Funnel cloud

condensation cloud extending from cumulonimbus with rotating column of air (vortex) that may/may not contact the ground

Weak tornado

path less than 1 mi long, 330 ft wide, lifetime of a few minutes

Violent tornado

path more than 100 mi long, 3000 ft wide, lifetime of 10 mi to over 2 hrs, wind speeds may be up to 300 mph

Where and when tornadoes occur?

occur in all 50 states, NS corridor stretching from N TX through OK, KS, and NE with secondary area in SE US termed by Dixie Alley, Terrain with weak tornadoes more likely over flat than rough terrain and strong to violent tornadoes largely unaffected by terrain, US typically has 1300 tornadoes/yr with peak activity in May and June, relative instability of lower atmosphere, favorable synoptic weather conditions with well-defined polar front and intense cyclones, center of maximum tornado frequency follows the sun

How are tornadoes a threat to people and property?

extremely high winds with blow down structures and flying debris main cause of death and injury, strong updraft, subsidiary vortices, abrupt drop in air pressure

Multi-vortex tornadoes

consist of 2+ subsidiary vortices that orbit each other/common center, most destructive

Enhanced F-scale

based on rotational wind speeds estimated from property damage, ranges from EF0 to EF5, EF5 tornadoes are rare, about 77% of tornadoes in US rated weak (0 to 1), 95% are below EF3, few violent storm responsible for most fatalities

Supercell thunderstorms

spawn most strong to violent tornadoes, updraft speeds sometimes in excess of 150 mph, can last for several hours and produce more than 1 tornado

Formation and characteristics of tornado-thunderstorm

horizontal wind exhibits strong vertical shear causing air to rotate about a horizontal axis, updraft tilts tube of rotating air to vertical and forms mesocyclone, wall cloud

Wall cloud

roughly circular lowered portion of the rain-free base of a thunderstorm, often accompanies a mesocyclone and forms in region of strong updraft with S/SW of main precipitation shaft, most wall clouds don’t produce tornadoes, tornadic wall clouds have strong and persistent rotation before the appearance of a tornado

Tornado-thunderstorm connection

mesocyclone circulation most intense at 20,000 ft with narrows and builds downward toward ground, low air pressure causes water vapor to condense, forming funnel cloud, spinning column of air narrows and circulation increases similar to ice skater spinning faster as she pulls in her arms, tornado appears near updraft and toward rear of a supercell, understanding/predicting which supercells produce tornadoes is active research area

Supercell

forward flank downdraft in NE, rear flank downdraft in S/SW, help/hinder tornado formation depending on temp, relatively warm RFD maintains mesocyclone, trnaposrting preexisting spin from cloud to ground

Non-supercell tornadoes

squall line tornadoes most common in cells just N of break in the line/S cell, many landfalling hurricanes have tornadoes in NE sector and after system is tracking toward the N and NE, some bases tornadoes develop without a mesocyclone present with circulation developing in boundary layer, forms when rotation centers on convergence lines collocated with strong updrafts in growing convective storms, landspouts, waterspouts

Landspouts

term for non-supercell tornadoes

Waterspouts

non-supercell tornadoes over water

Example of non-supercell tornadoes

on leading edge of thunderstorm gust front

Monitoring tornadic thunderstorms

direct monitoring not feasible, weather radar

Direct monitoring not feasible

digital cameras, balloon borne instruments monitor surrounding atmospheric conditions, portable doppler radar to resolve circulation within supercells, VORTEX, NWS-trainer storm spotters help monitor for severe weather

Weather radar

reflectivity mode shows a hook echo of a parent mesocyclone, velocity mode monitor circulation within system with tornado circulation may show as a tornado vortex signature (TVS) a small region of rapidly changing winds within a mesocyclone, dual-polarization radar detects tornado debris

Conclusions

thunderstorms are a product of convection currents surging to great altitudes within the troposphere, occur most frequently in warmest regions of Earth especially over continental interiors of tropical latitudes, most consist of more than 1 cell with each cell completes its life cycle within an hour, less than 1% of all thunderstorms produce tornadoes, tornado is short lived and small scale weather system with usually spawned by a severe thunderstorm, most frequent in spring over central US, most intense tornadoes develop in supercell thunderstorms