single cell thunderstorms metr 4803 hazardous weather detection and prediction spring, 2005 semester...

Single Cell Single Cell ThunderstormsThunderstormsMETR 4803METR 4803

Hazardous Weather Detection and Hazardous Weather Detection and PredictionPrediction

Spring, 2005 SemesterSpring, 2005 Semester

Kelvin K. DroegemeierKelvin K. Droegemeier

School of MeteorologySchool of MeteorologyUniversity of OklahomaUniversity of Oklahoma

ThunderstormThunderstorm DefinitionDefinition: By definition, a thunderstorm is a local : By definition, a thunderstorm is a local

storm, invariably produced by a cumulonimbus cloud, storm, invariably produced by a cumulonimbus cloud, that always is accompanied by lightning and thunder. that always is accompanied by lightning and thunder. It usually contains strong gusts of wind, heavy rain, It usually contains strong gusts of wind, heavy rain, and sometimes hail. Meteorologists often use the and sometimes hail. Meteorologists often use the word "convection" to describe such storms in a word "convection" to describe such storms in a general manner, though the term convection general manner, though the term convection specifically refers to the motion of a fluid resulting in specifically refers to the motion of a fluid resulting in the transport and mixing of properties of the fluid. To the transport and mixing of properties of the fluid. To be more precise, a convective cloud is one which be more precise, a convective cloud is one which owes its vertical development, and possibly its origin, owes its vertical development, and possibly its origin, to convection (upward air currents).to convection (upward air currents).

Thunderstorm ClimatologyThunderstorm Climatology At any given time there are an estimated 2000 At any given time there are an estimated 2000

thunderstorms in progress, mostly in tropical and thunderstorms in progress, mostly in tropical and subtropical latitudes. About 45,000 thunderstorms subtropical latitudes. About 45,000 thunderstorms take place each day. Annually, The U.S. experiences take place each day. Annually, The U.S. experiences about 100,000 thunderstorms. About 16 about 100,000 thunderstorms. About 16 millionmillion thunderstorms occur annually around the world! thunderstorms occur annually around the world!

Thunderstorm ClimatologyThunderstorm Climatology(storms per year)(storms per year)

Hail ClimatologyHail Climatology

Hail Days ClimatologyHail Days Climatology

Courtesy H. Brooks, National Severe Storms Laboratory

Lightning ClimatologyLightning Climatology

Wind Days ClimatologyWind Days Climatology

Courtesy H. Brooks, National Severe Storms Laboratory

Modes of Convection / Storm Modes of Convection / Storm ClassificationClassification

Although a Although a continuous spectrumcontinuous spectrum of of storms exists, meteorologists find storms exists, meteorologists find it convenient to classify storms it convenient to classify storms into into specific categoriesspecific categories according according to their to their structure, intensity, structure, intensity, environmentsenvironments in which they form, in which they form, and and weatherweather produced. produced.

Basic ClassificationBasic Classification Single-cell or air-mass stormSingle-cell or air-mass storm Typically lasts 20-30 minutes. Pulse Typically lasts 20-30 minutes. Pulse

storms can produce severe weather elements such as downbursts, hail, storms can produce severe weather elements such as downbursts, hail, some heavy rainfall and occasionally weak tornadoes. some heavy rainfall and occasionally weak tornadoes.

Multicell cluster stormMulticell cluster storm A group of cells moving as a single unit, with A group of cells moving as a single unit, with each cell in a different stage of the thunderstorm life cycle. Multicell each cell in a different stage of the thunderstorm life cycle. Multicell storms can produce moderate size hail, flash floods and weak storms can produce moderate size hail, flash floods and weak tornadoes. tornadoes.

Multicell Line (squall line) StormsMulticell Line (squall line) Storms - consist of a line of storms with a - consist of a line of storms with a continuous, well developed gust front at the leading edge of the line. continuous, well developed gust front at the leading edge of the line. Also known as squall lines, these storms can produce small to moderate Also known as squall lines, these storms can produce small to moderate size hail, occasional flash floods and weak tornadoes. size hail, occasional flash floods and weak tornadoes.

Supercells Supercells Defined as a thunderstorm with a rotating updraft, these Defined as a thunderstorm with a rotating updraft, these storms can produce strong downbursts, large hail, occasional flash storms can produce strong downbursts, large hail, occasional flash floods and weak to violent tornadoes.floods and weak to violent tornadoes.

Key Ingredients for Key Ingredients for ThunderstormsThunderstorms

Static InstabilityStatic Instability

Warm Air

Cold Air

Warm Moist Air

Cold Dry Air

Convection and BuoyancyConvection and Buoyancy

Convection: Convection: transport of fluid properties transport of fluid properties by motions within that fluidby motions within that fluid

Buoyancy: Buoyancy: vertically oriented force on a vertically oriented force on a parcel of air due to density differences parcel of air due to density differences between between the parcel and between between the parcel and surrounding airsurrounding air– Mathematically, the buoyancy force can be Mathematically, the buoyancy force can be

derived from the vertical equation of motion derived from the vertical equation of motion (we’ll do this later)(we’ll do this later)

Vertical Wind Shear;Vertical Wind Shear;Change in wind Change in wind speed and/or speed and/or direction with height;direction with height;Severe storms needSevere storms needstrong veering ofstrong veering ofwind with height andwind with height andstrong increase in strong increase in speedspeed


Mechanism to trigger the Mechanism to trigger the instabilityinstability– FrontFront– TerrainTerrain– DrylineDryline– Daytime heatingDaytime heating– Landmass inhomogeneitiesLandmass inhomogeneities


Types of ThunderstormsTypes of Thunderstorms Ordinary single stormsOrdinary single storms

– Most commonMost common– Last for less than an hourLast for less than an hour– Built-in self-destruct mechanism!Built-in self-destruct mechanism!– Occur all year long, mostly in summerOccur all year long, mostly in summer– Can produce strong winds, hail, and lightningCan produce strong winds, hail, and lightning

Air Mass ThunderstormsAir Mass Thunderstorms First studied just after World War IIFirst studied just after World War II Many commercial and military aircraft Many commercial and military aircraft

accidentsaccidents Newly developed radar was exploited Newly developed radar was exploited

for weather studiesfor weather studies The Thunderstorm ProjectThe Thunderstorm Project Resulted in first life cycle of a Resulted in first life cycle of a

thunderstormthunderstorm Air mass thunderstorms are also Air mass thunderstorms are also

referred to as “Garden Variety!”referred to as “Garden Variety!”

Conditions of Formation of Conditions of Formation of Air Mass ThunderstormsAir Mass Thunderstorms

Conditional instability (we’ll Conditional instability (we’ll come to that later)come to that later)

Warm, moist air near the Warm, moist air near the groundground

Localized source of lift (usually Localized source of lift (usually thermally driven)thermally driven)

Weak or no environmental Weak or no environmental vertical wind shearvertical wind shear

Wind ShearWind Shear Definition:Definition:

– The change in the direction or speed of the wind over a distance. Vertical Wind ShearVertical Wind Shear

– The change with height in the direction or speed of the horizontal wind. Low wind shear indicates little change in direction or speed of the wind over a distance.Low wind shear indicates little change in direction or speed of the wind over a distance.

Weak Wind ShearWeak Wind Shear

7 kts

6 kts

7 kts

6 kts

There is very littlechange in the speedor direction of thewind with height.

Height

Three stages of single-cell storm Three stages of single-cell storm developmentdevelopment

Developing stage Mature Stage Dissipating Stage

Example of Single-cell Life CycleExample of Single-cell Life Cycle

(a) – Developing

(b) – Mature

(c) Mature

(d) – Dissipating

Cumulus PhaseCumulus Phase Development of towering Development of towering

cumuluscumulus– Region of low level convergence– Warm moist air– Updraft driven by latent heating

Nearby cumulus may Nearby cumulus may merge to form a much merge to form a much larger cloudlarger cloud

Dominated by updraftDominated by updraft Mixing and entrainment Mixing and entrainment

occur in the updraftoccur in the updraft

Cumulus PhaseCumulus Phase

c William Zender (2001)

c David Shohami

EntrainmentEntrainment Entrainment is the process by Entrainment is the process by

which saturated air from the which saturated air from the growing cumulus cloud mixes with growing cumulus cloud mixes with the surrounding cooler and drier the surrounding cooler and drier (unsaturated) air.(unsaturated) air.

Entrainment causes evaporation of Entrainment causes evaporation of the exterior of the cloud and tends the exterior of the cloud and tends to reduce the upward buoyancy to reduce the upward buoyancy there.there.

Mature PhaseMature Phase Precipitation, Precipitation,

formed by the formed by the Bergeron cold rain Bergeron cold rain process, begins to process, begins to reach the ground.reach the ground.

The precipitation The precipitation drags some of the drags some of the surrounding air surrounding air down creating the down creating the downdraftdowndraft..

Mature PhaseMature Phase

DowndraftDowndraft The downdraft is the The downdraft is the

descending column of air in a descending column of air in a thunderstorm.thunderstorm.

Created and maintained by Created and maintained by three processesthree processes– Evaporational cooling of entrained air– Downward drag caused by falling

precipitation– Evaporational cooling of the air below

the cloud base

DowndraftDowndraft When the downdraft reaches the When the downdraft reaches the

ground, it spreads out in all directions.ground, it spreads out in all directions. The leading edge of this cold, often The leading edge of this cold, often

gusty wind is called the gusty wind is called the outflow outflow boundaryboundary or or gust frontgust front..

ReflectivityReflectivity Radial Velocity Radial Velocity 0.5 deg Elevation, 04:28 UTC

LIT LIT

ReflectivityReflectivity Radial Velocity Radial Velocity

0.5 deg Elevation, 04:34 UTC

LIT LIT


0.5 deg Elevation, 04:40 UTC

LIT LIT


0.5 deg Elevation, 04:34:12 UTC

LIT LIT

Main body ofstorm (second part)

Gust front;First part of storm

First part ofthe storm

Second part ofthe storm

LITRadar

Time = 04:34:12 UTC

RadarLIT

Heavy Precip

Doppler RadialVelocity

Reflectivity





LITRadar

Time = 04:40:02 UTC

LITRadar

Heavy Precip

Doppler RadialVelocity

Reflectivity



Gust Front Shelf CloudGust Front Shelf Cloud

National Severe Storms Laboratory

DowndraftDowndraft The outflow boundary behaves like a cold The outflow boundary behaves like a cold

front:front:– Strong wind shift (speed and direction)– Much colder air behind the gust front– Acts as a location for additional lift for future storm

development. New Storm

Mature PhaseMature Phase The mature phase represents The mature phase represents

the peak intensity of the the peak intensity of the storm.storm.

Updrafts and downdrafts are Updrafts and downdrafts are about equal in strength.about equal in strength.

Precipitation is typically heavy Precipitation is typically heavy and may contain small hailand may contain small hail

Gusty winds result from the Gusty winds result from the downdraft spreading out on downdraft spreading out on the ground.the ground.

The anvil, or cloud top, begins The anvil, or cloud top, begins to turn to ice, or to turn to ice, or glaciateglaciate..

Mature PhaseMature Phase

Dissipating PhaseDissipating Phase

Eventually the downdraft Eventually the downdraft overwhelms the updraft and overwhelms the updraft and convection collapses – convection collapses – becausebecausethe cloud is vertically-orientedthe cloud is vertically-oriented

Precipitation becomes lighter Precipitation becomes lighter and diminishes.and diminishes.

Cloud begins to evaporate from Cloud begins to evaporate from the bottom up often leaving the bottom up often leaving behind an “orphan anvil.”behind an “orphan anvil.”– Cirrus Spissatus cumulonimbogenitus

Air Mass ThunderstormsAir Mass Thunderstorms Usually weak (but can produce heavy Usually weak (but can produce heavy

rain in a short period of time).rain in a short period of time). Usually not severeUsually not severe Usually move slowly (weak winds aloft)Usually move slowly (weak winds aloft) Often develop and dissipate in less than Often develop and dissipate in less than

one hourone hour Form in a weakly sheared environment Form in a weakly sheared environment

and thus have a BUILT-IN SELF-and thus have a BUILT-IN SELF-DESTRUCT MECHANISM that guarantees DESTRUCT MECHANISM that guarantees a short lifetimea short lifetime

Life Cycle of Single Cell/Airmass StormV

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adar

Basic Concept of Basic Concept of BuoyancyBuoyancy

Vertically oriented force on a parcel of Vertically oriented force on a parcel of air due to density differences between air due to density differences between between the parcel and surrounding airbetween the parcel and surrounding air

Now to the blackboard!!Now to the blackboard!!

Impact of Pressure Gradient Impact of Pressure Gradient ForceForce

When an air parcel rises (due to buoyancy), it has to push When an air parcel rises (due to buoyancy), it has to push through air above it, through air above it, creating higher pressure (positive p’) creating higher pressure (positive p’) aboveabove (imagine pushing yourself through a crowd – or (imagine pushing yourself through a crowd – or drafting of race cars)drafting of race cars)

Below the rising parcel, a void is created, leading to Below the rising parcel, a void is created, leading to lower lower pressure at the cloud basepressure at the cloud base

H

L

PGF

The higher pressure The higher pressure above will push air to the sideabove will push air to the side, , making room for the rising parcel, while the lower making room for the rising parcel, while the lower pressure pressure below “attracts” surrounding airbelow “attracts” surrounding air to to compensate for the displaced parcelcompensate for the displaced parcel

Such a positive-negative pattern of p perturbation Such a positive-negative pattern of p perturbation creates a downward pressure gradient. The creates a downward pressure gradient. The PGF force PGF force therefore opposes the buoyancy forcetherefore opposes the buoyancy force, and therefore , and therefore acts to reduce the net upward forcing.acts to reduce the net upward forcing.


The degree of opposition to the buoyancy force depends upon the The degree of opposition to the buoyancy force depends upon the aspect aspect ratioratio of the cloud (L/H), or more accurately of the updraft. This aspect ratio of the cloud (L/H), or more accurately of the updraft. This aspect ratio dependence ties directly into the degree of validity of the hydrostatic dependence ties directly into the degree of validity of the hydrostatic assumption (see Bluestein Vol. II 433-434)assumption (see Bluestein Vol. II 433-434)

The The effect is larger for a widereffect is larger for a wider/large aspect-ratio /large aspect-ratio cloudcloud, and , and weakerweaker for a for a narrowernarrower/small aspect ratio /small aspect ratio cloud, becausecloud, because– For a narrow cloud, a small amount of air has to be displaced/attracted by the For a narrow cloud, a small amount of air has to be displaced/attracted by the

rising parcel, therefore the p perturbation needed to achieve this is smaller, so rising parcel, therefore the p perturbation needed to achieve this is smaller, so that the opposing pressure gradient is smaller (often << buoyancy) so a narrow that the opposing pressure gradient is smaller (often << buoyancy) so a narrow cloud can grow fastercloud can grow faster

– PGF is stronger for a wide cloud: as a result, the net upward force (buoyancy – PGF is stronger for a wide cloud: as a result, the net upward force (buoyancy – PGF) is significantly reduced, and the cloud can only grow slowly. When B and PGF PGF) is significantly reduced, and the cloud can only grow slowly. When B and PGF have similar magnitude, the vertical motion becomes quasi-hydrostatic – this is have similar magnitude, the vertical motion becomes quasi-hydrostatic – this is typical of large scale broad ascent.typical of large scale broad ascent.

Dynamic stability analysis of Dynamic stability analysis of inviscid flowinviscid flow shows that the shows that the infinitely narrow infinitely narrow clouds grow the fastestclouds grow the fastest, but in reality, the presence of turbulent , but in reality, the presence of turbulent mixing mixing prevents the cloud from becoming too narrowprevents the cloud from becoming too narrow, hence the typical aspect ratio , hence the typical aspect ratio of clouds is ~ 1.of clouds is ~ 1.


Hazards of Air Mass Hazards of Air Mass ThunderstormsThunderstorms

Heavy RainHeavy Rain HailHail

– Usually not terribly large– May be numerous

Downbursts or MicroburstsDownbursts or Microbursts– Exceptionally strong downdrafts that, when

they hit the earth, may have potentially destructive winds associated with them.

Hail Produced by an Hail Produced by an Ordinary ThunderstormOrdinary Thunderstorm

Downbursts and Downbursts and MicroburstsMicrobursts

MicroburstMicroburst– An anomalously strong, concentrated downdraft

that produces a pocket of dangerous wind shear near the ground over an area of 4 km or less in horizontal extent.

– Very short lived (last for 3-8 minutes)– Very small and isolated (city block)

Associated with cumulonimbus cloudsAssociated with cumulonimbus clouds– Can have heavy rain (Wet microbursts)– Can have vanishing sprinkles (Dry microbursts)

MicroburstMicroburst

Dry and Wet MicroburstsDry and Wet Microbursts

Dry MicroburstsDry Microbursts

A microburst with little or no precipitation.A microburst with little or no precipitation. Very dry air is located beneath the cloud Very dry air is located beneath the cloud

base.base. Hydrometeors falling into the dry air will Hydrometeors falling into the dry air will

evaporate causing a pool of cold air just evaporate causing a pool of cold air just below cloud base.below cloud base.

This cold pool descends rapidly forming This cold pool descends rapidly forming the dry microburst.the dry microburst.

Often you can’t detect them until it is too Often you can’t detect them until it is too late.late.

Dry MicroburstDry Microburst

Wet MicroburstsWet Microbursts Microbursts associated with moderate or heavy Microbursts associated with moderate or heavy

precipitation.precipitation. Some dry air above cloud top gets entrained in Some dry air above cloud top gets entrained in

the top of the thunderstorm.the top of the thunderstorm. This dry air mixes with cloud air causing some This dry air mixes with cloud air causing some

evaporation of the cloud.evaporation of the cloud. Evaporational cooling will form a pool of cold air Evaporational cooling will form a pool of cold air

near the top of the cloud.near the top of the cloud. This cold pool descends and adds to the This cold pool descends and adds to the

downdraft to form a microburst.downdraft to form a microburst. Often there is a “rain gush” coincident with the Often there is a “rain gush” coincident with the

microburst.microburst.

Wet MicroburstWet Microburst

Microburst Microburst DamageDamage

Visual DetectionVisual Detection

Detection of MicroburstsDetection of Microbursts Doppler Radar (Airport and Doppler Radar (Airport and

Aircraft)Aircraft)– Best when precipitation is present– Terminal Doppler Weather Radar (TDWR)

Integrated Terminal Weather Integrated Terminal Weather System (ITWS)System (ITWS)

Detection of MicroburstsDetection of Microbursts LLWASLLWAS

– Low level wind shear alert system– A network of wind sensors positioned

around the airport.– Does not detect elevated microbursts or

microbursts that are between sensors.

Microbursts and AviationMicrobursts and Aviation

Microbursts are extremely Microbursts are extremely hazardous to low-flying aircraft hazardous to low-flying aircraft because of because of – Low airpseedLow airpseed– Proximity to the groundProximity to the ground– ““Dirty” aerodynamic configuration Dirty” aerodynamic configuration

(flaps out, gear down)(flaps out, gear down)– Difficulty of visual microburst Difficulty of visual microburst

detectiondetection– Rapid onset and short durationRapid onset and short duration

MicroburstMicroburst

Glide Slope

Runway

Flight of Eastern 902Flight of Eastern 902

Flight of Eastern 66Flight of Eastern 66

1

136

115

154136

38

00

50

100

150

Nu

mb

er

of

Fa

tali

tie

s

'65-'69 '70-'74 '75-'79 '80-'84 '85-'89 '90-'94 '95-'98

Year of Accident

Fatalities Associated with Fatalities Associated with Aviation Wind Shear Aviation Wind Shear

AccidentsAccidents

Wind Shear R&D

Pilot Training

TDWR

single cell thunderstorms metr 4803 hazardous weather detection and prediction spring, 2005 semester...

Documents

line of storms

multicell storms

heightsevere storms

pulse storms

continuous spectrum

weak tornadoes

moderate size hail

large hail