MFE 658 Lecture 2bSevere Thunderstorms

Severe Thunderstorms:– Atmospheric Ingredients– Large Hail,– Strong Winds and Downbursts– Tornados

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2011 Year of Environmental Disasters

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2011 Year of Environmental

Disasters

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2011 Year of Environmental Disasters

$52 Billion in damage from weather disasters in 2011.

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Bad Tornado Season in 2011

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MFE 658 Lecture 2b

Severe Thunderstorms

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Stability in the AtmosphereEssentially, warm (cold) air relative to its surroundings will rise (sink). To determine the atmosphere’s stability for vertical motions we need to know and compare the temperature of rising/sinking air parcels with the temperature in the surrounding environment.

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Stability in the Atmosphere

StableReturns

UnstableKeeps Going

NeutralStays atnew level

An InitialPerturbation

How do we diagnose stability in the atmosphere?What assumptions are implicit to our analysis of soundings for stability?

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What conditions support taller cumulus development ?

• A less stable atmospheric (steeper lapse rate) profile permits greater vertical motion

• Lots of low-level moisture permits latent heating to warm parcel, accelerating it upward

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When is a Thunderstorm Severe?

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Severe Thunderstorms

NWS Criteria: to qualify as a severe thunderstorm at least one of the following must be present: • Large Hail > 1 Inch• Strong straight line winds >50 kt • Presence of a Tornado

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Air Mass ThunderstormsEnvironment: Air Mass thunderstorms form in regions of relatively light winds and light wind shear. Thus they form away from fronts and jet streams.

LOW WIND SHEAR HIGH WIND SHEAR

IN AN AIR MASS NEAR FRONTS OR JET STREAMS

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Severe Thunderstorms:

• Differ from air mass thunderstorms: • Updraft may remain strong for long periods. • May reach great heights and strength.

– Rotating or sloped updraft.– Updraft size reduces the effects of entrainment.

– Precipitation falls free of the updraft.

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Deep ConvectionAlmost all severe weather events are associated with deep convection. To achieve deep convection there are three necessary ingredients:1. A moist layer of sufficient depth in the lower troposphere.2. A steep enough lapse rate above the moist layer to allow for

a substantial "positive area".3. Sufficient lifting of a parcel from the moist layer to allow it to

achieve a temperature warmer than the environment (level of free convection (LFC)).

Once deep convection is found to be possible, the potential for severe events to occur with this convection becomes a primary forecast concern. To assess the potential for deep convection, the forecaster must analyze the current thermodynamic profile of the troposphere and forecast changes resulting from thermal and moisture advection and vertical motion fields.

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> 3/4 inch diameter

Large Hail

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Large HailHail DamageOne Colorado hailstorm caused ~$1 billion damage in July 1990.Hailstones have high density and fall fast (up to 90 mph)• Hail damages• Crops• Cars, trucks…• Livestock

This is one of the largest hailstones ever observed in the United States. The stone fell on June 22, 2003 in Aurora, Nebraska.

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Large Hail

Large hailstones make several trips up into the cloud. Layers in the hailstone tells about its history.

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Large Hail

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Large Hail

Large hail forms in clouds with• High supercooled water content• Very strong updrafts

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Large Hail• The maximum hailstone size attained aloft is related to the

strength of the updraft (Doswell, 1985).

• A necessary ingredient for the development of large hail is a strong updraft, one that is capable of supporting the weight of a hailstone long enough for it to reach a large size.

• A primary contributor to strong updrafts is ample instability (large positive area).

• In general, the greater the thermal buoyancy of the environment, the greater the potential for large hail.

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Large HailA factor that affects the strength of updrafts is vertical wind shear. Numerical simulations show that vertical pressure gradient accelerations induced by the interaction of the updraft with the environmental winds can contribute substantially to updraft speeds. Another important factor affecting hailstone size at ground level is melting as the hailstones descend from the freezing level to the surface.

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Large Hail: Cold Low Aloft

Kona lows are associated with cold lows aloft and can produce hail in Hawaii.

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Strong Straight-line Winds

>50 knots (57 MPH)

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Strong straight-line winds in thunderstorm are often the result of precipitation produced downdrafts called downbursts.

Strong Straight-line Winds

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Downburst

• Downbursts can reach wind speeds of ~200 mph!• Damage to forests and man made structures.• Cause of numerous aviation disasters.

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Downbursts can reach wind speeds of ~200 mph!Damage to forests and man made structures.Cause of numerous aviation disasters.

Downburst

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Downbursts

Airline crashes related to downbursts prior.

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Schematic diagram of Andrews AFB microburst 8/1/83

Downbursts

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Andrews AFB microburst (8/1/83) winds to 130 kt

Downbursts

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Strong Straight-line WindsDamaging straight-line winds associated with deep convection almost always are generated by outflow that occurs at the base of the downdraft. Ingredients necessary for damaging winds at the surface are ones that promote very strong downdrafts.

These include1. Precipitation loading 2. Evaporative cooling3. Downward transport of momentum from the strong

flow aloft

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Bow Echo

Curved bow echo structure reflects the diverging outflow winds associated with the systems very strong downdraft

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Tornadoes

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TORNADOES

• Source of Spin• Anatomy• Life Cycle• Nowcasting• Climatology• Damage

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Nature’s most Violent Storms

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Tornado

The word "tornado" is an altered form of the Spanish word tronada, which means "thunderstorm". This in turn was taken from the Latin tonare, meaning "to thunder". It most likely reached its present form through a combination of the Spanish tronada and tornar ("to turn"); however, this may be a folk etymology. A tornado is also commonly referred to as a twister, and is also sometimes referred to by the old-fashioned colloquial term cyclone.

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Tornado

A violently rotating column of air, in contact with the ground, either pendant from a cumuliform cloud or underneath a cumuliform cloud, and often (but not always) visible as a funnel cloud.

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1st Tornado Photograph

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First Tornado ForecastOn March, 25 1948, Major Fawbush and Captain Miller determined that the conditions of the atmosphere just west of Tinker AFB, OK were suitable for tornado development. A tornado forecast was then issued, the first of its kind. A few hours later, a tornado arrived with the storm, causing significant damage to the base. However, no deaths and only a few injuries occurred because many had been warned by the tornado forecast.

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First Tornado Forecast

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Texas Tornadic Supercells

Visible imagery shows location of overshooting tops, anvil spreading, and storm motion.

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TORNADOS

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Marilee Thomas of Beaver City, NE took this photograph of her daughter Audra about two miles from a Furnas County tornado in April 1989.

Tornados

• Environment• Storm Structure• Life Cycle• Source of Spin• Climatology• Forecasting• Damage

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1st Tornado Photograph

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Tornado Environment

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A mesocyclone is a rotating vortex in conjunction with the updraft in a supercell storm. Supercells develop mesocyclones by tilting environmental and/or locally generated horizontal spin (vorticity).

Tornadic Thunderstorm Structure

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Wall Cloud

The wall cloud usually exhibits a lot of rotation.

Tornado Life Cycle

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Early stage of large tornado; stubby funnel

Young Large Tornado

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Mature Tornado

Rotation has reached the ground. Debris cloud kicked up by the winds is visible.

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Tornado funnel and debris cloud

Mature Tornado

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Mature Tornado

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Tornado in Final Rope Stage

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Source of Spin

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Updraft Stretches Rotating Column

Source of Spin

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Simulation of Tornado Formation

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Tornados can occur whenever severe thunderstorms occur, including in hurricanes.Tornados are most common in spring and early summer and are most common in the latter half of the afternoon. However, tornados have occurred during every month of the year and during every hour of the day in the U.S.

Tornado Climatology

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Tornado Climatology

Tornados are most common in spring and early summer.

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Tornados are most common in the mid to late afternoon.

Tornado Climatology

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Tornado Climatology

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Tornado Climatology

Tornado Deaths in the U.S.

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Tornado Climatology

Tornado Deaths in the U.S. per Million People

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Tornado Forecasting

• Tornados form quickly so forecasting is a matter of nowcasting.

• Tools to observe tornado formation include spotters (people go out looking and call in reports), radar, and satellite.

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Tornado Forecasting is a matter of Nowcasting• Radars can see impact of rotation on reflectivity (hook echo).• Doppler radars can see wind shear over short distances.• Radars can track storm motion.• Satellite imagery can document rapid anvil growth and

unusual storm motion.• Radiosonde and aircraft soundings show environmental

conditions: large instability and wind shear.

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Hook Echo in Radar Reflectivity

A hook shaped echo in the radar reflectivity PPI indicates rotation associated with larger circulation associated with tornados.

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Satellite Imagery as a ToolTornadic thunderstorms form ahead of an upper-level trough. Anvils usually blow to the NE by strong SW jet.

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Visible and IRImageryVisible and IR imagery shows overshooting top, anvil spreading, and storm motion.

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NWS Tornado Advisories! The NWS issues two levels of advisories to the

public concerning tornados.

1. Tornado Watch: when the conditions are favorable for the formation of tornadic thunderstorms.

2. Tornado Warning: when a tornado or evidence for a tornado circulation has been observed. The location and track of the feature is given and if it is headed your way head for the basement!!

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Tornado Safety Tips• Take cover on the lowest floor of a sturdy building,

preferably a basement.• Go to a windowless, inner room such as a closet, hallway

or bathroom.• Avoid windows.• Avoid buildings such as strip malls with large unsupported

roof spans.• Get under a mattress, table, desk, etc.• Avoid automobiles and mobile homes.• If caught in the open, lie flat in a ditch or depression.• Move at right angles to the tornado's motion, if possible. • Be careful of downed power lines, broken gas lines, etc.

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Tornado Damage

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F-0 Light damage. Wind up to 72 mph. F-1 Moderate damage. Wind 73 to 112 mph. F-2 Considerable damage. Wind 113 to 157 mph. F-3 Severe damage. Wind 158 to 206 mph. F-4 Devastating damage. Wind 207 to 260 mph. F-5 Incredible damage. Wind above 261 mph.

Fujita Tornado Intensity Scale

Tornado Damage

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Cause of Tornado DamageRecall that the winds in a tornado funnel are nearly in cyclostrophic balance where the pressure gradient force (directed inward) is balanced by centrifugal force (directed outward).

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Cause of Tornado Damage

Suction vortices cause maximum damage

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Severe Thunderstorm Damage

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Severe Thunderstorm Damage

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Tornado Damage Oklahoma-Kansas Tornadoes • May 1999. Outbreak of F4-F5 tornadoes hit the states

of Oklahoma and Kansas, along with Texas and Tennessee,

• Oklahoma City area hardest hit• At least $1.1 billion damage/costs; 55 deaths.

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1999 OK Tornados

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Tornado paths in and around Oklahoma City on May 3, 1999. Individual tornado tracks are identify by different colors. Tornado intensity is given by Fujita Scale values. Note that the largest tornado track, that cuts through the center of the city, has intensity values recorded along its entire path.

1999 OK Tornados

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Tornado Damage Swath

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‘89 Raleigh, NC F-4 Tornado

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‘89 Raleigh, NC F-4 Tornado

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‘89 Raleigh, NC F-4 Tornado

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‘89 Raleigh, NC F-4 Tornado

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‘89 Raleigh, NC F-4 Tornado

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Parting Shots

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Questions?

Tornado Star Knife85

Questions?

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Tornado over Kapolei

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