Severe Weather

ATS 351 Lecture 10 November 9, 2009

Types of Severe Weather

      Thunderstorms Hail Lightning Flood Tornado Severe Wind (Straight-Line Winds)

Thunderstorm Distribution

Favorable Conditions

     Instability Fuel Initial Lift Shear Capping Inversion

Instability

    Steep lapse rate Means warm, moist air near the surface Colder air above it Needs to be calculated from a sounding

Fuel

   Just like any other weather phenomenon, a storm needs fuel to sustain itself The fuel for a storm is just a continued supply of what started it - Heat - Moisture - Lift The storm needs to remain in areas of warm, moist air. If storm moves into a colder region, it will die

Sources of Lift

    Convective lifting Boundaries  Fronts  Drylines  Outflow boundaries Orographic Convergence

Shear

   Because of the way a thunderstorm works, it needs to be tilted to remain strong Therefore, winds need to change with height Two kinds of shear  Speed Shear: Wind is faster as you go up  Directional Shear: Wind changes direction with height

Capping Inversion

   If the atmosphere is unstable all the way up, you get a constant updraft It is more effective when the energy is held back and released all at once  This can happen by having a stable layer near the surface that suppresses convection As ground heats during the day, energy builds up until it can “break the cap”  Also referred to as a “capping inversion”  CIN

Back to the Skew-T

   Meteorologists have formulated various numbers that can tell how favorable the weather is for a storm. These quantities can describe things such as:  Instability  Shear  Or a combination of both CAPE (Convective Available Potential Energy  How unstable atmosphere is LI (Lifted Index; normally at 500mb levle)  LI = T environment – T parcel

Thunderstorm Development

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Stages

 Cumulus  Mature  Dissipation

Cumulus Stage

     Warm moist air rises, condenses Latent heat release keeps air in cloud warmer than environment Grows to a towering Cu Cloud particles grow larger, begin to fall No precipitation at surface

Mature Stage

      Marked by appearance of downdraft  Falling cloud drops evaporate, cooling the air Storm is most intense during this stage Cloud begins to form anvil May have an overshooting top Lightning and thunder may be present Gust front forms   Downdraft reaches the surface and spreads out in all directions Gust front forces more warm, humid air into the storm

Dissipation Stage

    Usually follows mature stage by ~15-30 min Gust front moves out away from the storm, and moist air is no longer lifted into the storm.

Downdrafts become dominant Low level cloud drops can evaporate rapidly, leaving only the anvil as evidence of the storm’s existence

Types of Thunderstorms

   Thunderstorms come in many varieties Likelihood of severity proportional to storm lifetime NWS definition of severe (one or more of the following elements)  ¾” or larger diameter hail  50 kt (58 mph) or greater winds  tornadoes

Single Cell Thunderstorms

    Also referred to as ordinary, pulse, or air mass thunderstorms Typically do not produce severe weather Three stages   Cumulus Mature  Dissipating Life span: ~45-60 min.

Multi-Cell Storms

     Cluster of storms moving as a single unit Stronger wind shear than the ordinary cell case More organized multi-cells   Bow Echoes Squall Lines New cells tend to form on the upwind (W or SW) edge of the cluster, with mature cells located at center and dissipating cells found along the downwind (E or NE) portion of the cluster Multiple cells compete for warm, moist low-level air so not incredibly strong and have short life spans

Multi-cell Storms

   Cell 1 dissipates while cell 2 matures and becomes dominant Cell 2 drops heaviest precipitation as cell 3 strengthens Severe multicell storms typically produce a brief period of hail and/or downbursts during and immediately after the strongest updraft stage

Updrafts and Downdrafts

 Degree of instability and moisture determine the strengths of updrafts and downdrafts

Vertical Wind Shear

   Change of wind speed and/or direction with height Weak vertical wind shear: short-lived since rainy downdraft quickly undercuts and chokes off the updraft Sheared environments are associated with organized convection

Vertical Wind Shear

      

Gust fronts

An area of high pressure created at the surface by cold heavy pool of air from downdraft called a mesohigh Gust front: leading edge of cold air from downdraft Passage noted by calm winds followed by gusty winds and a temperature drop then precipitation Convergence region between cold outflow and warm, moist inflow Can generate new cells Leads to multi-cell storms Production of shelf and roll clouds

Downbursts

Overshooting tops

Mesoscale Convective Systems (MCS ’s)

• • • • • Individual storms can grow and organize into a large convective system (weak upper level winds) • Definition: 100km contiguous group of t-storms • Range of lifetimes New storms grow as older ones dissipate (reinvigorates itself) Provide widespread precipitation Can spawn severe weather • Hail, high winds, flash floods, tornadoes Formation (in U.S.) • Usually during summer when a cold front stalls beneath an upper level ridge of high pressure • Surface heating and moisture can generate thunderstorms on the cool side of the front

Squall Lines

    Multicell storms can form as a line of storms extending for hundreds of km, called a squall line Squall lines often form along or just ahead of a cold frontal boundary (called pre-frontal squall lines) Supercells may be embedded within prefrontal squall lines Leading line of thunderstorms may be followed by large region of stratiform precipitation where the anvil cloud trails behind the main storm.

Bow Echoes

      Bow Echo – a bowed convective line (25 – 150 km long) with a cyclonic circulation at the northern end and an anticyclonic circulation at the southern end Strong jet in from behind Can produce long swaths of damaging winds Form in conditions of large instability and strong low level shear Observed both as isolated convective systems or as substructures within much larger convective systems (such as a squall line) May contain strong winds or tornadoes

Supercells

      Characterized by rotating updrafts (called a mesocyclone) Differ from multicell cluster because of rotation and that updraft elements merge into a main rotated updraft rather than developing separate and competing cells Can persist for 12 hours and travel hundreds of miles Forms in environments of strong winds aloft Winds veer with height from the surface Can be classified as either High Precipitation (HP) or Low Precipitation (LP)

Hail

• Storms contain updraft and downdraft – Not same strength everywhere • Hail that swept upwards in a region of lesser updraft • Begins to fall, can fall into stronger updraft • Cycling may occur • Important contributors to creating charged regions in clouds

Lightning

    Inside a cloud, updrafts and turbulence toss ice particles around Each collision creates a small amount of electric charge After a few million of those, the charge is too much to be held back by the air Discharges all at once in a flash of lightning

Lightning

   The temperature of lightning is roughly 30,000 degrees C  The surface of the sun is only about 5700 degrees C One bolt of lightning carries enough electricity to power the entire United States for 0.1 seconds Lightning has been known to strike up to 15 miles from the actual storm

Lightning Misconceptions

    Lightning comes down from the clouds    It actually comes down AND goes up. As a bolt begins the trip down, a “streamer” from the ground shoots upward toward the oppositely charged cloud. The flash happens when they meet in the middle.

Entire process happens in under 0.001 seconds Lightning always hits the tallest object   Not true. It may seem that way, but lightning simply takes the “path of least resistance”.

If you conduct electricity better than the 30 ft. tall tree next to you, you will get hit Lightning never hits the same place twice  That ’s just wrong.

 There are many documented cases of lightning hitting twice in the same spot  Sometimes only a few seconds apart!

Lightning Fatalities

Thunder

 If air is heated from 75 to 90 degrees, it will expand  If air is heated from 75 to 50,000 degrees, it will expand quickly  • Thunder is a compression wave due to this rapid heating The thunder you hear is not lightning “hitting the ground” but actually a sonic boom

      Formation Life Cycle Definition Types Damage EF-scale

Tornadoes

Wall clouds

    Lowering of cloud base Visible manifestation of the mesocyclone at low levels (contains significant rotation) Develop when rain-cooled air is pulled upward, along with more buoyant air  Rain-cooled air usually very humid so upon being lifted, will quickly saturate to form the lowered cloud base Tornado often forms from within wall cloud

Formation

   Tornadogenesis is the formation of tornadoes  We know relatively little about this process Basic formation steps are known Details are missing, but they are very crucial details

• Vertical wind shear crucial Rotation tilting • After horizontal rotation is established, the storm’s updraft works to tilt it upright • Now the storm has a vertically rotating component

Mesocyclone

   The new rotating storm is called a mesocyclone Characterized by rotating updraft At this point, the rotation can be picked up on Doppler radar if it is strong enough

Supercell Tornado Formation

Funnel Cloud

  Area of rotation that does not touch the ground Often mistaken for a tornado

Ground Contact - Tornado

   Once the rotation reaches the ground, the downward moving air will spread out Some will go back toward the center of the funnel, converging and forcing it back up The upward motion will begin to kick up debris

Suction Vortices

• • • Many violent tornadoes contain smaller whirls that rotate inside them Rotate faster, and do a great deal of damage How these form is still not completely understood

Damage

   The highest (strongest) winds on Earth are found inside tornadoes The strongest tornado ever recorded had winds over double that of the strongest hurricane Damage can be devastating

Fujita Scale

   In 1973, Ted Fujita of the Univ. of Chicago devised a scale for rating the intensity of a tornado Subjective damage scale that classified a tornado on a scale from F0 to F5 Assessed by going to damage sites and using a checklist

Enhanced Fujita Scale

• Proposed in early 2005, adopted in 2007 • Replaces Fujita Scale • Uses more criteria to assess damage • Has 28 “damage indicators” that surveyors look at

FUJITA SCALE F Numbe r 0 1 2 3 4 5 Fastest 1/4 mile (mph) 40-72 73-112 113-157 158-207 208-260 261-318 3 Second Gust (mph) 45-78 79-117 118-161 162-209 210-261 262-317 DERIVED EF SCALE EF Number 0 1 2 3 4 5

OPERATIONAL EF SCALE

3 Second Gust (mph) 65-85 86-109 110-137 138-167 168-199 200-234

EF Number 3 Second Gust (mph) 0 1 2 3 4 5 65-85 86-110 111-135 136-165 166-200 Over 200

http://www.spc.noaa.gov/efscale/ef-scale.html

EF0 “Light damage” EF1 “Moderate damage” EF2 “Considerable damage” EF3 “Severe damage” EF4 “Devastating damage” EF5 “Incredible damage”