Thunderstorms

ASTR /GEOL 1070

Physics of Thunderstorms

• Two fundamental ideas:

• Convection

• Latent heat of vaporization/condensation

Energy Source

• Energy difference between– Warm, moist surface air– Cool, dry upper air

Humid Surface Air

• Some energy is “latent” in humidity

• Lower temperature than if dry

• Will not immediately rise

• Unstable when condensation starts

Lifting a Surface Parcel

• Air rises, expands, and cools at the dry adiabatic lapse rate (fast T drop)

• Until it reaches its dew point

• Then rises and cools at the moist adiabatic lapse rate (slower T drop)

• Warmer than the surrounding air

Lifting Mechanism

• “Kick” to bring air to LCL

• Uplift over mountains

• Advancing cold front

• Late afternoon heating

End Result

• When moist air finally begins to condense, it becomes very buoyant

• Continues rising

• If there is enough warm, moist air, it rises all the way to the tropopause

Anvil

• Air in the stratosphere becomes warmer with altitude

• Cloud stops rising

• Piles up at neutral buoyancy

• May have overshooting top if energetic

Potential for Convection

Lifted Index

Lifted Index

• Compares theoretical lifted temperature of surface air to actual temperature of upper air

• If (lifted temperature) > (upper-air temperature), parcel unstably rises

Lifted Index

• LI = (upper-air temp)−(lifted air temp)• Meanings

> 0: stable air; no thunderstorms

0 to −2: possible thunderstorms with lifting mechanism

−2 to −6: thunderstorms likely, possibly severe

< −6: severe thunderstorms likely

Determine a L.I.!

• “Lift” parcel, cool at dry lapse rate until saturated

• Continue to “lift,” but at saturated lapse rate (less T drop)

• Compare to actual air temp at 500 mbar

Education

al

!Easy!

Fun!

Making a Stüve Plot

Potential for convective storms

Thermodynamic Plot

• Solid slanted lines (dry adiabats) show “lapse rate:” temperature drop with elevation gain

• If you know surface T,p you know it for the rising parcel

Thermodynamic Plot

• Rising moisture-saturated air condenses

• Releases heat

• Temperature drop is inhibited

• Slanted dashed curves: saturated adiabats

Thermodynamic Plot

• Mixing ratio: (mass of water vapor)/(mass of air) (g/kg)

• Dotted lines: saturation mixing ratios

• Dew point T, p at that mixing ratio

Lifting a Surface Parcel

• Until it reaches its dew point

• Then rises and cools along a saturated adiabat

• Air rises, expands, and cools along a dry adiabat…

Finding the Dew Point

• Lift along mixing ratio until it meets the dry adiabat

• That is when the moisture begins to coondense

Lifting a Surface Parcel

• Then lift along the saturated adiabat

• Lift along the dry adiabat and the saturation mixing ratio until they meet

Lifted Index

• Lift surface air along dry adiabat until saturation

• Then lift along saturated adiabat to 500 mb

• Lifted index = (air temp at 500 mb) − (lifted parcel temp at 500 mb)

• Best chance of severe thunderstorms when L.I. < −6 °C

Task

• Plot the temperatures and dew points

• Lift the surface parcel to 500 mb pressure

• Determine the lifted index

Thunderstorm Varieties

Single-Cell Storm

• Begins as a simple cumulus cloud (Cumulus stage)

Single-Cell Storm

• Grows into a towering cumulus cloud

• Falling rain creates a downdraft

• Mature stage

Single-Cell Storm

• Cool air sinks into updraft

• Cuts off storm’s energy source (dissipating stage)

• Storm dies in a few hours

Multicell Storm: Squall Line

• Cold front initiates lifting

• Storms appear in a line

COOL WARM, MOIST

L

Multicell Storm: MSC

• Wind shear displaces downdraft

• Downdraft from one storm spawns another

Supercell

• Requires unstable atmosphere and strong vertical wind shear

• “Capping inversion” prevents gradual energy release

• Entire storm rotates

• Updraft and downdraft in different positions

Thunderstorm Effects

U.S. Weather-Related Deaths

Average deaths per year

Flood 136

Lightning 85

Tornado 73

Hurricane 25

Hail 1

Source: Ackerman and Knox, Meteorology: Understanding the Atmosphere

Lightning and Hail

• Wind shear (rising and falling air) causes static charges → lightning

• Rain caught in updrafts can freeze—sometimes repeatedly → hail

Hail

• Destroy 1% of world agricultural production annually

• “Hail Alley:” Denver basin

Angular Momentum

• Moving toward a rotational axis causes spin to speed up!

Tornadoes

• Usually arise in supercells

• Horizontal wind shear causes horizontal-axis rotation

• Updraft re-orients vortex