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Physics of Thunderstorms
• Two fundamental ideas:
• Convection
• Latent heat of vaporization/condensation
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
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!
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
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
Supercell
• Requires unstable atmosphere and strong vertical wind shear
• “Capping inversion” prevents gradual energy release
• Entire storm rotates
• Updraft and downdraft in different positions
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