Notes on Severe Weather and Tornadoes: Lecture 4/30

Sunstorms and Severe Thunderstorms

Sunstorms are more tolerable and lead to heavy rain.
Long life cycle of storms increases rain totals and hail size.
Damaging winds and tornadoes are also related to longer life cycles.
Mystery: What contributes to the long life of these storms?
Answer: Vertical shear in the environment and separation of precipitation production.
The vertical shear controls the separation, contrasting with MS thunderstorms that arise from daytime heating.

Tornadoes are intense, Earth-oriented vortices rotating under thunderstorms.
The rotation source is a key question.
It's not just random swirling but organized and on a larger scale.
Severe thunderstorms that spawn tornadoes develop in environments with vertical wind shear.
Vertical wind shear means wind speed changes with height.
Horizontal temperature contrasts often cause vertical wind shear at mid-latitudes.

Initial State: Horizontally oriented tube of air.
Updraft Influence: Intense updraft distorts the tube.
Vertical Orientation: Parts of the tube become vertically oriented.
Vorticity Increase: The distortion leads to increasingly vertical vorticity, resembling a tornado.
Low Pressure: Localized low-pressure regions form, drawing in surrounding air.
Spin Up: Inrushing air increases rotation, similar to a figure skater pulling arms in.

The exact transition from mesocyclone to tornado funnels is still under debate.
Difficulty in modeling and observing at relevant scales contributes to the uncertainty.
Pressure differences near the tornado center can be intense (10-20 millibars over a mile).
These intense pressure differences cause very high winds.
Even without a tornado, a rotating mesocyclone can sometimes be visually identified by a dark, rotating cloud base.

Vortex Rings: Paul Markowski and Yvette Richardson's theory involves 3D vortex tubes and vortex rings ingested into the updraft, leading to localized vorticity.
Vortex Sheets: Colleague Greg's retired theory on vortex sheets adjusted and rearranged in the updraft.

Funnel clouds are tornadoes that don't reach the ground.
Whether a funnel cloud becomes a tornado depends on intensity and randomness.
Larger initial "oomph" may allow some tornadoes to sustain themselves, but this is not well understood.

Tornadoes occur in many places worldwide, but frequency and intensity vary.
The United States and Southern Canada are primary locations.
Other regions include the maritime continent, parts of Japan, South Korea, China, Europe.
Many land masses never experience tornadoes.

Canada: Concentration in Southern Plains and along the Great Lakes.
Europe: Most intense tornado on the border of France and Belgium (F4). Tornadoes occur along the coasts of Italy, Croatia, and Serbia.

The Central Plains of the U.S. experience the highest rate (5-10 per 10,000 square miles).
Central Florida also has a high frequency, though not as intense.

Gulf of Mexico: High dew point temperatures (70s °F) due to warm water (75-80°F) leading to high humidity.
Mexican Plateau: High elevation (10,000 feet) bringing dry air.
Upper Level Trough: Divergence aloft leading to surface convergence and cyclone development.
Surface Cyclone: Develops east of the upper-level trough.

Parcel A (moist ascent): cools at moist adiabatic lapse rate (approximately 6^"{C}/km). Final temperature: 20^"{C} - 6^"{C} = 14^"{C}.
Parcel B (dry ascent): cools at dry adiabatic lapse rate (approximately 10^"{C}/km). Final temperature: 21^"{C} - 10^"{C} = 11^"{C}.

The lapse rate between the lifted Parcels A & B would be \frac{14^"{C}-11^"{C}}{0.25 km} = 12^"{C}/km at this point, greater than dry adiabatic lapse rate (10^"{C}/km). This indicates an unstable condition.
Initially stable due to inversion, lifting and differential cooling create instability.

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