Chapter 12: Thunderstorms and Tornadoes Study Notes

Chapter 12: Thunderstorms and Tornadoes

Thunderstorm Overview

  • Thunderstorm: Defined as convective rainfall accompanied by thunder and lightning.

  • Severe Thunderstorm: According to the National Weather Service (NWS), a severe thunderstorm has any of the following characteristics:

    • Hail of 1 inch or greater in diameter.

    • Winds reaching or exceeding 58 mph.

    • Tornadoes.

  • Note: Heavy rain, flooding, or lightning are not included in the NWS definition of a severe thunderstorm.

  • For our purposes, we will classify “severe thunderstorms” as those with intense updrafts.

Thunderstorm Ingredients

  • Moisture: Acts as fuel or energy necessary for the storm's development and sustenance.

  • Instability: Provides buoyancy required for rapid upward vertical motions within the storm.

  • Lift: Essential for raising air parcels to their Level of Free Convection (LFC).

  • Conclusion: All three ingredients are essential to thunderstorm formation; none is considered the most important.

Model of a Single Cell Thunderstorm

  • Developed from the Thunderstorm Project established in 1949.

  • Represents the simplest form of a thunderstorm.

  • Contains critical components observed in all types of thunderstorms.

  • Functions as a fundamental building block for more complex thunderstorm systems.

Single-Cell Thunderstorm Life Cycle

Cumulus Stage

  • Characterized by rising motions producing an updraft that is buoyant, leading to instability.

  • Vertical motions occur at the cloud top, where evaporation leads to cooling effects, which include:

    • Enhanced instability.

    • Increased relative humidity.

Mature Stage

  • Features both updrafts and downdrafts.

  • Downdrafts develop from a cold pool, often becoming prominent as precipitation begins.

  • Fully developed storms extend to the equilibrium level (EL), which can coincide with the tropopause.

  • Notable features include:

    • Overshooting tops: Indicate the strength of updrafts.

    • Jet stream flow contributes to the formation of an anvil cloud at the storm's top.

    • Inflow creates shelf clouds, which can form ahead of the storm.

    • Downdraft contact with the ground generates a gust front that affects surrounding air.

Dissipation Stage

  • Occurs when the expanding gust front disrupts the updraft, leading to a decline in the storm's strength.

  • Characterized by:

    • Exclusively downdraft activity.

    • The storm “rains out” as precipitation ceases.

  • Typical specifications of single-cell storms:

    • Lifetime: 30 minutes to 1 hour.

    • Size: Approximately 10 km.

Single vs Multi-Cell Thunderstorms

  • Observations show that thunderstorms often last longer than 1 hour and can exceed 10 km in size.

  • Most storms contain multiple cells:

    • Quasi-Linear Convective System (QLCS): A linear arrangement of thunderstorms.

    • Bow Echoes: A specific phenomenon where thunderstorm clusters bow outward.

    • Mesoscale Convective Complexes (MCC): Larger systems composed of multiple thunderstorms.

  • Supercells: Refer to the long-lived single-cell thunderstorms.

Multi-Cell Thunderstorms

  • Multi-cell thunderstorms have distinctive features, two predominant being:

    • Tilted Updrafts: Result from wind shear, which is defined as the change in wind speed/direction with height.

    • Intense Downdraft and Gust Front: Characteristic of multi-cell structures.

Tilted Updrafts

  • A tilted updraft design allows the updraft to be spaced apart from the downdraft, prolonging the updraft's life.

  • The tilt is attributed to the presence of wind shear.

Wind Shear

  • Wind shear is the variation in wind both in direction and speed with height in the atmosphere. For example:

    • At 21,120 ft: 60 MPH

    • At 15,000 ft: 35 MPH

    • At 10,000 ft: 17 MPH

    • At 5,000 ft: 12 MPH

    • At 0 ft: 2 MPH

Cold Pools and Downdrafts

  • Strong downdrafts are associated with significant evaporative cooling, resulting from dry air aloft creating a heavy cold air pocket falling to the surface.

Generating New Cells

  • The expanding gust front acts as a new lifting mechanism for new thunderstorm cells to form, particularly near where moisture flows into the existing storm.

Storm vs Cell Motion

  • It's essential to differentiate between the overall storm motion and the motion of individual cells, as they can differ in direction and speed.

Thunderstorm Motion

  • To correctly forecast storm behavior, tracking cells and shifting them eastward may only provide accurate results for 15-30 minutes.

  • Subsequently, the evolution of new cells can change the storm's overall motion, usually characterized by right-movers, meaning new cells form on the storm’s right flank.

Lightning

  • Charge Separation: Occurs due to violent collisions between ice and supercooled water within the storm system.

  • A typical lightning bolt requires approximately 1 million volts to bridge the gap and conduct electricity in the atmosphere.

Lightning Formation

  • It begins with the formation of stepped leaders descending from the cloud, which connects with a return stroke from the surface.

  • This return stroke neutralizes the charge difference between the cloud and ground, commonly involving clouds with negative charges near the surface and positive charges at higher elevations.

Severe Thunderstorms

  • NWS Definition: A severe thunderstorm must meet the following criteria:

    • Inclusion of hail (indicates strong updraft conditions).

    • Winds of 58 mph or stronger (caused by downdrafts or strong winds aloft).

    • Tornadoes are often associated with severe storms due to their rotation.

  • For analysis, severe thunderstorms are strong systems with a potential to produce the listed impacts.

Types of Strong Thunderstorms

  • Pulse Thunderstorms: Rapidly building single-cell systems that tend to collapse shortly after forming, with high winds and hail as the main risks.

  • QLCS (Quasi-Linear Convective Systems): Linear thunderstorms influenced by strong cold pools and shear, posing risks of high winds, hail, and tornadoes (formerly termed as squall lines).

  • Supercells: Single-cell thunderstorms that exhibit strong shear and inherent instability, associated with significant wind, hail, and tornado risks.

Summary of Supercell Thunderstorms

  • A supercell is defined as a single-cell thunderstorm possessing a rotating updraft.

  • Supercells are infrequent but require an optimal balance of wind shear and instability to develop successfully.

  • Wind shear typically alters both the speed and direction of the winds involved.

  • Supercells are characterized by two distinct downdrafts:

    • Forward Flank Downdraft: Occurs ahead of the updraft zone.

    • Rear Flank Downdraft: Found behind the updraft.

  • Supercells serve as the predominant sources for major hail storms and significant tornado occurrences.

Classic Supercell Example

  • A notable supercell occurrence was documented on May 1, 2018, over Kansas, showcasing features like:

    • Classic hook reflecting typical supercell structure.

    • Notable forward and rear flank downdrafts with identified tornado locations.

Tornado Development

  • A tornado is characterized by its circulation that is in contact with the ground; otherwise, it remains a funnel cloud when aloft.

  • The parent rotation of a tornado, termed a meso-cyclone, is derived from distinct wind shear forces.

  • Tornado formation occurs when the meso-cyclone ingests and stretches vorticity (spin) from convective activity.

  • Tornado intensity is assessed using the Enhanced Fujita (EF) Scale, which gauges the damage produced by tornado forces.

Radar Systems Overview

  • Basic Radar Operations: Radars emit pulses that receive energy returns, providing data on storm severity and internal structure.

  • The magnitude of returned energy is highly dependent on the size of raindrops, affecting turbulence detection and rainfall estimates:

    • A 3mm droplet returns 729 times more energy than a 1mm droplet despite containing only 27 times more mass.

    • Reflectivity increases sharply with rising drop sizes.

Radar Reflectivity**

  • The radar's reflectivity typically indicates the intensity of rainfall within the storm, where higher reflectivity suggests heavier precipitation.

Radar Velocity**

  • Velocity radar provides insight into storm motion:

    • Shows convergence and divergence of air within storm cells.

    • Differentiates motion toward and away from the radar, useful for identifying rotation and potential severe weather.

Overall, these details represent the critical knowledge required for understanding thunderstorms, their formation, development, dynamics, and associated risks.