Quiz 5 Severe and Unusual Weather

Gust front speed is a function of the _________________________ AND __________________________ difference across the front.

Depth of the cold pool; Temperature

A widespread convectively induced straight-line wind storm. Specifically, a family of downburst clusters that produce surface wind gusts greater than 26 m/s over a concentrated area of at least 400 km2.

Derecho

_______________________________ is the process by which saturated air from the growing cumulus cloud mixes with the surrounding cooler and drier (unsaturated) air.

Entrainment

Dissipating Stage

Eventually the downdraft overwhelms the updraft and convection collapses - because the cloud is vertically-oriented.

Air mass thunderstorms are also referred to as ___________________________.

Garden Variety

The ________________ the temperature difference, the _________________ the buoyancy force -> _________________ the upward acceleration of the parcel -> the _________________ the updraft!

Greater; Greater; Greater; Stronger

Gust fronts propagate due to the ____________________________________ across the front - created mainly by the temperature difference across the front.

Horizontal Pressure Gradient

Most Thunderstorms Occur Along the __________________________________________________.

Intertropical Convergence Zone (ITCZ)

The _________________________________ represents the peak intensity of the storm.

Mature Phase

Just behind the gust front is an area of high pressure. This is called a ___________________________.

Mesohigh

When the downdraft reaches the ground, it spreads out in all directions. The leading edge of this cold, often gusty wind is called the _______________________ or _______________________.

Outflow Boundary; Gust Front

Mature Stage

Precipitation, formed by evaporative cooling in the downdraft, and by dragging the air downwards as it falls, begins to reach the ground.

Typically lasts 20-30 minutes. They can produce severe weather such as downbursts, hail, heavy rain and occasionally weak tornadoes.

Single-cell or air-mass storm

Key Ingredients for Thunderstorms

1. Instability 2. Mechanism to trigger the instability 3. Vertical wind shear

Thunderstorm

A local storm, invariably produced by a cumulonimbus cloud, that always is accompanied by lightning and thunder. It usually contains strong gusts of wind, heavy rain, and sometimes hail.

As precipitation forms in the storm, it begins to fall - dragging ______________________ down with it.

Air

Self-Destruct Mechanism

Because the horizontal winds are weak and do not change speed or direction with height (that is, weak shear), the storm updraft is vertically oriented. When precipitation forms aloft, it has no place to go except to fall back through the updraft, killing it off by dragging air down and by evaporative cooling of air near the rain. As the rain-cooled air spreads along the ground, it STABILIZES the atmosphere (remember, we want warm, moist air at the ground, not cool air) and aids in the storm's demise.

_______________________is a vertically oriented force acting on an air parcel due to the temperature (density) difference between the parcel and the surrounding air.

Buoyancy

Meteorologists often use the word ___________________ to describe storms in a general manner.

Convection

The strength of _____________________________________, and thus rising motion at the gust front, is due to the combined effects of the wind speed behind the gust front and that of the environment out ahead of it.

Convergence

The precipitation also __________________ the air by evaporation, making it _________________ (denser) than the surrounding air.

Cools; Heavier

The word _____________________ is used o describe the transport of some quantity.

Advection

_______________________________________ are relatively small (20-120 km long), bow-shaped systems of convective cells that are noted for producing long swaths of damaging surface winds.

Bow Echos

A _______________________ cloud is one which owes its vertical development, and possibly its origin, to convection (upward air currents).

Convective

Consist of a line of storms with a continuous, well developed gust front at the leading edge of the line. These storms can produce small to moderate size hail, occasional flash floods and weak tornadoes.

Multicell Line (squall line) Storms

A group of cells moving as a single unit, with each cell in a different stage of the single-cell life cycle. Multicell storms can produce moderate size hail, flash floods and weak tornadoes.

Multicell cluster storm

18% of U.S. tornadoes are _____________________________.

Quasi-Linear Convective Systems (QLCS)

A type of multicell storm - consisting of a line of storms with a continuous, well developed gust front at the leading edge of the line.

Squall Lines

A very large, isolated storm with one principal updraft.

Supercell

Defined as a thunderstorm with a main rotating updraft, these storms can produce strong downbursts, large hail, occasional flash floods and weak to violent tornadoes.

Supercells

Bow echoes that develop within a squall line have been referred to as ____________________________.

line echo wave patterns (LEWPs)

The mesohigh is caused by:

o Evaporation of precipitation o Melting of precipitation

Mechanisms that can trigger instability:

o Front o Terrain o Dryline o Daytime heating o Landmass inhomogeneities

Characteristics of a "cell"

o Independent o Has a complete life cycle. o Has a life cycle of 30 minutes to an hour. o Is usually weak.

Single Cell "Air Mass" Storms

o Most common o Last for less than an hour o Built-in self-destruct mechanism! o Occur all year long, mostly in summer. o Can produce strong winds, hail, and lightning.

Characteristics of a Gust Front

o Strong wind shift (speed and direction) o Much colder air behind the gust front o Acts as a location for additional lift for future storm development.

Multi-Cell Storms

o These storms are made up of many cells. o Each individual cell goes through a life cycle, but the group persists. o These storms consist of a series of evolving cells with each one, in turn, becoming the dominant cell in the group.

What is the circulatory system of thunderstorms?

Updrafts and downdrafts

Which direction is buoyancy oriented for unstable air parcels?

Vertically

Conditions for Long-Lived Squall Lines

With surface cold pool and low-level vertical shear, the updraft is vertically oriented again - the OPTIMAL condition for long-lived squall lines.

Conditions of Formation of Air Mass Thunderstorms

· Conditional instability · Warm, moist air near the ground · Localized source of lift (usually thermally driven) · Weak or no environmental vertical wind shear.

The National Weather Service's definitions of a thunderstorm include:

· Hail greater than or equal to 1 inch in diameter (quarter-size) · Wind gusts ≥ 50 knots (58 mph) · and/or ... a tornado!

The four basic thunderstorm types from the storm spectrum are:

· Single Cell · Multicell Cluster · Multicell Line (Squall Lines) · Supercells

Formation of Stratiform Precipitation

· The updraft is at the leading edge of the storm · The cloud tops radiate energy to space, thereby cooling the top of the stratiform region. · Radiation from the ground is absorbed by the cloud bases of the stratiform region and warms the bases. · This combination acts to destabilize the atmosphere in the stratiform region and cause weak rising motion à widespread light/moderate precipitation.

Three Stages of Single-Cell Storm Development

· Towering Cumulus Stage - Updraft · Mature Stage - "cell" - Both updraft and downdraft · Dissipating Stage - Downdraft

Conditions Necessary for Squall Line Development

· Warm, moist air at low levels · Relatively cold air aloft · Mid-level dry air that can enhance the downdraft and the vigor of the line. · "Linear" forcing.