Thunderstorms

Ø  Ordinary Thunderstorms

̶        Define: Isolated thunderstorms that form in the absence of strong vertical wind shear.

̶        Often form along weak boundaries, such as cool air outflows from other thunderstorms

̶        Tend to form in hot afternoons

̶        Difference between environments for ordinary thunderstorms and severe is the strength of the winds aloft.

̶        Winds aloft are typically weak and the vertical wind shear minimal

̶        Far from frontal boundaries

̶        Three stages of growth:

1.    Cumulus stage: cloud consists of a warm, buoyant plume of rising air - the updraft (increases rapidly with height)

2.    Mature stage: can be identified visually by examining the upper part of the storm. Air from the updraft exhausts horizontally at the tropopause forming the anvil, Mammatus clouds often form on the bottom of the anvil. Precipitation forming in the upper part has to fall vertically with through the updraft.

3.    Dissipation stage: typically produce and outflow of cool air. Clouds (shelf or roll) will typically form over the gust front as warm air is lifted over the spreading cold pool. As the cool air rushes out more thunderstorms cells may trigger near the cool air’s leading edge as the warm air is lifted.

Ø  Mesoscale Convective Systems

̶        Produce much of the summer rainfall on the Central Plains of North America

̶        Damaging straight line winds and occasional weak tornados

̶        Lifting of air along weak airmass boundaries often will trigger thunderstorms, particularly in the late afternoon.

̶        As the storm progresses, the storms will begin to organize often becoming more intense and progressively aligning into an arc-shaped line (Squall Line)

̶        One or more segments along the squall line may bow outward, producing a bow echo on radar. During this time, a region of widespread less-intense precipitation develops to the rear of the squall line.

̶        Eventually, the thunderstorms decay, leaving their wake a wide stratiform region that itself may continue to produce rain for several more hours. During this time, new thunderstorms may develop.

Ø  Frontal Squall Lines

̶        Form in the warm, moist air ahead of surface cold fronts and dry lines, or just ahead of the upper-level front.

̶        Typically hundreds of kilometers long

̶        Commonly form the “tail” of the comma-cloud pattern in extratropical cyclones, particularly those that form over land.

̶        Long lifelines, lasting many hours or days

̶        Can produce weak tornados and hail but tend to be most associated with strong straight-line winds.

Ø  Supercell Thunderstorms

̶        A supercell is a storm with a mesocylone

̶        Most intense thunderstorms in Earth’s atmosphere.

̶        Always rotate (entire storm behaves as a single entity, rather than as a group of cells. example: MSC)

̶        Produces virtually most all severe tornados, large hail (can grow as large as grapefruits), and damaging straight-line winds (typically 20-40 meters/second or 45-90 mph)

̶        Four key ingredients to form:

1.    An environment that is conditionally unstable (humid and warm at the surface and dry and very cold aloft)

2.    Very moist air in lower troposphere.

3.    Moderate to strong wind shear (through the depth of the unstable layer)

4.    A trigger mechanism (lifting along a boundary)

̶        The larger the CAPE the stronger a thunderstorms updraft may become (all else being equal)

̶        It is critical that the environment winds increase rapidly with height within the layer of maximum instability.

̶        Supercells have a tendency to first erupt where boundary intersections occur (such as, the intersection of the dry line and warm front)

̶        New cells often trigger in rapid succession along the boundaries, creating a line of supercells.

̶        Precipitation rarely appears in the core of the updraft region, leaving a rain-free base.

̶        Tornados within classic supercells nearly always occur outside of the heavy precipitation region of the storm because they occur in the vicinity of the intense updraft.

̶        Typically occur where strong middle and upper tropospheric jet streams are present.

̶        Inversion: Temperature is warmer outside the cloud, not unstable

o   How do I know there is an inversion? Bob

o   Lifted Index (LI): measure of instability