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Air Mass Thunderstorms

We shall examine three phases of an air mass thunderstorm– Cumulus Phase (Growth Phase)– Mature Phase– Dissipating Phase

Life cycle can last from 45 minutes to over an hour

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Air Mass Thunderstorms

Three Stages of Air Mass Thunderstorms

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Cumulus Phase

Development of towering cumulus– Region of low level

convergence– Warm moist air

Nearby cumulus may merge to form a much larger cloud

Dominated by updraft Mixing and entrainment

occur in the updraft

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Entrainment

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

Entrainment causes evaporation of the exterior of the cloud and tends to reduce the upward buoyancy there.

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Mature Phase

Precipitation, formed by the Bergeron cold rain process, begins to reach the ground.

The precipitation drags some of the surrounding air down creating the downdraft.

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Downdraft

The downdraft is the descending column of air in a thunderstorm.

Created and maintained by three processes– Evaporational cooling of entrained air

– Downward drag caused by falling precipitation

– Evaporational cooling of the air below the cloud base

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Downdraft

When the downdraft reaches the ground, it spreads out in all directions.

The leading edge of this cold, often gusty wind is called the outflow boundary or gust front.

The outflow boundary behaves like a cold front:– Strong wind shift (speed and direction)– Much colder air behind the gust front– Acts as a location for additional lift for

future storm development.

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Mature Phase The mature phase represents

the peak intensity of the storm. Updrafts and downdrafts are

about equal in strength. Precipitation is typically heavy

and may contain small hail Gusty winds result from the

downdraft spreading out on the ground.

The anvil, or cloud top, begins to turn to ice, or glaciate.

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Dissipating Phase

Eventually the downdraft overwhelms the updraft and convection collapses.

Precipitation becomes lighter and diminishes.

Cloud begins to evaporate from the bottom up often leaving behind an “orphan anvil.”– Cirrus Spissatus

cumulonimbogenitus

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Air Mass Thunderstorms

Usually weak (but can produce heavy rain in a short period of time).

Usually not severe. Usually move slowly. Often develop and dissipate in less than

one hour. Form in a weakly sheared environment.

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Air Mass Thunderstorms

Outflow may trigger additional storms nearby.

New Storm

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Hazards of Air Mass Thunderstorms

Heavy Rain Hail

– Usually not terribly large– May be numerous

Downbursts or Microbursts– Exceptionally strong downdrafts that, when

they hit the earth, may have potentially destructive winds associated with them.

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Conditions of Formation of Air Mass Thunderstorms

Conditional Instability Weak or no environmental vertical

wind shear Warm, moist air Localized source of lift (usually

thermally driven)

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Wind Shear Example of little or no vertical wind

shear:

7 kts

6 kts

7 kts

6 kts

There is very littlechange in the speedor direction of thewind with height.

Height

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Wind Shear Definition:

– The change in the direction or speed of the wind over a distance. Vertical Wind Shear

– The change in the direction or speed of the wind with height. Low wind shear indicates little change in direction or speed of the wind over a distance.