Convective Weather

Thunderstorms

Lightning

Tornadoes…

…and more

Today

• Thunderstorms

• Journal returns and comments (important)

• Other comments (also important)

• New Homework

We’ve done convection

• So now:• How thunderstorms form

– Simple isolated storms– More organized storms– Big nasty storms

• Severe weather– Lightning– Downbursts– Tornadoes

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

Air Mass Thunderstorms

• First studied just after World War II

• Many commercial and military aircraft accidents

• Newly developed radar was exploited for weather studies

• The Thunderstorm Project

• Resulted in first life cycle of a thunderstorm

Thunderstorms• We shall now look at the life-cycle of a

typical single cell thunderstorm.

Cumulus StageInitially: Free Convection

If conditions are favorable (conditional or absolute instability),cumulus clouds can grow both vertically and laterally.

Warm

The sun warms the groundwhich warms a parcel of airabove the surface.

ThunderstormsAs the afternoon wears on, the surface is continued to beheated by the sun. Convective bubbles (parcels) continue torise from the surface of the earth. These convective parcels (thermals) bring more moisture from the surface allowing the Cumulus clouds to grow.

Thunderstorms

p

TSunrise

p

TAfternoon

Mixing RatioDry AdiabatSounding

ConvectiveTemperature

Air Mass Thunderstorms

Three Stages of Air Mass Thunderstorms

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

Thunderstorms

• The rising stream of moist air below the cloud and feeding into it is called the updraft.

• The updraft is sufficiently strong to keep water droplets and ice crystals aloft.

• There is no precipitation in the cumulus stage.

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.

Air Mass Thunderstorms

Mature Stage

• The mature stage begins when precipitation occurs. Cu transitions into a Cb.

• Evaporation caused by entrainment causes regions of cooler air that begin to descend.

• This is the beginning of the downdraft.

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

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.

Downdraft and Gust Front

• When the downdraught hits the ground, it spreads out.

• The leading edge of this spreading cold air is called the gust front.

Gust Front

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.

Thunderstorms

• The thunderstorm reaches its maximum intensity during the mature phase.

• Elements of the mature thunderstorm– Lightning– Thunder– Heavy Rain– Small Hail– Gusty Winds

Thunderstorms

Dissipating Stage

• The downdraft and precipitation fall down through the updraft cutting off the updraft.

• The gust front surges out in front of the storm cutting off the inflow of warm, moist air.

• The storm begins to die of “starvation.”

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.”

Thunderstorms

• The previous discussion was a description of an ideal single cell thunderstorm.

• These are the common variety of thunderstorms seen in the summer.

• Warm Humid Air Mass• Widely Scattered• Usually Weak• Usually Short Lived

• Produce a good portion of summertime rainfall.• Produces temporary cooling• Slight potential for producing severe weather.

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.

Hazards of Air Mass Thunderstorms

• Heavy Rain

• Hail– Usually not terribly large– May be numerous

• Downbursts or Microbursts– Exceptionally strong downdraughts that, when

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

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)

Journals

• We’ve been kind

• Expect more detail

• Can be used for extra credit

• Will be returned

Homework

• Read chapter 15 of Ahrens and answer the following questions for thought on page 434: Questions 2 and 7.