Air Mass Thunderstorms

1. 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

2. Air Mass Thunderstorms Three Stages of Air Mass Thunderstorms

3. 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

4. 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.

5. 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.

6. 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

7. 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.

8. 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.

9. 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

10. 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.

11. Air Mass Thunderstorms • Outflow may trigger additional storms nearby. New Storm

12. 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.

13. 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)

14. Wind Shear • Example of little or no vertical wind shear: Height 7 kts There is very little change in the speed or direction of the wind with height. 6 kts 7 kts 6 kts

15. 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.