1. Stability and Thunderstorms
ESS 111 – Climate and Global Change

2. Atmospheric Stability & Clouds
Parcel Theory
Parcel of air expands and contracts freely
Remains as a single unit
No heat exchange with the outside air (Adiabatic Process)

3. Atmospheric Stability & Clouds
Adiabatic Process
Parcel expands when lifted due to lessening pressure
Molecular action slows = cools at a steady rate
Compresses when forced down = Warms at a steady rate

4. Atmospheric Stability & Clouds
Lapse Rate: Rate of cooling (or warming) of the atmosphere:
Environmental Laps Rate = actual temperature as measured by a radiosonde (weather balloon)
Dry Adiabatic Lapse Rate (DALR) – Unsaturated parcel of air = 10°C per km
Moist Adiabatic Lapse Rate (MALR) – Saturated parcel of air = 6°C per km.

5. Cloud & Storm Updraft Development
Mechanisms
Surface Heating – Free Convection
Topography
Convergence of Surface Air
Fronts
Free Convection: Ground heated by radiation
Thermal Forms (Convective Updraft)
Air in contact with warmest ground is warmer and therefore less dense than surrounding air
Air rises…..it cools as it rises….it expands due to lessening pressure acting on it (Adiabatic Processes)
If the rising (and cooling) parcel that started at the surface continues to be warmer than the surrounding air, then the parcel is unstable and will continue to rise.
Numerous unstable parcels over time (tens of minutes) can eventually form a thunderstorm updraft.

6. Cloud Development Triggers

7. Convection & Clouds
Pockets of warm air rise as thermals with invisible water vapor, and at the dew point temperature condensation creates the cloud base.
Rising air from below is replaced by sinking air from above, creating areas of blue sky.

8. Ingredients for thunderstorm initiation:
Thunderstorms
A cumulonimbus cloud that produces lightning (and hence thunder)
Very common (globally)
Transport heat and moisture vertically in the atmosphere
Most are not severe
Severe thunderstorm:
Hail > 1 inch diameter (¾ inch prior)
Wind gust >50 kt (58 mph)
Tornado
Ingredients for thunderstorm initiation:
Water vapor
Instability
Lifting mechanism
For a severe thunderstorm, a 4th ingredient is:
d) Strong vertical wind shear
(change in wind speed / direction)

9. Thunderstorm Facts & Climatology
At any given time there are 2000 thunderstorms in progress
About 45,000 thunderstorms take place each day
Annually, The U.S. experiences about 100,000 thunderstorms.
About 16 million thunderstorms occur annually around the world!
The lightning from these storms strikes Earth about 100 times each second

10. Thunderstorm Frequency Map

11. Global Lightning Frequency

12. Upscale development of thunderstorms
Individual thunderstorms
Single cell thunderstorm
Multicell thunderstorm
Supercell thunderstorm
Larger-scale systems with embedded showers and/or thunderstorms:
Mesoscale Convective System (MCS)
Squall line (quasi-linear convective system, QLCS)
Hurricane
– will examine this first

13. Single-Cell Thunderstorm

14. Single-Cell Thunderstorm
Life span of about 20 to 30 minutes
Usually not strong enough to produce severe weather
Single Cell Pulse Storms often produce severe weather
Usually strong winds as it collapses
Sometimes small hail

15. Life cycle of a single cell (air mass) thunderstorm
3 stages of development 1 2 3
Fig. 18.3
Vertical shear is weak (as in the SE during summer)
Form along weak boundaries or convergence zones.
Small size: <10 km diameter
Short lifetime: <1 h
Microbursts (strong downdrafts and their outflows) may form during first part of the mature stage.

16. Towering Cumulus (Cumulus Congestus)

17. Is the Cumulus Cloud still growing?
Wispy soft top =
Weaker updraft & dying storm
Cauliflower-type hard top =
Strong updraft & growing storm

18. Life Cycle – Mature Stage

19. Life Cycle – Dissipating (Orphan Anvil)

20. Air Mass Thunderstorms Summary
Usually weak (but can produce heavy rain in a short period of time).
Usually not severe
Usually move slowly (weak winds aloft)
Often develop and dissipate in less than one hour
Form in a weakly sheared environment and thus have a BUILT-IN SELF-DESTRUCT MECHANISM that guarantees a short lifetime (downdraft collapsing down on the updraft).