Cloud Development Introduction to Atmospheric Stability

1
Cloud Development - Introduction to Atmospheric
Stability

• Ever wonder why clouds form on some days and not
  on others?
• Why does the atmosphere sometimes produce stratus
  clouds (thin layered) while other times we get
  cumulus, or cumulonimbus clouds to form?
• The answer depends on concept of atmospheric
  stability.....

2
Stable Environment

• Consider a marble in the bottom of a bowl
• If you push the marble up the side of the bowl,
  it will roll back down to the bottom, to its
  original position

3
Stable Atmosphere

• Parcels in a stable environment will not rise
• Vertical motion is inhibited
• If clouds form, they will be shallow, layered
  clouds like stratus

4
Unstable environment

• If the marble is on the top of the bowl and you
  give it a little push, it rolls off the bowl....
  does NOT come back to it's original position
• This is an unstable situation

5
Unstable Atmosphere

• Unstable air (parcel) - vertical motion occurs
• Commonly produces Cu, Cb clouds
• So, how do we determine the stability of the
  atmosphere?

6
Rising air parcels and adiabatic cooling

• Consider a rising parcel of air
• As the parcel rises, it will adiabatically expand
  and cool
• Adiabatic - a process where the parcel
  temperature changes due to an expansion or
  compression alone, (no heat is added to or taken
  away from the parcel)
• The parcel expands since the lower pressure
  outside allows the air molecules to push out on
  the parcel walls
• Since it takes energy for the parcel molecules to
  "push out" on the parcel walls, they use up some
  of their internal energy in the process.
• Therefore, the parcel cools since temperature is
  proportional to molecular internal energy

7
Sinking air parcels and adiabatic warming

• A sinking parcel of air
• As the parcel sinks, it will adiabatically
  compress and warm
• The parcel compresses since it is moving into a
  region of higher pressure
• Due to the parcels compression, the air
  molecules gain internal energy
• The mean (average) temperature of the parcel
  increases

8
Dry adiabatic lapse rate

• Dry adiabatic cooling
• 10oC/1000m
• What will the temperature of the parcel be if it
  is raised to 1 km?

30oC
9
Moist Adiabatic Lapse Rate

• At 2 km, the temperature and the dew point
  temperature lines intersect
• The parcel has become saturated
• After saturation is reached, the parcel will cool
  at a smaller rate
• A saturated parcel of air, cools at the moist
  adiabatic lapse rate 6C/km (3.5oF/1000ft)
• What will be the parcel's temperature be at 3 km?
  

10
Moist Adiabatic Rate

• What will be the parcel's temperature be at 4 km?

14oC
11
Moist Adiabatic Rate

• Why does the parcel cool at a slower rate
  (6C/km) when it is saturated and a faster rate
  (10C/km) when it is unsaturated?

8oC
12
Dry versus Moist-Adiabatic Process

• The moist adiabatic lapse rate is less than the
  dry adiabatic lapse rate because as vapor
  condenses into water in a saturated parcel,
  latent heat is released into the
  parcel--partially offsetting the adiabatic
  cooling

13
Applying this to determine the stability of the
atmosphere

• To this point, weve learned that a parcel of air
  will cool at either the dry or moist adiabatic
  rate when it is lifted.
• We now have to compare the temperature of the
  parcel to the temperature of the atmosphere that
  surrounds it
• If the parcel is warmer (lighter, less dense)
  than the atmosphere surrounding it--it will rise
• Unstable atmosphere
• If the parcel is cooler (heavier, more dense)
  than the atmosphere surrounding itit will sink
• Stable atmosphere

14
Assessing Atmospheric Stability

• The bottom line - 
• To determine whether or not a parcel will rise or
  sink in the atmosphere, we must compare the
  parcels temperature (Tp) with that of the
  environment (Te) at some altitude
• if Tp gt Te what will the parcel do?
• if Tp Te what will the parcel do?
• if Tp lt Te what will the parcel do?

Te
15
How do we find the temperatures in the atmosphere
above us?

• Vertical profiles of atmospheric temperature,
  winds and dew point are collected at 12 and 00
  UTC every day from over 1000 locations worldwide
  by launching weather balloons (rawinsondes)
• Temperature, dew point and winds are plotted on a
  diagram called a Skew-T, Log-P diagram

16
RAWINSONDE LAUNCH--UZBEKISTAN
100 gram balloon
Combat Weather HUAH
Rawinsonde
17
1. Absolute Stability

• If a parcel is lifted from the surface it will
  cool either dry or moist adiabatically
• But in either case, the parcel will be cooler
  than the environment.
• This is an example of absolute stability

18
Absolutely Stable

• So an absolutely stable parcel (whether it is
  unsaturated or saturated) will always be cooler
  than the environment and will sink back down to
  the ground
• The condition for absolute stability is
  GeltGmltGd (G is gammalapse rate)
• Gd is the dry adiabatic lapse rate (10C/km)
• Gm is the moist adiabatic lapse rate (6C/km)
• Ge is the environmental lapse rate (In this
  example 4C/km)

19
Stability of Inversion Layers

• How would you characterize the stability of an
  inversion layer?
• Inversions are absolutely stable
• Note that the absolute stability criteria
• GeltGmltGd
• How do stable layers form in the atmosphere?

20
Formation of Stable Layers

• How does the atmosphere form stable layers?
• 1. Radiational Cooling - radiation inversion
• 2. Cold air advection at low levels
• Behind a cold front (over land)
• 3. Warm air moving over cold ground
• Fog forming over snow fields

21
2. Absolute Instability

• This is an example of absolute instability
• Everywhere on this diagram an unsaturated or a
  saturated parcel will always be warmer than the
  environment and will continue to rise
• The condition for absolute instability is Gegt Gd
  gtGm
• Ge is the environmental lapse rate (12C/km)
• Gd is the dry adiabatic lapse rate (10C/km)
• Gm is the moist adiabatic lapse rate (6C/km)

22
3. Conditional Instability

• This is an example of conditional instability
• An unsaturated parcel will be cooler than then
  environment and will sink back to the ground
• The saturated parcel will be warmer than the
  environment and will continue to ascend
• The condition for conditional instability is Gd
  gt Ge gt Gm
• Gd is the dry adiabatic lapse rate (10C/km)
• Ge is the environmental lapse rate (8C/km)
• Gm is the moist adiabatic lapse rate (6C/km)

23
Conditional Instability - example

• Lets start with a parcel on the surface with a
  temperature and dew point of 30 C and 10C,
  respectively
• The parcel is initially forced to rise in an
  environment where the environmental lapse rate
  (Ge) is 8C/km up to 8 km.
• Let's follow the parcel upward

24
Conditional Instability - 1km

• The parcel must rise dry adiabatically (10C/km)
  because it not saturated
• The parcel temperature lt Te, so something is
  forcing the parcel upward
• Onward to 2km .....

25
Conditional Instability - 2km

• The parcel reaches saturation at 2km
• The temperature of the parcel is still lt Te, so
  something is still forcing the parcel upward...
• Onward to 3km .....

26
Conditional Instability - 3km

• The parcel now rises moist adiabatically (6C/km)
• The parcel temperature is still cooler than the
  environment, so something is still forcing it
  upward....
• Upward to 4km .....

27
Conditional Instability - 4km

• The parcel continues to rise moist adiabatically
  (6C/km)
• Notice that now parcel temperature Te
• What happens if the parcel is pushed upward just
  a little???

28
Conditional Instability - 4km

• The height where the parcel temperature becomes
  equal to or larger than its environment is level
  of free convection
• The parcel is still rising moist adiabatically
  (6C/km)
• The parcel will continue to rise until the parcel
  becomes cooler than the environment (at 9km above
  the ground)
• Above that point, parcel temperature lt Te , so
  the parcel will rise no further
• Below 4 km where parcel is cooler than the
  environment, the atmosphere is stable and there
  will be no upward parcel movement (something must
  push the parcel upwards to the level of free
  convection)
• Above 4 km where parcel is warmer than the
  environment, the atmosphere is unstable, and the
  parcel will rise on its own
• This is an example of a conditionally unstable
  atmosphere... the condition is lifting the parcel
  above 4 km where it can then rise on it's own

29
The real atmosphere

• What are the stabilities of each layer?
• (Recalling that
• Gd10oC/km
• Gm6oC/km)

Gm6oC/km)
Gd10oC/km
30
Processes that destabilize the atmosphere

• 1. Cold air advection aloft
• This often occurs when an extratropical cyclone
  (a winter low pressure system) passes overhead
• 2. Surface Heating
• Tells us that the atmosphere will be most
  unstable at time of maximum surface heating

31
Processes that destabilize the atmosphere

• 3. Warm air advection at low levels
• This often occurs ahead of a cold front
• 4. Cool air moving over a warm surface
• A common example is after a cold front passes us
  and goes into the Gulf of Mexico

32
Atmospheric Instability and Cloud Development

• Where will the base (bottom) of a cloud form?
• What determines the height to which the cloud
  will grow?
• Using the previous example of a rising air parcel
• On this diagram, where is cloud base?
• On this diagram, where is cloud top?

33
Atmospheric Instability and Cloud Development

• On this diagram, where is cloud base?
• Where the parcel reaches saturation -- 2 km
• On this diagram, where is cloud top?
• Where the parcel will no longer be able to rise
  -- 9 km

34
Abs Unstable
Abs Stable
35
Atmospheric Instability and Cloud Development -
lifting mechanisms

• Two questions should arise at this point
• 1. How are vertical parcel motions that create
  clouds generated naturally in the atmosphere?
• 2. What kind (if any) clouds will you visually
  observe in
• an absolutely stable environment?
• a conditionally unstable environment?
• an absolutely unstable environment

36
Atmospheric Instability and Cloud Development -
Convection

• Convection usually occurs when the surface is
  heated and a surface parcel becomes warmer than
  the environment
• The vertical extent of the cloud is largely
  determined by the stability of the environment
• In an absolutely stable environment, no clouds
  will likely form

37
Atmospheric Instability and Cloud Development -
Shallow Convection

• In a shallow conditionally unstable or absolutely
  unstable environment, we might expect clouds to
  develop, but their vertical growth will be
  limited...
• We may observe
• Cumulus Humilis (shallow cumulus)
• Stratocumulus

38
Atmospheric Instability and Cloud Development -
Deep Convection

• In a deep conditionally unstable or absolutely
  unstable environment, we would expect clouds with
  significant vertical development to form
• We might observe
• Cumulus congestus
• Cumulonimbus