Stability of the Atmosphere


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Background on Atmospheric Stability

How do we assess stability

How are vertical motions forced?

Background on Atmospheric Stability

What is stability?
Stability refers to a condition of equilibrium...

  • A stable equilibrium: if marble pushed up side of bowl and let go, it will return to original position.

  • An Unstable equilibrium: if marble on top of bowl receives a push, it will roll off bowl and not return to original position.
How does this relate to the atmosphere? When the atmosphere is stable, a parcel of air will want to return to its original position after being raised or lowered.
    Vertical motions are inhibited.
When the atmosphere is unstable, a parcel will want to continue on its path away from its original position it pushed upward or downward.
    Vertical motions are favored.

How do we assess stability?

We must compare the parcel's temerature Tp with the temperature of the surrounding environment Te.

  • Tp > Te The parcel is positively buoyant, it is less dense and will rise.
  • Tp < Te The parcel is negatively buoyant, it is more dense and will sink.
  • Tp = Te The parcel is neutrally buoyant, it will not rise or sink.

What about a layer of the atmosphere?

The atmosphere is absolutely stable when the environmental lapse rate is less than the moist adiabatic lapse rate.
A saturated OR unsaturated parcel will be cooler than the surrounding environment and will sink, if raised.

An inversion layer is always absolutely stable.

An unstable layer would have a lapse rate greater than the dry adiabatic lapse rate. An unsaturated OR saturated parcel will always be warmer than the surrounding environment and will continue to ascend, if raised

In conditionally unstable layer, the environmental lapse rate is between the moist adiabatic and the dry adiabatic lapse rates. If the parcel is unsaturated it will be cooler than the surrounding environment and sink if raise. If a parcel is saturated it will be warmer than the surrounding environment and ascend if raised.

Putting it all together...

How are vertical motions forced?

Stability is very important when there is forcing for vertical motion. So our next question is:
How are vertical (lifting) motions forced?

There are several ways to get a parcel to rise

  1. Lifting by Convection
      As the earth is heated by the sun, thermals (bubbles of hot air) rise upward from the surface. The thermal cools as it rises, losing some of its buoyancy (its ability to rise). The vertical extent of the thermal, which can form a cloud if it reaches the LCL, is largely determined by the stability of the environment.

      A deep stable layer restricts continued vertical growth. A deep unstable layer will likely lead to development of rain-producing clouds. These clouds are more vertically developed than clouds developed by convergence lifting.
  2. Lifting by Convergence
      Convergence exists when there is a horizontal net inflow into a region. The air has no where else to go so, when air converges along the surface, it is forced to rise.

      Large scale convergence can lift air hundreds of kilometers across. Vertical motions associated with convergence are generally much weaker than ones due to convection. Generally, clouds developed by convergence are less vertically developed.
  3. Lifting due to Topography
      This type of lifting occurs when air is confronted by a sudden increase in the vertical topography of the Earth. When air comes across a mountain, it is lifted up and over, cooling as it is rising. The type of cloud formed is dependent upon the moisture content and stability of the air.
  4. Lifting along Frontal Boundaries
      A front is the transition zone between two air masses of different densities. Lifting occurs along two different types of fronts,
      1. Cold Front
        A colder, denser air mass lifts the warm, moist air ahead of it. As the warm air rises, it cools and condenses, producing clouds and precipitation. The steep slope of the cold front leads to more vigorous rising motion. Hence, cold fronts are often associated with thunderstorms

      2. Warm Front
        A warmer, less dense air mass rises up and over the cold air ahead of the warm front Air rises, cools and condenses. Warm fronts have gentler slopes and move slower than cold fronts. Generally, precipitation is more steady and widespread, a steady rain shower results.