Raed Rayeq Abu Saadeh Weather Forecaster Jordan...

Raed Rayeq Abu Saadeh

Weather ForecasterJordan Meteorological Department

BSc in Physics – The Hashemite University

[email protected]

Atmospheric Stability and Instability , JMD(Raed Abu Saadeh) Jun 2011 ( facebook.com/raed.abusaadeh )

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Objectives

• Define lapse rate and distinguish between dry adiabatic, moist adiabatic, and environmental lapse rates.

• Describe stable, unstable and neutral conditions using air parcel theory .

• Process that destabilize the atmosphere.• The lifting process and cloud developing .• Indicators of stable and unstable atmosphere .• Thunderstorms.• Stability indices. .

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The Concept of Stability

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What happens if we push the ball to the left or right in each of these figures?


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Air Parcel

• body of air (a constant number of molecules) that acts as a whole.

• It does not readily mix with the surrounding air.• The exchange of heat between the parcel and its

surroundings is minimal, and the temperature within the parcel is generally uniform.

• The air inside a balloon is an analogy for an air parcel.

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Air Parcel

• a rising parcel of air expands and cools, while a

• sinking parcel is compressed and warms.

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Air parcels and stability

• Stable: if the parcel is displaced vertically, it will return to its original position.

• Neutral: if the parcel is displaced vertically, it will remain in its new position.

• Unstable: if the parcel is displaced vertically, it will accelerate away from its original position in the direction of the initial displacement.

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Adiabatic Process

• The term "adiabatic process" which simply means warming by compression, or cooling by expansion, without a transfer of heat or mass into a system .

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Temperature distribution of vertically moving air

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Atmospheric density

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Lapse Rates

• The lapse rate is defined as the rate at which air temperature changes with height.

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T

Y

∆−∆


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Dry Adiabatic Lapse Rates

• A dry air parcel rising in the atmosphere cools at the dry adiabatic rate of 9.8°C/1000m and has a lapse rate of −9.8°C/1000m.


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Moist Adiabatic Lapse Rates

• A rising parcel of dry air containing water vapor will continue to cool at the dry adiabatic lapse rate until it reaches its condensation temperature, or dew point.

• At this point the pressure of the water vapor equals the saturation vapor pressure of the air, and some of the water vapor begins to condense.


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Moist Adiabatic Lapse Rates

• Condensation releases latent heat in the parcel, and thus the cooling rate of the parcel slows.

• This new rate, called the moist adiabatic lapse rate,


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Why does unsaturated air parcel cool more quickly (10 º C / km) than a saturated air parcel cools (6 º C / km)?

• When a moist, saturated air parcel rises:1. The air parcel begins to cool as it rises.2. Because the air parcel is saturated, as it cools

water vapor begins to condense into liquid water (a cloud forms).

3. As the water vapor condenses, latent heat is released.

4. The latent heat offsets the cooling that occurs as the parcel rises. The amount of latent heat released varies, but is about 4 º C / km. Therefore, the net cooling is approximately 6 º C / km (10 º C / km - 4 º C / km = 6 º C / km).


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Environmental Lapse Rates

• the actual temperature profile of the ambient air shows the environmental lapse rate.

• Sometimes called the prevailing or atmospheric lapse rate.


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Mixing Height

• the point at which the air parcel cooling at the dry adiabatic lapse rate intersects the ambient temperature profile “line” is known as the mixing height.


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Atmospheric Stability

• The resistance of the atmosphere to vertical motion .

• The degree of stability of the atmosphere is determined by the temperature difference between an air parcel and the air surrounding it.

• This difference can cause the parcel to move vertically (i.e., it may rise or fall).


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Stable Conditions

• When the environmental lapse rate is less than the adiabatic lapse rate (cools at less than 9.8oC/1000 m), the air is stable and resists vertical motion.


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Stable ConditionsAbsolutely

StableAtmosphere.


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Neutral Conditions

• When the environmental lapse rate is the same as the dry adiabatic lapse rate, the atmosphere is in a state of neutral stability.

• Neutral stability occurs on windy days or when there is cloud cover such that strong heating or cooling of the earth's surface is not occurring.


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Unstable Conditions

• This assumes that the surrounding atmosphere has a lapse rate greater than the adiabatic lapse rate (cooling at more than 9.8oC/1000 m), so that the rising parcel will continue to be warmer than the surrounding air.


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Unstable Conditions

• The degree of instability depends on the degree of difference between the environmental and dry adiabatic lapse rates.


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Unstable ConditionsAbsolutely Unstable

Atmosphere.


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Conditionally Unstable


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Vertical Air Movement and Diurnal Changes


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Processes that destabilize the atmosphere

1. By cooling of the air aloft and Warming of the surface air.

• Example : Red-sea Trough with upper trough.


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2.Destabilize the atmosphere by mixing.


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3. Destabilize the atmosphere by lifting


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The lifting processes

• The four lifting processes which can cause thunderstorm development are:

• Convection or thermal .• Orographic .• Frontal .• Convergence.


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Thermal lifting


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Orographic lifting


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Orographic lifting


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Frontal lifting


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Relationship of lifting and convergence


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Determining Atmospheric Stability

• Clouds in layers, no vertical motion. • Stratus type clouds .• Smoke column drifts apart after limited rise. • Poor visibility in lower levels due to

accumulation of haze and smoke. • Fog layers .

• Steady winds.


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Indicators of Stable Air

Smoke / no vertical motion


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Clouds in layers


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Fog layers


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Haze and smoke


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Indicators of Unstable Air

• Clouds grow vertically and smoke rises to great heights .

• Cumulus type clouds.

• Upward and downward currents .

• Gusty winds.

• Dust .


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Smoke rises


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Cumulus clouds


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Atmospheric Instability and CloudDevelopment


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Thunderstorms

• a storm containing lightning and thunder.• Produces gusty surface winds with heavy rain

and hail.• The storm itself may be a single cumulonimbus

cloud, or several thunderstorms may form a cluster, or a line of thunderstorms may form that in some cases may extend for hundreds of kilometers.


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Thunderstorms Factors

• Very Humid Air mass – we need a cloud.• Instability – a rising parcel needs to be buoyant.• Lift (trigger action ) – some means to get the

parcels rising (orographic lift, frontal lift, convergence, etc.)


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Thunderstorms life cycle

• Three stages of its life cycle:

– Cumulus Stage – all updraft, cloud building.– Mature Stage – precipitation falls at the

surface, most lightning, rain and hail occurs, cloud has an updraft and a downdraft.

– Dissipating Stage – dominated by downdraft.


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Thunderstorms life cycle


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Stability Indices

• LI (Lifted Index) is an index used to assess low level parcel stability of the troposphere.

• LI formula = Temperature of Environment at 500 mb - Parcel temperature at 500 mb

Lifted Index

(500 ) (500 )Environment ParcelLI T mb T mb= −


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Stability Indices

LIFTED INDEX

Positive number Stable

0 to -4 Marginal instability

-4 to -7 Large instability

-8 or less Extreme instability


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Stability Indices

• The SI (Showalter Index) is an index used to assess 850 mb parcel stability.

Showalter Index

850(500 ) (500 ) Environment Parcel raised from mbLI T mb T mb= −


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Stability Indices

SHOWALTER INDEX

Positive number Stable

0 to -4 Marginal instability

-4 to -7 Large instability

-8 or less Extreme instability


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Stability Indices

• The KI (K INDEX) is an index used to assess convective potential.

K Index

( ) ( )850 500 850 700KI T T Td Tdd= − + −


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Stability Indices

K INDEX

15-25 Small convective potential

26-39 Moderate convective potential

40 + High convective potential


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Stability Indices

• The TT (Total Totals) is an index used to assess storm strength.

Total Total’s

( ) ( )850 500 850 500TT T T Td T= − + −


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Stability Indices

TOTAL TOTALS

>44 Convection not likely

44-50 Likely thunderstorms

51-52 Isolated severe storms

53-56 Widely scattered severe

<56 Scattered severe storms


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Stability Indices

• SW (SWEAT= Severe Weather Threat Index) uses several variables to determine the likeliness of severe weather and tornadoes.

SWEAT


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Stability Indices

SWEAT• SWEAT= 12(850Td) + 20(TT - 49) + 2(V850) + (V500) +

125(sin(dd500 - dd850) + 0.2)

* If TT less than 49, then that term of the equation is set to zero * If any term is negative then that term is set to zero* Winds must be veering with height or that term is set to zero

850Td = 850 mb dewpoint temperatureTT = Total Totals IndexV850 = 850 mb wind speedV500 = 500 mb wind speeddd500 - dd850 = Directional veering of wind with height


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Stability Indices

SWEAT INDEX

150-300 Slight severe

300-400 Severe possible

400 + Tornadic possible


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References

• Essential of Meteorology 3rd Edition- C.Donald .

• Handbook of Atmospheric Science(Principles and Applications).

• Atmospheric Science 2nd Edition, C.N. Hewitt Andrea V. Jackson

• An Introduction to Atmospheric Thermodynamics 2nd Edition Anastasias A. Tsonis.


Atmospheric Stability and Instability , JMD(Raed Abu Saadeh) Jun 2011 (facebook.com/raed.abusaadeh)

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