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IUB GEOG-G 109 - 8-Stability

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G109: Weather and Climate 8: StabilityStabilityTopics1. Lapse Rates1. Environmental LR2. Dry Adiabatic LR3. Saturated Adiabatic LR2. Atmospheric Stabilitya. Definitionb. In Principle & Practicei. Stable Stabilityii. Unstable Stabilityiii. Neutral Stabilityiv. Conditional Stabilityv. Inversionsc. Sample Problem3. Stability and Cloud Formation4. Modifying StabilityReadingsA&B: Ch.5 (p. 142-146), Ch.6 (p. 160-168)CD Tutorial: Stabilitywww: 8 StabilityLab 8G109: Weather and Climate 8: StabilityLapse Rates• Lapse rate: • in [K / km] or [oC/ km] a positive value indicates decrease of T with height• In the Troposphere - general decrease in T with height• Three types of lapse rates:a. ELR –b. DALR –c. SALR –G109: Weather and Climate 8: StabilityLapse Ratesa. Environmental Lapse Rate Varies in space and with timeG109: Weather and Climate 8: StabilityLapse Ratesb. Dry Adiabatic Lapse Rate Rate at which  Rate:G109: Weather and Climate 8: StabilityLapse Ratesb. Dry Adiabatic Lapse Rate Why does this decrease in T occur?• Adiabatic Process: physical change of the state of the air parcel that does not involve with the air surrounding the air parcel.• Pressure and density → As air parcel rises, it G109: Weather and Climate 8: StabilityLapse Ratesc. Saturated Adiabatic Lapse Rate When air is saturated - condensation occurs • Release of latent heat of vaporization (LV)• Keeps the parcel warmer than it would otherwise• T decrease with height is:  Range:  Dependent on the temperature• Warmer air →→→G109: Weather and Climate 8: Stability0200400600800100012008 101214161820Temperature (oC)Height (m)Adiabatic Lapse RatesDALRSALRG109: Weather and Climate 8: StabilityLifting Condensation Level• Start with air that is unsaturated• Lift air, cooling at the DALR• When air cools to the Tdew(saturation!), then switch from DALR to SALR• Lifting Condensation Level (LCL): This is the height of the0200400600800100012008 101214161820Temperature (oC)Height (m LCLDALRSALRG109: Weather and Climate 8: StabilityAtmospheric Stability• We have noted that air parcels do move up & down in the atmosphere. But WHY?• Stability: Unstable air:  Stable air:  Neutral air: • Atmospheric stability is dependent on T and ρ The thermal stratification of the air: ELR Rising air parcels assumed to be adiabaticG109: Weather and Climate 8: StabilityAtmospheric Stability: In Principle• Temperature differences between an air parcel and its surrounding lead to density differences and thus to buoyancy forces in upward (positive) or downward (negative) direction.→ Air warmer than its surroundings will tend to → Air cooler than its surroundings will tend to → Air at the same temperature as its surroundings will tend toG109: Weather and Climate 8: StabilityAtmospheric Stability: In Practice• Can use the lapse rates to determine: Which direction the air will move? (up/down) How far the air will move?• Method to determine stability: Start at a given point on the ELR (given T at given height) From that point, move upward, along the dry (or saturated) adiabat (i.e., draw a line with the slope of the DALR or the SALR) Compare the environmental air T (given by the ELR) with the DALR or SALR Note: ELR is not a single line• Different layers in atmosphere have different slopes and different stabilityG109: Weather and Climate 8: Stability0200400600800100012008 101214161820Temperature (oC)Height (m)Atmospheric Stability: Stable• If T on the adiabat is less than ELR → The atmosphere is STABLEDALRELRG109: Weather and Climate 8: Stability0200400600800100012008 101214161820Temperature (oC)Height (m)Atmospheric Stability: Unstable• If T on the adiabat is greater than ELR → The atmosphere is UNSTABLEDALRELRG109: Weather and Climate 8: Stability0200400600800100012008 101214161820Temperature (oC)Height (m)Atmospheric Stability: Neutral• If T on the adiabat is the same as ELR → The atmosphere is NEUTRALDALRELRG109: Weather and Climate 8: Stability0200400600800100012008 101214161820Temperature (oC)Height (m)Atmospheric Stability: Conditional StabilityDALRSALRELR• If T on the DALR is less than on the ELR• But T on the SALR is greater than on ELR→ The air is CONDITIONALLY UNSTABLE• Stability depends on whether air is saturateG109: Weather and Climate 8: StabilityAtmospheric Stability: Inversions• Inversion:  Radiation inversion – due to nighttime cooling Frontal inversion – cold wedge of air under warm air Subsidence inversion – sinking air warms adiabatically• Opposite of an inversion = Lapse: • Air pollution will get trapped under an inversion – unable to mix upward→G109: Weather and Climate 8: StabilityAtmospheric Stability: Sample Problem• An air parcel at the surface is not saturated and has a Temperature of 30°C and Tdew=25°C. Assuming it moves adiabatically and the SALR is -7°C km-1, what will the air T be at 3000 m?05001000150020002500300035000 5 10 15 20 25 30 35Temperature (oC)Height (m)• Steps:1. Start at 2. Lift and cool air parcel, using 3. …until 4. Continue to lift and cool using 5. What is Tairat 3000 m?G109: Weather and Climate 8: StabilityAtmospheric Stability: Sample Problem05001000150020002500300035000 5 10 15 20 25 30 35Temperature (oC)Height (m)• At what temperature will air parcel be at saturation?• At what height will this occur?• What is this height called? This is DALRSALRG109: Weather and Climate 8: StabilityAtmospheric Stability & Cloud Formation• If there are unstable conditions → Localized convection• Zones of rising air: cooling• Zones of sinking air: warmingG109: Weather and Climate 8: StabilityModifying Atmospheric Stability• The stability of air can be modified if either the top or bottom of the air layer is warmed or cooling• Stability is increased (becomes more stable): Bottom of layer is •• Advection of (influx of air that has moved over a ) Top of layer is • Advection ofG109: Weather and Climate 8: StabilityModifying Atmospheric Stability• Stability is decreased (becomes more unstable): Bottom of layer is •• Advection of (influx of air that has moved over a ) Top of layer is • Advection of  Mixing of the entire layer• Bottom of layer & Top of layer . Forceful lifting• Uplift over a mountain (orographic lifting)• Convergence: around a Low pressure system•


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