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Assessing Atmospheric Stability…

Assessing Atmospheric Stability…. …Use of the Skew-T/log P diagram… (what does it all mean?). What is stability?. The atmosphere tends to resist vertical motion due to hydrostatic balance: dP / dZ = - ρ g This is why horizontal motions are generally >> than vertical motions.

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Assessing Atmospheric Stability…

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  1. Assessing Atmospheric Stability… …Use of the Skew-T/log P diagram… (what does it all mean?)

  2. What is stability? The atmosphere tends to resist vertical motion due to hydrostatic balance: dP/dZ = -ρg This is why horizontal motions are generally >> than vertical motions. Stability: The degree to which the atmosphere resists vertical motion.

  3. Causes of vertical motion: Forced / mechanical: fronts lake breeze orography jet streams short waves Thermal: differential surface heating warm advection in lower levels and/or cold advection aloft cooling on top of moist layers.

  4. Stable: Lifted parcel of air is negatively buoyant. Unstable: Lifted parcel of air becomes positively buoyant

  5. Instability = Convection A convective cell:

  6. pic of cu field

  7. Adiabatic Process • From the first law of thermodynamics: dE= dQ + dW, where dQ = 0… • …Changes in the internal temperature of an air parcel (hypothetical “bubble” of air) are due entirely to expansion (rising…cooling) and compression (sinking…warming) • Applying the hydrostatic equation to the gas law yields: γd = -g/Cp or 10 C ⁰ / Km, or 5.4 F ⁰/1K’ • ISA standard lapse rate ≈ 3.6 F ⁰ / 1K’

  8. Expansion/Compression example

  9. Moist Ascent: pseudo-adiabatic process Adiabatic cooling by expansion is partially offset by release of latent heat of condensation: γm = -g/Cp + Lvdqv/CpdZ The moist (or saturation) lapse rate varies depending on: • Dew Point (how much water vapor is present) • Buoyancy ( rate of condensation)

  10. Saturation lapse rates range from about 1 F ⁰/1K’ in very warm, moist tropical air…and approach the dry adiabatic lapse rate in very cold, dry environments. (gives saturation adiabats a curve with altitude)

  11. Evaluating Stability Uses parcel theory: Parcel of air: A hypothetical bubble of air, homogeneous in temperature and dew point. Treated as a discrete feature. Assumes no mixing with “outside” environmental air. T environment Tp Tdp

  12. Parcel can ascend either dry or pseudo adiabatically:

  13. Lift the parcel to a desired altitude:

  14. A parcel’s ascent is usually a combination of dry…then moist ascent:

  15. Next, compare the temperature of the parcel to the temperature of the environment at the top of the ascent: In this example, the temperature of a parcel lifted from the surface to 500 mb would be 4 C⁰ warmer than the temperature of the environment. It would be positively buoyant, it would be unstable…

  16. Quantifying stability: Stability Indices Lifted Index (LI) >0 = stable; 0 = neutral; <0 unstable Showalter Index (SI) Total Totals Index (TT) K-Index (KI) SWEAT Index CAPE: Convective Available Potential Energy. (This “index” is most comprehensive since it integrates the amount of buoyancy over the entire vertical extent of the sounding) <1000 J/KG = weak instability 1000-2000 = moderate instability 2000-4000 = strong instability > 4000 = explosive

  17. Two examples:

  18. A couple more examples:

  19. Values of stability indices:

  20. Sources for sounding data: • weather.uwyo.edu/upperair/sounding.html • weather.cod.edu/analysis/analysis.sound.html • www.crh.noaa.gov/lot/?n=firewx

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