Post on 01-Jan-2016
description
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 = -ρg
This is why horizontal motions are generally >> than vertical motions.
Stability: The degree to which the atmosphere resists vertical motion.
Causes of vertical motion:
Forced / mechanical:
fronts
lake breeze
orography
jet streams
short waves
Thermal:differential surface
heatingwarm advection in lower
levels and/or cold advection aloft
cooling on top of moist layers.
Stable: Lifted parcel of air is negatively buoyant.
Unstable: Lifted parcel of air becomes positively
buoyant
Instability = Convection
A convective cell:
pic of cu field
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’
Expansion/Compression example
Moist Ascent: pseudo-adiabatic process
Adiabatic cooling by expansion is partially offset
by release of latent heat of condensation:
γm = -g/Cp + Lv dqv/Cp dZ
The moist (or saturation) lapse rate varies depending on: Dew Point (how much water vapor is present) Buoyancy ( rate of condensation)
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)
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 TpTdp
Parcel can ascend either dry or pseudo adiabatically:
Lift the parcel to a desired altitude:
A parcel’s ascent is usually a combination of dry…then moist ascent:
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…
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
Two examples:
A couple more examples:
Values of stability indices:
Sources for sounding data:
• weather.uwyo.edu/upperair/sounding.html• weather.cod.edu/analysis/analysis.sound.html• www.crh.noaa.gov/lot/?n=firewx