Download Understanding Air Parcel Movement & Atmospheric Conditions: Stability & Skew-T Diagrams and more Study notes Climatology in PDF only on Docsity!
ATS 351ATS 351
Lecture 7
March 4-5, 2008
Stability & Skew-T Diagrams
Air ParcelAir Parcel
• To demonstrate
stability, a parcel
of air is used
• Expands and
contracts freely
• Always has
uniform properties
throughout
Air Parcel Movement: Air Parcel Movement:
Why does rising air expand and cool?
• Lift parcel: pressure lowers ⇒ air
molecules push outward ⇒ EXPANDS
- Energy is used to expand so molecules
slow down ⇒ COOLS
• Lower parcel: pressure increases ⇒
COMPRESSES parcel
- Compressing increases molecular energy
⇒ WARMS
Adiabatic ProcessAdiabatic Process
- Adiabatic Process: when a parcel expands
and cools or compresses and warms
WITHOUT exchange of heat with the
surrounding environment.
- In unsaturated air, a parcel of air cools or
warms at the Dry Adiabatic Rate (about
10ºC/km)
- The dew point also decreases as a parcel is
raised “Dry Adiabatically”
- Dew Point Lapse Rate: 2ºC/km
Moist Adiabatic ProcessMoist Adiabatic Process
- As the parcel rises, temperature and dew
point get closer together and are eventually
equal ⇒ condensation
- T (^) d decreases at a slower rate than T
- Since latent heat is released inside the parcel
during condensation, the temperature will
now decrease at a slower rate
- Moist Adiabatic Lapse Rate: ~6ºC/km
StabilityStability
- Stable Equilibrium
- If the ball is displaced it will return to it’s original position
- Unstable Equilibrium
- If the ball is displaced it will accelerate away from the equilibrium point
- Neutral Equilibrium
- If the ball is displaced it will stay in it’s new location.
Unstable AtmosphereUnstable Atmosphere
- Buoyant parcels are accelerated upward
- As parcels rise and cool, they are still warmer than the environment since the environment is cooling faster than the adiabatic lapse rate
- Larger instabilities lead
to larger updrafts
- Large updrafts lead to the formation of cumulonimbus clouds and thunderstorms
Causes of InstabilityCauses of Instability
- Cooling of the air aloft:
- Winds bringing in colder air (cold advection)
- Clouds (or the air) emitting IR radiation to
space (radiational cooling)
- Warming of the surface air:
- Daytime solar heating of the surface
- Winds bringing in warm air (warm advection)
- Air moving over a warm surface
Conditionally UnstableConditionally Unstable
- Environmental lapse rate is between moist
and dry adiabatic lapse rates (common in
atmosphere)
- Ex: environmental rate of 7ºC/km
- Conditional instability means that if
unsaturated air (stable) could be lifted to a
level where it becomes saturated, instability
would result
- Figure on next slide demonstrates conditional
instability
Conditional InstabilityConditional Instability
SkewSkew--T/LogT/Log--P DiagramP Diagram
Reminder:
Stability on a SkewStability on a Skew--TT
Lifting a ParcelLifting a Parcel
- Once a parcel has reached the LCL, it will continue to rise, but instead cool at the Moist Adiabatic Lapse Rate
- Often the temperature of the parcel at the LCL is still cooler than the temperature of the environment (negative area)
- If the parcel is lifted further it will reach its Level of Free Convection (LFC), the point at which the parcel becomes warmer than the environment and will be accelerated upward by buoyancy (positive area)
- As it continues to rise it will eventually reach a point where it is cooler than the environment again. This is the Equilibrium Level (EL)
Lifting a Parcel Lifting a Parcel
Sources of LiftSources of Lift
- 4 ways to lift a parcel to the LCL
- Frontal Boundary
- Orographic
- Convergence
- Convection
CAPECAPE
- CAPE = Convective Available Potential Energy
- CAPE is the energy available to a rising parcel to accelerate it
- On a Skew-T, CAPE is proportional to the area between the parcel’s temperature and the environment’s when the parcel is warmer
- CAPE gives an upper limit on how high updraft speeds can get in a severe storm
- High values of CAPE are associated with the possibility of strong convection - Large hail requires very high CAPE values 2,500+ Extreme
1,500-2,500 Large
1 - 1,500 Positive
CAPECAPE
CINCIN
- CIN = Convective Inhibition
- This is the energy the must be overcome in order to lift a parcel to its LFC
- On a Skew-T, CIN is proportional to the area between the parcel’s temperature and the environment’s when the parcel is colder
- Large values of CIN will prevent the formation of storms, but often the presence of some CIN can add strength to a storm if this energy is overcome
More Uses for Skew-More Uses for Skew-TT’’ss
- Forecasting precipitation type
The 0^0 C isotherm in this skew-T shows that the precipitation will fall through a layer which is above freezing, thus implying that freezing rain is possible
More Uses for Skew-More Uses for Skew-TT’’ss
- Forecasting maximum/minimum temperature
More Uses for Skew- More Uses for Skew-TT’’ss
- Forecasting the possibility of microbursts
The “inverted V” shape is a sign of possible dry microbursts (isolated pockets of strong winds associated with thunderstorms)
Parcels Movement on SkewParcels Movement on Skew--TT
Parcels Movement onParcels Movement on SkewSkew--TT
- A few skew-T reminders:
- Plot the temperature (or dew point) ON the pressure line that is given. - i.e. 25C at 900mb
- When plotting temperature, remember the temperature lines (isotherms) are slanted. - i.e. 25C at 300mb is NOT going to be directly above 25C at 1000mb
- The parcel of air begins at the surface temperature but follows either the dry or moist adiabatic lapse as it rises in the atmosphere (NOT the plotted temperature profile = environmental lapse rate)
