2 Air Parcel To demonstrate stability, a parcel of air is used Expands and contracts freelyAlways has uniform properties throughout
3 Air Parcel Movement: Why does rising air expand and cool? Lift parcel: pressure lowers air molecules push outward EXPANDSEnergy is used to expand so molecules slow down COOLSLower parcel: pressure increases COMPRESSES parcelCompressing increases molecular energy WARMS
4 Adiabatic ProcessAdiabatic 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
5 Moist Adiabatic Process As the parcel rises, temperature and dew point get closer together and are eventually equal condensationTd decreases at a slower rate than TSince latent heat is released inside the parcel during condensation, the temperature will now decrease at a slower rateMoist Adiabatic Lapse Rate: ~6ºC/kmBullet 2: LH release offsets the adiabatic cooling that is occurring because of the rising motion - temperature decreases at a slower rate.
6 Stability Stable Equilibrium Unstable Equilibrium Neutral Equilibrium If the ball is displaced it will return to it’s original positionUnstable EquilibriumIf the ball is displaced it will accelerate away from the equilibrium pointNeutral EquilibriumIf the ball is displaced it will stay in it’s new location.
7 Stability in the Atmosphere At any height, if the temperature of the parcel is greater than the environment, the parcel will rise (and vice versa).Temperature profile of the environment is received from radiosonde data.We can look at the lapse rate of the environment to see what an air parcel will do if it is displacedIn a stable atmosphere: a displaced parcel will return to its initial position.In an unstable atmosphere: a displaced parcel will continue to move in the initial direction of motion.
8 Conditions for Stability Absolutely StableEnvironmental lapse rate is less than moist adiabatic lapse rate.Lapse rate < 6ºC/kmAbsolutely UnstableEnvironmental lapse rate is greater than dry adiabatic lapse rate.Lapse rate > 10ºC/kmConditionally UnstableEnvironmental lapse rate lies between moist and dry lapse rates.Lapse rate between 6- 10ºC/km
9 Stable AtmosphereThe parcel of air is colder than the environment since its lapse rate is greater.Therefore, a displaced parcel will return to its original position: vertical motion is suppressed.What conditions produce a stable atmosphere?:Air aloft warms (by warm advection) and surface air cools (by radiative cooling at night or cold advection)Subsiding air (frequently associated with a ridge of high pressure)Inversions represent very stable air.Tropopause is often very stable, as the stratosphere is warmed due to ozone.Bullet 1: The PARCEL follows either the dry or moist adiabatic lapse rate (depending on if it is saturated or not).Before bullet 3: A stable atmosphere means that the difference between the surface air and air aloft is SMALL.Inversion is an example of very stable air.If clouds WERE to form by forced vertical motion, the type of clouds that would be produced are thin layers of clouds with flat tops (stratus, cirrostratus, altostratus, nimbostratus).
10 Unstable 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 rateLarger instabilities leadto larger updraftsLarge updrafts lead tothe formation ofcumulonimbus cloudsand thunderstorms
11 Causes 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 surfaceWinds bringing in warm air (warm advection)Air moving over a warm surfaceDaytime solar heating: this is one of the reasons you see convection occur a lot in the later afternoon because the sun has reached its max of heating the surface of the earth
12 Conditionally Unstable Environmental lapse rate is between moist and dry adiabatic lapse rates (common in atmosphere)Ex: environmental rate of 7ºC/kmConditional instability means that if unsaturated air (stable) could be lifted to a level where it becomes saturated, instability would resultFigure on next slide demonstrates conditional instabilityIf environmental lapse rate is 7ºC/km: FIGUREIf it is unsaturated, the parcel will follow the dry adiabatic lapse rate and it will be colder than the environment, so it is stable.However, if it reaches a level where it is saturated now, it will cool at the moist adiabatic rate and therefore will be warmer than the environment and will be come unstable and rise.
15 Stability on a Skew-TDetermine the stability of the atmosphere by looking at the PLOTTED temperature profile.There will be varying layers of stability throughout the atmosphere.
16 Examples Layer between 700mb and 800mb is absolutely stable ABSOLUTELY UNSTABLELayer between 700mb and800mb is absolutely stableLayer between 850mb and950mb is absolutely unstable
17 Examples Layer between 600mb and 700mb is conditionally unstable
18 Lifting a ParcelInitially, a parcel being lifted will cool at the Dry Adiabatic Lapse RateWhen the dry adiabat from the surface temperature meets the saturating mixing ratio line from the surface dew point, the parcel will have reached saturation and condensation can occurThis is called the Lifted Condensation Level (LCL)Green: Mixing ratio lines…FOLLOW UP FROM SURFACE DEW POINTPink: Dry adiabat lines…FOLLOW UP FROM SURFACE TEMPERATUREINTERSECTION: LCL
19 Lifting a ParcelOnce a parcel has reached the LCL, it will continue to rise, but instead cool at the Moist Adiabatic Lapse RateOften 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 itsLevel 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)
21 Sources of Lift 4 ways to lift a parcel to the LCL Orographic Frontal BoundaryConvergenceConvection
22 CAPE CAPE = Convective Available Potential Energy CAPE is the energy available to a rising parcel to accelerate itOn a Skew-T, CAPE is proportional to the area between the parcel’s temperature and the environment’s when the parcel is warmerCAPE gives an upper limit on how high updraft speeds can get in a severe stormHigh values of CAPE are associated with the possibility of strong convectionLarge hail requires very high CAPE valuesExtreme2,500+Large1,500-2,500Positive1 - 1,500
24 CIN CIN = Convective Inhibition This is the energy the must be overcome in order to lift a parcel to its LFCOn a Skew-T, CIN is proportional to the area between the parcel’s temperature and the environment’s when the parcel is colderLarge 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
26 More Uses for Skew-T’s Finding cloud levels Forecasting precipitation typeForecasting max/min temperaturesForecasting the possibility of microbursts
27 More Uses for Skew-T’s Finding cloud levels – useful for aviation Clouds are mostlikely present at 3layers in thisskew-T. Can youfind them?Right at surface, 680mb, and 480mb
28 More Uses for Skew-T’s Forecasting precipitation type The 00C isotherm in this skew-T shows that the precipitation will fall through a layer which is above freezing, thus implying that freezing rain is possibleRain is falling through above freezing layer but right at surface, temperature is just at 0 degrees C…therefore the rain may freeze.
29 More Uses for Skew-T’s Forecasting maximum/minimum temperature 12Z on left (Oct 3); 0Z on right (Oct 4)How to do it:From the morning sounding (12Z): if there is an inversion (like there is here): find the temperature at the warmest point of the inversion.From that point, follow the dry adiabat down to the surface and that corresponding temperature is your Tmax
30 More Uses for Skew-T’s Forecasting the possibility of microbursts The “inverted V” shape is a sign of possible dry microbursts (isolated pockets of strong winds associated with thunderstorms)
31 Parcels Movement on Skew-T Just explain that if you are looking for the PARCELS temperature, the orange line is what you will follow. The temperature profile is the ENVIRONMENT.
33 A few skew-T reminders: Plot the temperature (or dew point) ON the pressure line that is given.i.e. 25C at 900mbWhen plotting temperature, remember the temperature lines (isotherms) are slanted.i.e. 25C at 300mb is NOT going to be directly above 25C at 1000mbThe 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)
34 900mb1000mb500mbJust pointing out that your temperatures are going to be SLANTED, even if they are the same value as you go up in the atmosphere.