Chapter 2Air Parcel 2StabilityStability cont.Lapse ratesStableNeutralUnstableStability - visualVisual cont.ThetaMoistureConditional VerticalChangesDiagramsSkew-TMovementTemperaturesvariablesmorelevelsCAPEIndiciesex1ex2ex3ex4Meteorology 311Atmospheric Stability Fall 2010Meteorology 311Air Parcel●Consider a parcel of infinitesimal dimensions that is:●Thermally isolated from the environment so that its temperature changes adiabatically as it sinks or rises.●Always at the same pressure as the environmental air at the same level, assumed to be in hydrostatic equilibrium.●Moving slowly enough that its kinetic energy is a negligible fraction of its total energy.Meteorology 311Stability●Stability describes how air parcels react to an initial vertical push by some external force.●Forced to return to its original position: stable.●Continues to accelerate away from its originial position without outside help: unstable.●Continues to move away from its original position without accelerating: neutral.Meteorology 311Stability cont.●Consider a small disturbance from equilibrium....–Note: Primed values refer to the PARCEL.●P = P'●Adiabatic, displacements on small time scales.Meteorology 311Lapse Rates●Dry adiabatic lapse rate–Rate at which “dry” parcel changes temperature if raised or lowered in the atmosphere.–10 °C/km●Moist adiabatic lapse rate–Rate at which “moist” parcel changes temperature if raised or lowered in the atmosphere.–6 °C/km●Environmental lapse rate, Γ–Temperature structure of the environment.Meteorology 311Γ < Parcel Lapse rate●Buoyant acceleration < 0.●Buoyant force is opposite the displacement (negatively buoyant).●Positive restoring force.●Hydrostatically stable or positive stability.Meteorology 311Γ = Parcel Lapse Rate●Buoyant acceleration = 0.●No restoring force.●Displacements are met without opposition.●Hydrostatically neutral or neutral stability.Meteorology 311 Γ> Parcel Lapse Rate●Buoyant acceleration > 0●Buoyant force in direction of displacement.●Negative restoring force.●Hydrostatically unstable or negative stability.Meteorology 311Stability - VisuallyMeteorology 311Stability – Visual cont.Meteorology 311Stability - ThetaMeteorology 311Moisture●Γ < Γm < Γd–Absolutely stable.●Γ > Γd > Γm–Absolutely unstable.● Γm< Γ < Γd–Conditionally unstable.–Stable for unsaturated conditions.–Unstable for saturated conditions.Meteorology 311Conditional StabilityMeteorology 311Vertical Motion●Stability determines a layers ability to support vertical motion and transfer of heat, momentum, and constituents.●How do you get vertical motion?–Frontal boundaries (airmass differences)–Topography–Convergence (continuity equation)–Differential heatingMeteorology 311Changes in Lapse Rate●Environmental Lapse Rate can change over time.●Non-adiabatic heating and cooling●Solid advection●Differential advection●Vertical motionMeteorology 311Thermodynamic Diagrams●Let us plot the vertical structure of the atmosphere.●Tephigram●Stuve Diagram–Pseudo-adiabatic chart ●Skew-T, log P diagram–Most used operationally by forecasters.Meteorology 311Skew-T Diagram●Y-Axis is logarithmic in pressure.●Isotherms are “skewed” 45° from lower left to upper right.●Dry adiabats: slope from upper left to lower right. Label in degrees Celcius.●Saturation or “moist” adiabats – curved–(green on official charts)●Mixing ratio lines: dashed and slope a little from lower left to upper right (g/kg).Meteorology 311Movement●If air is dry (not-saturated), θ is conserved.–Adiabatic, move along a dry adiabat or line of constant θ.–Mixing ratio does not change.●If air is saturated, moisture condenses or evaporates, heat released impacts the temperature.– θe and θw keep the same value.–Mixing ratio changes.Meteorology 311Temperatures●Potential tempertature–Conserved in an adiabatic process–Dry adiabat●Wet-bulb temperature–Conserved in a moist adiabatic process–Moist adiabat●Equivalent potential temperature–Raise parcel until all moisture has condensed out and bring parcel back to 1000mb.–Used to compare parcels with different mositure contents and temperatures.Meteorology 311Important Variables●Mixing ratio (w)–Use w line through Td.●Saturated mixing ratio (ws).–Use w line through T.●RH = 100% (w/ws)●Vapor pressure (e)–Go from Td up isotherm to 622mb and read off mixing ratio in mb.●Saturation vapor pressure–Use T, not Td.Meteorology 311More Variables●Wet-bulb temperature (Tw).●Wet-bulb potential temperature (θw).●Equivalent temperature (Te).●Equivalent potential temperature (θe).Meteorology 311Important Levels●LCL – lifting condensation level–Where lifted air becomes saturated.●LFC – level of free convection–Where lifted air becomes positively buoyant.●EL – Equilibrium level–Where lifted air becomes negatively buoyant up high.●CCL – Convective condensation level.–Height to which a parcel of air would rise adiabatically to saturation from surface heating.Meteorology 311CAPE●CAPE = Convective Available Potential Energy●Positive area between parcel path and environmental profile.●Gives energy available to be converted to kinetic energy and upward motion.Meteorology 311Stability Indicies●LI – Lifted Index●SI – Showalter Index●K Index●TT – Total totals●Precipitable waterMeteorology 311Example #1Meteorology 311Example #2Meteorology 311Example #3Meteorology 311Example