4 Rising Air Cools-Sinking Air Warms Rising air parcel expandsExpansion requires work against outside airAir molecules rebound from “walls” at a slower speed, resulting in a cooler temperatureAssuming no transfer of heat across parcel walls (adiabatic expansion), cooling rate is 10oC/kmno heat transfer494 m/s10oC502 m/s502 m/s494 m/s8 m/s1 kmRising ExpandingSinking Contracting20oC502 m/sAhrens, Fig 5.2
5 Adiabatic Cooling-Warming Dew point decreases with height at a rate of 2oC/km b/c DP varies less with Pressure than Temp.The rate is much less than cooling rate for air.Thus, unsaturated air can become saturated IF it rises far enough.10C 10C20C 12C30C 14CAhrens, Fig 5.2Red=Temperature Blue=Dew Point
6 Rising, Saturated Air Cools Less As a saturated parcel rises and expands, the release of latent heat mitigates the adiabatic coolingCooling for saturated air varies with mixing ratio.We will use an average value of 6oC/km for moisture lapse rateNote: sinking clear air always warms at dry lapse rateno heat transfer497 m/s14oC502 m/slatent heating502 m/s497 m/s5 m/s1 kmRising ExpandingSinking Contracting20oC502 m/sAhrens, Fig 5.2
7 Moist Flow over a Mountain -6C -6CMAR+10C +2CDAR-6C -6CMARsaturatedunsaturated+10C +2C DARunsaturated-10C -2CDAR+10C +2CDARAhrens, Fig 5.12These concepts can be applied to understand Temp and DP changes for moist flow over a mountain
8 Brain Burners Rising and sinking unsaturated (clear) air Temp changes at Dry Adiabatic Rate (DAR) of 10oC/kmDew point changes at rate of 2oC/kmRising and sinking saturated (cloudy) airTemp cools at Moist Adiabatic Rate (MAR) of 6oC/kmDew point decreases at rate of 6oC/km
9 Concept of Stability Stable Rock always returns to starting point Unstable Rock never returns to starting pointConditionally Unstable Rock never returns if rolled past top of initial hillAhrens, Fig 5.1
10 Archimedes’ Principle Archimedes' principle is the law of buoyancy.It states that "any body partially or completely submerged in a fluid is buoyed up by a force equal to the weight of the fluid displaced by the body."The weight of an object acts downward, and the buoyant force provided by the displaced fluid acts upward. If the density of an object is greater/less than the density of water, the object will sink/float.Demo: Diet vs. Regular Soda.
11 Absolutely Stable: Top Rock Stable air strongly resists upward motionExternal force must be applied to an air parcel before it can riseClouds that form in stable air spread out horizontally in layers, with flat bases-topsAhrens, Fig 5.3
12 Absolutely Unstable: Middle Rock Unstable air does not resist upward motionClouds in unstable air stretch out verticallyAbsolute instability is limited to very thin layer next to ground on hot, sunny daysSuperadiabatic lapse rateAhrens, Fig 5.5
13 Conditionally Unstable: Lower Rock Ahrens, Fig 5.7
14 Environmental Lapse Rate (ELR) ELR is the Temp change with height that is recorded by a weather balloon6.5o C/km6.0o C/kmELR is 6.5o C/km, on average, and thus is conditionally unstable!10.0o C/kmELR is absolutely unstable in a thin layer just above the ground on hot, sunny daysAhrens, Meteorology Today 5th Ed.
15 Lapse Rates and Cumulus Types Ahrens, Meteorology Today 5th Ed.The ELR and depth of unstable layer modulates the type of Cu.As depth increases, the vertical extent of Cu generally increases.As temp difference between the air parcel and the environment increases, the updraft speed and severity of Cb typically increase.
16 Summary: Key Concepts I Rising unsaturated air, and all sinking airTemp changes at DAR of 10oC/kmDP changes at rate of 2oC/kmSaturation occurs with sufficient liftingRising saturated airLatent Heating Mitigates Adia. CoolingTemp and DP cools at MAR of 6oC/kmNote that MAR is always less than DAR
17 Summary: Key Concepts II Vertical Stability Determined by ELRAbsolutely Stable and UnstableConditionally UnstableTemp Difference between ELR and Air Parcel, and Depth of Layer of Conditionally Instability ModulatesVertical Extent and Severity of Cumulus
18 Assignment for Next Lecture Topic - Precipitation ProcessesReading - Ahrens pProblems , 5.16, 5.17