NATS 101 Lecture 12 Vertical Stability

Tennis Basics Piece of cake, right? Forehand: Drop Shot: Racket velocity before hit Ball speed approaching Ball speed leaving Forehand: Racket velocity before hit Ball speed approaching Ball speed leaving Drop Shot: Piece of cake, right?

Air Molecules Act Similarly Molecule speed approaching Boundary velocity Molecule speed leaving Contracting Boundary: Expanding Boundary:

Rising Air Cools-Sinking Air Warms Rising air parcel expands Expansion requires work against outside air Air molecules rebound from “walls” at a slower speed, resulting in a cooler temperature Assuming no transfer of heat across parcel walls (adiabatic expansion), cooling rate is 10oC/km no heat transfer 494 m/s 10oC 502 m/s 502 m/s 494 m/s 8 m/s 1 km Rising Expanding Sinking Contracting 20oC 502 m/s Ahrens, Fig 5.2

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 10C 20C 12C 30C 14C Ahrens, Fig 5.2 Red=Temperature Blue=Dew Point

Rising, Saturated Air Cools Less As a saturated parcel rises and expands, the release of latent heat mitigates the adiabatic cooling Cooling for saturated air varies with mixing ratio. We will use an average value of 6oC/km for moisture lapse rate Note: sinking clear air always warms at dry lapse rate no heat transfer 497 m/s 14oC 502 m/s latent heating 502 m/s 497 m/s 5 m/s 1 km Rising Expanding Sinking Contracting 20oC 502 m/s Ahrens, Fig 5.2

Moist Flow over a Mountain -6C -6C MAR +10C +2C DAR -6C -6C MAR saturated unsaturated +10C +2C DAR unsaturated -10C -2C DAR +10C +2C DAR Ahrens, Fig 5.12 These concepts can be applied to understand Temp and DP changes for moist flow over a mountain

Brain Burners Rising and sinking unsaturated (clear) air Temp changes at Dry Adiabatic Rate (DAR) of 10oC/km Dew point changes at rate of 2oC/km Rising and sinking saturated (cloudy) air Temp cools at Moist Adiabatic Rate (MAR) of 6oC/km Dew point decreases at rate of 6oC/km

Concept of Stability Stable Rock always returns to starting point Unstable Rock never returns to starting point Conditionally Unstable Rock never returns if rolled past top of initial hill Ahrens, Fig 5.1

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. http://www.onr.navy.mil/focus/blowballast/sub/work2.htm

Absolutely Stable: Top Rock Stable air strongly resists upward motion External force must be applied to an air parcel before it can rise Clouds that form in stable air spread out horizontally in layers, with flat bases-tops Ahrens, Fig 5.3

Absolutely Unstable: Middle Rock Unstable air does not resist upward motion Clouds in unstable air stretch out vertically Absolute instability is limited to very thin layer next to ground on hot, sunny days Superadiabatic lapse rate Ahrens, Fig 5.5

Conditionally Unstable: Lower Rock Ahrens, Fig 5.7

Environmental Lapse Rate (ELR) ELR is the Temp change with height that is recorded by a weather balloon 6.5o C/km 6.0o C/km ELR is 6.5o C/km, on average, and thus is conditionally unstable! 10.0o C/km ELR is absolutely unstable in a thin layer just above the ground on hot, sunny days Ahrens, Meteorology Today 5th Ed.

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.

Summary: Key Concepts I Rising unsaturated air, and all sinking air Temp changes at DAR of 10oC/km DP changes at rate of 2oC/km Saturation occurs with sufficient lifting Rising saturated air Latent Heating Mitigates Adia. Cooling Temp and DP cools at MAR of 6oC/km Note that MAR is always less than DAR

Summary: Key Concepts II Vertical Stability Determined by ELR Absolutely Stable and Unstable Conditionally Unstable Temp Difference between ELR and Air Parcel, and Depth of Layer of Conditionally Instability Modulates Vertical Extent and Severity of Cumulus

Assignment for Next Lecture Topic - Precipitation Processes Reading - Ahrens p121-134 Problems - 5.14, 5.16, 5.17