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Stability and Cloud Development

Stability and Cloud Development. Chapter 7. Cloud Development Introduction to atmospheric instability. Q: How and why do clouds form on some days and not on others????

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Stability and Cloud Development

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  1. Stability and Cloud Development Chapter 7

  2. Cloud Development Introduction to atmospheric instability • Q: How and why do clouds form on some days and not on others???? • Q: Why does the atmosphere sometimes produce stratus clouds (thin layered) while other times we get cumulus, or cumulonimbus clouds to form?? • The answer is largely related to the concept of atmospheric stability.....

  3. Cloud Development - stable environment • Consider this simple situation of a marble in the bottom of a bowl • If you push the marble up the side of the bowl, it will fall back down to the bottom, to it's original position • Stable air (parcel)- vertical motion is inhibited • If clouds form, they will be shallow, layered clouds like stratus.

  4. Cloud Development - unstable environment • If the marble is on the top of the bowl and you give it a little push, it rolls off the bowl.... does NOT come back to it's original position. • This is an unstable situation. • Unstable air (parcel)- vertical motion occurs. • This commonly produces cumulus or cumulonimbus clouds. • So, the question becomes, how does one determine the stability of the atmosphere?

  5. Assessing Atmospheric Stability • To determine whether or not a parcel will rise or sink in the atmosphere, one must compare the parcels temperature (Tp) with that of the environment (Te) at some altitude: • If Tp > Te what will the parcel do? • If Tp = Te what will the parcel do? • If Tp < Te what will the parcel do?

  6. Assessing Atmospheric Stability  • The bottom line - compare an air parcels temperature (Tp) with the environmental temperature (Te) at a given altitude • if Tp > Te, parcel rises • if Tp = Te parcel does not move up or down • if Tp < Te parcel sinks So, to assess stability, what two pieces of information do we need? 

  7. Assessing Atmospheric Stability • The vertical temperature profile of the environment and the temperature of the parcel of air • Vertical profiles of atmospheric temperature are collected at 12, 00 UTC every day at select NWS offices by launching balloon soundings: 

  8. Rising air parcels • Consider a rising parcel of air, as the parcel rises, it will adiabatically expand and cool • Adiabatic - a process where the parcel temperature changes due to an expansion or compression, no heat is added or taken away from the parcel

  9. Adiabatic cooling • Since it takes energy for the parcel molecules to "push out" on the parcel walls, they use up some of their internal energy in the process. • The parcel expands since the lower pressure outside allows the air molecules to push out on the parcel walls • Therefore, the parcel also coolssince temperature is proportional to molecular internal energy

  10. Sinking air parcels • As the parcel sinks, it will adiabatically compress and warm • Adiabatic - a process where the parcel temperature changes due to an expansion or compression, no heat is added or taken away from the parcel

  11. Adiabatic Warming • The parcel compresses since it is moving into a region of higher pressure • Due to the parcel compression, the air molecules gain internal energy. • Hence, the mean temperature of the parcel increases.

  12. Dry adiabatic lapse rate • As a parcel of air rises, it cools, but at what rate??? • Rate of temperature change with height is called the lapse rate. • Units of lapse rate are °C km-1. • Let's first consider an unsaturatedparcel of air • Unsaturated parcels cool at a rate of 10°C km-1 - this is called the dry-adiabatic lapse rate.

  13. Dry adiabatic lapse rate • What will be the parcel's temperature be at 1 km? • What will be the parcel's temperature be at 2 km? 20 0 C 30 0 C

  14. Question for Thought • Suppose the air pressure outside a conventional jet airliner flying at an altitude of 10 km is 250 mb. • Further, suppose the air inside the aircraft is pressurized to 1000mb. • If the outside air temperature is -50 ºC, what would be the temperature of this air if brought inside the aircraft and compressed at the dry adiabatic rate to a pressure of 1000mb? • Assume that a pressure of 1000mb is equivalent to an altitude of 0 m.

  15. Moist Adiabatic Lapse Rate • For a saturated parcel of air, that is when it's T=Td, then it cools at the moist adiabatic lapse rate = 6°C km-1 • What will be the parcel's temperature be at 3 km? • What will be the parcel's temperature be at 4 km? 14 0 C 8 0 C

  16. Moist Adiabatic Lapse Rate QUESTION Why does the parcel cool at a slower rate, 6 °C km-1 when it is saturated than at10°C km-1 when it is unsaturated?

  17. Dry versus Moist-Adiabatic Process • The moist adiabatic lapse rate is less than the dry adiabatic lapse rate because as vapor condenses into water (or water freezes into ice) for a saturated parcel, latent heat is released into the parcel, counteracting the adiabatic cooling.

  18. Cloud Development – Localized Convection Unequal heating of surfaces causes parcels to rise.

  19. Cloud Development – Orographic Lifting Windward side Leeward side LCL – lifting condensation level Rain Shadow Air is forced to rise over mountains.

  20. Cloud Development – Convergence Whenever air flows together, lifting results

  21. Cloud Development – Frontal Wedging Warmer, less dense air, is forced over cooler, denser air.

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