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Meteorology 2603

Meteorology 2603. Lecture 10 3 February 1999. Today. Exam 1: Wednesday 10 February Review Session: Monday 8 February 1999 7:00 PM EC 1410 Thermodynamic Diagram Stability. Moist Adiabatic Lapse Rate.

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Meteorology 2603

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  1. Meteorology 2603 Lecture 10 3 February 1999

  2. Today • Exam 1: Wednesday 10 February • Review Session: • Monday 8 February 1999 • 7:00 PM • EC 1410 • Thermodynamic Diagram • Stability

  3. Moist Adiabatic Lapse Rate • In a moist adiabatic process, assume that the relative humidity of the air is equal to or greater than 100 %. • Recall: The dry adiabatic lapse rate is • -10oC km-1 • For every 1 km you lift a parcel, the temperature of that parcel will fall 10oC. Example: An unsaturated parcel with a temperature of 5oC is lifted 4.5 km. What is the new temperature of the parcel? 5oC + (-10oC km-1)(4.5 km) = 5oC - 45oC = -40oC

  4. Moist Adiabatic Lapse Rate • With a moist parcel, the RH100% so we assume condensation. • With condensation there is a release of latent heat. • This release of latent heat, warms the parcel. • The parcel does not cool with height as fast as does the dry parcel.

  5. Moist Adiabatic Lapse Rate • The moist (or wet) adiabatic lapse rate is • -6oC km-1 • Note: This is an average value!!! • Since the concentration of water vapor decreases with height, the release of latent heat decreases with hight. • The moist adiabatic lapse rate is not constant with height!!! It is curved.

  6. Thermodynamic Diagram • Moist (pseudo) adiabats (red): p Moist adiabats tell you what happens to the temperature of moist air (RH=100%) as it rises. If there is no condensation, the process is reversible. T Any time you are going up a moist adiabat, you are making a cloud. If you condense out all the moisture, you go down dry adiabat. Problem: (a) Moist air rising from the surface (T=12oC) will have a temperature of _________ at 1 km. (b) If dry, the temperature will be? Why? (a) T = 12oC + (-6oC km-1) x (1 km) = 6oC (b) T = 12oC + (-10oC km-1) x (1 km) = 2oC

  7. Stability • Stable - Lift an unsaturated parcel and the parcel cools dry adiabatically. In a stable environment, the parcel will be cooler than the environment so the parcel will sink and return to it’s original level. Parcel Environment 10C 1 km 13C 20C 20C

  8. Stability • Neutral - Lift an unsaturated parcel and the parcel cools dry adiabatically. In a neutral environment, the parcel will be the same temperature as the environment so the parcel will stay at the new level. Parcel Environment 10C 1 km 10C 20C 20C

  9. Stability • Unstable - Lift an unsaturated parcel and the parcel cools dry adiabatically. In an unstable environment, the parcel will be warmer than the environment so the parcel will move away from the original level. Parcel Environment 10C 1 km 8C 20C 20C

  10. Thermodynamic Diagram • Stability: To determine the stability you must plot a sounding. A sounding is the temperature at various heights as measured by a balloon-borne radiosonde. p The sounding is also called the environmental lapse rate (ELR). COLD WARM T Note: We also plot dew point on the chart -- we’ll get to that later.

  11. Thermodynamic Diagram • Stability: • We can evaluate the stability of an atmospheric layer by comparing the sounding (blue) to the dry (green) and moist (red) adiabats. CONDITIONALLY UNSTABLE STABLE UNSTABLE

  12. Absolute Stability • Special Kinds of Absolute Stability • Lapse -- The sounding is absolutely stable. The lapse rate is just the environmental sounding. • Isothermal -- The sounding is absolutely stable. The temperature is constant with height. • Inversion -- The sounding is absolutely stable. The temperature increases with height.

  13. Absolute Stability Lapse Isothermal Pressure Inversion Dry Adiabatic Lapse Rate Temperature Moist Adiabatic Lapse Rate

  14. Factors to Increase Stability • Warming Aloft • Radiational absorption • Latent heat release • Sensible heat transport into a region • Cooling Below • Radiational Cooling • Latent heat “absorption” • Sensible heat transfer out of a region • Air mass moving over a cold surface

  15. Factors to Increase Stability • Compression of a Layer of Air • Top of the layer will heat (by compression) more quickly than the bottom of the layer. • The lapse rate of the layer decreases.

  16. Factors to Increase Instability • Warming Below • Radiational absorption • Latent heat release • Sensible heat transport into a region • Air mass moving over a warm surface • Cooling Aloft • Radiational Cooling • Latent heat “absorption” • Sensible heat transfer out of a region

  17. Factors to Increase Instability • Lifting of a Layer of Air • Top of the layer will cool (by expansion) more quickly than the bottom of the layer. • The lapse rate of the layer increases.

  18. Neutral Stability • Displace a parcel and it remains where you put it. • It doesn’t return to it’s original position. • It doesn’t fly away from it’s original position. • Dry Neutral • Environmental lapse rate equals the dry adiabatic lapse rate. • Can result from thoroughly mixing air. • Moist Neutral • Environmental lapse rate equals the dry adiabatic lapse rate.

  19. Convective Instability • Lift layer with a dry top and a moist bottom. • The top layer cools dry adiabatically. • The bottom layer cools moist adiabatically. • Since the top layer cools faster than the bottom layer, we see a net cooling aloft. This tends to destabilize the atmosphere.

  20. Thermodynamics • Thermodynamic Diagram: • Handy little diagram -- Has on it: • pressure (P), altitude (Z) • temperature (T) • dry adiabatic lapse rates • (10C/100 m) • moist pseudoadiabatic lapse rates • (~0.60C/100 m) • saturation mixing ratio lines (for RH stuff)

  21. Thermodynamics • Thermodynamic Diagram (cont.): • Is used everyday in forecasting offices • clouds: type, time of formation, bases, tops • thunderstorms: hail, wind gusts, tornadoes • precipitation: type, intensity, amount • turbulence and icing for aircraft • temperatures: max, min • fog, freezing rain, rain vs. snow, etc.

  22. Norman Sounding3 February 1999 Temperature Sounding Dew Point Sounding

  23. Thermodynamic Diagram • Construction: Altitude in Km or 1,000’s of feet Pressure levels in mb. -400 C +300 C Temperature How high is the 500 mb level?

  24. Thermodynamic Diagram • Saturation mixing ratio line (yellow): p The saturation mixing ratio line tells how much H2Ov is in the parcel at a particular P, T if RH = 100%. T What is ws at 1000 mb and -100 C? What is the RH at 1000 mb when T=240 C and Td=130 C? If T=200 C and RH = 70%, what is Td at 1000 mb?

  25. Thermodynamic Diagram p • Dry adiabats (green): The dry adiabats tell us what happens to the temperature of a dry air parcel as it rises or sinks. (RH<100%) T What is the temperature of a parcel at 1000 mb and T=200C if lifted to 900 mb? to 600 mb? What will the temperature of a parcel at 600 mb and T= -200C be if it sinks to 1000 mb?

  26. Temperature of a parcel lifted dry adiabatically to 600 mb. Tparcel = -20C Temperature of a parcel at 1000 mb Tparcel = 20C

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