Atmospheric Motions & Climate

Transcription

1 Atmospheric Motions & Climate 20-1 Vertical Atmospheric Motion Hydrostatic Balance Non-hydrostatic Balance Science Concepts Newtonʼs Laws of Motion Vertical Forces Pressure Gradient Force Gravitational Force Friction Force Buoyancy Vertical Dry Adiabatic Motion Moist Adiabatic Motion Skew-T Log-p Diagram Rules Changes in The Earth System (Kump, Kastin & Crane) Chap. 4 (pp ) Latent Heat

2 Atmospheric Motions 20-2 What keeps this balloon in the air? Scientist Benjamin Franklin witnessed brothers Montgolfiers launch the first manned balloon flight on November 21, 1783 in France, after he had negotiated the end of the Revolutionary War. The Montgolfiers believed the balloonʼs lift was caused by hot air and smoke, so plied the fire with wet straw and wool. Ten days later, Jacques Charles (Charlesʼ Law fame) flew a silk balloon filled with hydrogen for two hours while traveling 21 miles. Franklin helped finance Charlesʼ flight. Franklin Parody - Walter Isaacson, 2003: Benjamin Franklin - An American Life, Simon and Schuster, NY, pp If you want to fill your balloons with an element ten times lighter than inflammable air, you can find a great quantity of it, and ready made, in the promises of lovers and of courtiers.

3 Atmospheric Motions 20-3 Vertical Motion Vertical Forces and Acceleration - Gravitational force (GF) > Generally points toward the center of the Earth > Depends on the mass of the object or in our case the air parcel - Pressure gradient force (PGF) > As before, points toward lower pressure, i.e., upward in the vertical > Depends on the mass of the displaced fluid, in our case the mass of the displaced environmental air - Friction force (FF) > Not very important because friction depends on the object's speed and vertical velocities are in general small

4 Atmospheric Motions 20-4 Vertical Motion (Con t) Hydrostatic Balance - As a first approximation the PGF is equal to and in the opposite direction to the GF. Thus these two forces cancel and the net force is zero. Therefore, the acceleration is zero. This state is called hydrostatic balance. Non-Hydrostatic Balance - In cases with large imbalances between the PGF and GF, air parcels are accelerated vertically either up or down. - This acceleration is referred to as buoyancy or Archimedean acceleration. - Warm air accelerating upward and cold air accelerating downward, i.e., convection, are examples of non-hydrostatic balance.

5 Atmospheric Motions 20-5 Hydrostatic Balance Non-Hydrostatic Balance PGF PGF PGF PGF = Gravity PGF Gravity Gravity Gravity Zero Acceleration Surface Upward Acceleration Gravity Downward Acceleration

6 Atmospheric Motions 20-6 In a Thunderstorm PGF PGF Downdraft: PGF<< Gravity Updraft: PGF >> Gravity Gravity Gravity Surface

7 20-7 Vertical Consequences of the Gas Law revisited Boyleʼs Law T Constant T Changes p Changes V Constant Gas Law Dry Adiabatic Ascent T V T V T V 15 C C C 1.91 Altitude (m) Pressure (mb) C C C C C C C C C 1.00

8 20-8 Vertical (Con t) Important to determine the occurrence and strength of convection and afternoon showers. Also important to determine the vertical mixing of pollution. Adiabatic Diagrams Use to determine atmospheric stability Plot of temperature versus pressure - Several types - Skew-T Log-p diagrams Compare measured lapse rate with dry or moist adiabatic parcel lapse rates - Adiabatic - No energy (heat) added or subtracted

9 20-9 Simple Skew-T Log-p Diagram Pressure (mb) Temperature ( C)

10 20-10 Skew-T Log-p Diagram with Dry Adiabats Pressure (mb) Temperature ( C)

11 20-11 Stable Atmosphere Pressure (mb) Dry Adiabat T p <T e Observed or Measured Lapse Rate T p =T e Temperature ( C) Parcel beginning at 1050 mb is lifted to 600 mb dry adiabatically. Note the parcel is colder than its environment, thus it is accelerated back toward its original position. This atmosphere is considered to be stable.

12 20-12 Unstable Atmosphere 400 Pressure (mb) Dry Adiabat Observed or Measured Lapse Rate Continues to accelerate upward until T p =T e T p >T e Temperature ( C) Parcel beginning at 1050 mb is lifted to 600 mb dry adiabatically. Note the parcel is warmer than its environment, thus it is accelerated away from its original position. This atmosphere is considered to be unstable.

13 20-13 Unstable Atmosphere 400 Pressure (mb) Dry Adiabat Unstable Stable Observed or Measured Lapse Rate Temperature ( C)

14 20-14 Rules When a parcel is displaced (moved) from its original position to a new position, > If the net force accelerates the parcel back toward its original position then the atmosphere is considered stable (T p <T e ) > If the net force accelerates the parcel away from its original position then the atmosphere is considered unstable (T p >T e ) > If the net force is zero then the atmosphere is considered neutral (T p =T e )

15 20-15 Vertical Consequences of the Gas Law revisited again Altitude (m) Pressure (mb) Boyleʼs Law T Changes p Changes Gas Law Dry Adiabatic Ascent T Constant V Constant T V T V T V 15 C 15 C C -99 C C C Gas Law Moist Adiabatic Ascent T V (M s ) C 2.14 (0.76) -7.8 C 1.52 (3.42) C C C C 1.23 (6.64) C C C C 1.00 (10.70)

16 20-16 Skew-T Log-p Diagram with Moist Adiabats Pressure (mb) Temperature ( C)

17 20-17 Stable Moist Atmosphere Pressure (mb) Moist Adiabat T p <T e accelerates downward Observed or Measured Lapse Rate A saturated parcel beginning at 1050 mb is lifted to 600 mb moist adiabatically. Note the parcel is colder than its environment, thus it is accelerated back toward its original position. This atmosphere is considered to be stable T p =T e Temperature ( C)

18 20-18 Unstable Moist Atmosphere Pressure (mb) Observed or Measured Lapse Rate Moist Adiabat Continues to accelerate upward until T p =T e T p >T e A saturated parcel beginning at 1050 mb is lifted to 600 mb moist adiabatically. Note the parcel is warmer than its environment, thus it is accelerated away from its original position. This atmosphere is considered to be unstable Temperature ( C)

19 20-19 Unstable Moist Atmosphere 400 Pressure (mb) Moist Adiabat Unstable Stable Observed or Measured Lapse Rate Temperature ( C)

20 20-20 Combined Regions 400 Pressure (mb) Dry Adiabat Conditional Moist Adiabat Absolute Stablility Three Observed or Measured Lapse Rates Absolute Instablility Temperature ( C)

21 Skew-T Log-p Chart Pressure (mb) Temperature ( C)

22 20-22 Conditional Note: In this region the stability criterion depends on if the parcel is saturated or not - If the parcel is saturated, then the atmosphere is considered to be Unstable and if the parcel is unsaturated, then the atmosphere is considered to be Stable Causes of Changes in Destabilizes - Solar heating - Cold air advection over a warm surface Stabilizes - Radiational cooling - Warm air advection over a cold surface As it usually does on the Colorado Plateau, night defeated the storm. It drifted northeastward, robbed of the solar power that fed it, and exhausted its energy in the thin, cold air over the Utah canyons and mountains of northern New Mexico. Tony Hillerman, 1986: Skinwalkers, p. 261.

23 20-23 Effects on Convective Clouds 21 November 2005 cloud streets over Hudson Bay NewImages/images.php3?img_id=17108 Wind Direction Cloud streets are parallel lines of cumulus clouds that align with the wind Result of thermals, or rising columns of warmed air formed when the surface is a little warmer than the air above Here a cold northwest wind (red arrow) is blowing off the ice covered land over the still warmer water of Hudson Bay This destabilizes the air and creating convective clouds through lifting the air to its saturation level These clouds are then carried by the steady wind forming lines of clouds aligned along the direction of the wind

24 20-24 Effects on Convective Clouds Surface solar heating destabilizes the air sometimes allowing afternoon convective clouds and showers to form 7 September 1999 Radar Reflectivity

25 20-25 Effects on Convective Clouds Lightning intensity Diurnal distribution Beginning at midnight local time

26 20-26 Effects on Convective Clouds Lightning intensity Diurnal distribution Clock & 2400 up Minimum ~7:00 am Maximum ~5:00 pm

27 20-27 Effects on Convective Clouds Global Lightning vs Time Lightning intensity Blue = less Green = more Latitudinal Lightning Distribution vs Time White Line = Annual Mean Latitudinal Lightning Distribution Month/Year Longitudinal Lightning Distribution White Line = Annual Mean Latitudinal Lightning Distribution

28 20-28 Effects on Convective Clouds Lightning Intensity June May 1999 distribution

29 20-29 Effects on Convective Clouds Lightning intensity Annual distribution (Jan-Dec)

30 20-30 Effects on Convective Clouds Lightning intensity Note difference between February and August

31 20-31 Effects of Air Pollution Fanning Fumigation Looping Coning Lofting Trapping