Atmospheric DynamicsBuffalo State College：大气动力学布法罗州立学院.ppt

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1、Atmospheric Dynamics,The atmosphere is a fluidWhat are the properties of fluids?,Flow under the influence of forcesImportant forces for the atmosphere(and ocean):GravityFrictionPressure gradientThe so-called“fictitious forces”:Coriolis,CentrifugalGenerally considered continuous,although they are in

2、fact composed of discrete atoms/moleculesCome in two general forms:LiquidsGases,Forces and Motions,Gravity“heavy”fluid sinks/“light”fluid risesExample:buoyancy,like in a lava lamp“BLOOP BLOOP”Friction drag force acting on the fluid on account of its coming in contact with a rigid surfaceExample:exce

3、ss drag on one side of a spinning baseball causes a net force(in this case,on the ball)in that direction“STRIKE ONE!”Pressure Gradient motion ensues from high-pressure to low-pressure under this forceExample:fill your hose,set it on the ground,step on one end:what happens?Under your foot is high pre

4、ssure,at the open end the pressure is low“SQUIRT!”,Forces and Motions,FICTICIOUS FORCESCentrifugal Force if you wish to move in a circle,you must overcome your inertia,i.e.your tendency to move in straight lines unless you are acted on by a forceExample:sharp left turn in your car“WHOA!”Coriolis For

5、ce motion relative to a rotating surface is apparently affected by this forceExample:rather like playing catch on a merry-go-round“SMACK!”,Coriolis Force,Forces and Motions,Newtons Second Law:In words,a net force acting on an object causes it to accelerate by an amount proportional to the sum of for

6、ces and inversely proportional to its mass.If the net force acting on an object is zero,the object will move in a straight line at constant speed(Newtons First Law).,Density,Pressure and Temperature,DensityMass per unit volume,mks units:kg m-3“kilograms per cubic meter”cgs units:g cm-3“grams per cub

7、ic centimeter”,Density,Pressure and Temperature,PressureForce per unit area,mks units:kg s-2 m-1 Pa“Pascals”other units:bar,mb,atm,hPa,Density,Pressure and Temperature,Temperature(and the Ideal Gas Law)Temperature:Average kinetic energy of moleculesunits:Kelvins(K),degrees Celcius(oC),:pressure exer

8、ted by vapor:volume occupied by vapor:mass of vapor:specific gas constant of substance:temperature of vapor(must be in K),Density,Pressure and Temperature,Use definition of density to formulate yet another version of the Ideal Gas Law,:pressure exerted by the vapor:density of the vapor:specific gas

9、constant of the substance:temperature of the vapor,Buoyancy,Archimedes principleVitruvius(De architectura IX.912)recounts the famous story of Archimedes making this discovery while in the bath.He was given the task of finding out if a goldsmith,who worked for the king,was carefully replacing the kin

10、gs gold with silver.While doing this Archimedes decided he should take a break so went to take a bath.While entering the bath he noticed that when he placed his legs in,water spilled over the edge.Struck by a moment of realisation,he shouted Eureka!He informed the king that there was a way to positi

11、vely tell if the smith was cheating him.Knowing that gold has a higher density than silver,he placed the kings crown and a gold crown of equal weight into a pool.Since the kings crown caused more water to overflow,it was,therefore,less dense,Archimedes concluded that it contained silver,causing the

12、smith to be executed.The actual record of Archimedes discoveries appears in his two-volume work,On Floating Bodies.source:http:/en.wikipedia.org/wiki/Buoyancy,Buoyancy,Archimedes principleBuoyancy Force=weight of the displaced fluidWeight:force of gravity,Example Problem:The volume of a lake freight

13、er hull is V=300 m X 30 m X 20 m=1.8 x 105 m3.If the density of lake water is 1000 kg m-3,what is the buoyancy force acting on the ship if it is nearly submerged(i.e.the water line is at the deck)?What is the cargo capacity,assuming the mass of the ship is negligible compared to the mass of the carg

14、o it is carrying(this is probably a fairly bad assumption)?Solution:The mass of displaced water is 1.8 x 108 kg and the weight of this water is then about 1.8 x 109 N.Thus,the buoyancy force acting on the ship will be about 2 billion Newtons.The cargo capacity would be somewhat less than 1.8 x 108 k

15、g,depending on exactly how massive the empty ship itself is.,How does buoyancy affect motion in fluids?,ambient fluid,parcel,Will the parcel rise,fall or remain still?,How does buoyancy affect motion in fluids?,Forces acting on the parcel:GravityBuoyancy,Apply Newtons Second Law:,So,if the parcel is

16、 more dense than the ambient fluid(),it will sink.Examples:ocean convection/deep water formationHowever,if the parcel is less dense than the environment(),it will rise.Examples:hot air balloons,dirigibles,moist air in atmospheric convection,How is buoyancy generated in the atmosphere?,There are two

17、main ways that buoyancy(and therefore motion)is generated in the atmosphere:Add/Subtract moisture,since H2O has a lower molecular weight than dry air composed of N2,O2 and ArIncrease/Decrease temperature(or a combination of the two,of course),Global Energy Budget(revisited),INSOLATION,Atmospheric Mo

18、tion,The Tropics:The Hadley Circulation,Atmospheric Motion,The Tropics:Precipitation Patterns,Atmospheric Motion,The Tropics:Precipitation Patterns(Seasonal Cycle in Soil Moisture)http:/geography.uoregon.edu/envchange/clim_animations/gifs/prate_web.gif,Why do we have jet streams?,Angular Momentum Co

19、nservation,angular momentum per unit mass,distance to rotation axis,east-west wind speed,Figure Skater,Atmosphere,Atmospheric Motion,The Extra-tropics:The Jet Streams,Atmospheric Motion,The Hadley Cell and Angular Momentum,What is weather?,Midlatitude weather systems are instabilities of the zonal j

20、et.A perfectly symmetric jet is unstable and begins to“meander”,producing mobile high and low pressure systems.Weather is the response of the fluid atmosphere to a local imbalance in the energy budget.,The meandering jet stream,Jet Stream and Weather Systems,What is weather?,Midlatitude weather syst

21、ems are instabilities of the zonal jet.A perfectly symmetric jet is unstable and begins to“meander”,producing mobile high and low pressure systems.Weather is the response of the fluid atmosphere to a local imbalance in the energy budget.,The function of large-scale atmospheric motions,including“weat

22、her”phenomena,is to transport thermal energy from the equator to the poles,thereby balancing the GLOBAL ENERGY BUDGET.,Global Energy Budget(revisited),Global Energy Budget,Balanced global energy budget comprises radiative energy input and output as well asSensible heat transport(temperature)Latent h

23、eat transport(water vapor)We just spent some time on the sensible heat budget;lets look at water vapor transport,Moving warm air to the poles and cold air to the tropics,The Hydrological Cycle,The Hydrological Cycle,The Hydrological Cycle,The Hydrological Cycle,How in the world does evaporation and

24、precipitation constitute heat transport in the atmosphere?!?dump some radiative energy into tropical ocean surface and evaporate water the energy is not used to heat the ocean but to evaporate the watermove the newly evaporated water vapor toward the poles in atmospheric circulationscondense the wat

25、er vapor in extra-tropical clouds/rain energy released by condensation warms extra-tropical airNet Result:Radiative energy deposited in the tropics has been used to warm extra-tropical air!,Schematic of the General Circulation,The General Circulationof the Atmosphere,The General Circulationof the Atmosphere,Some other climate animationshttp:/geography.uoregon.edu/envchange/clim_animations/,