Good evening everyone. My topic is section five point four, secondary circulation and spin down. [click]

There are five parts in my presentation, key words, secondary circulation, spin down, Physical meaning,and summary.

[click]Firstly, let us see the key words. The secondary circulation is a circulation superposed on the primary circulation by the physical constraints of the system. And the spin down is a mechanism for destroying vorticity in a rotating atmosphere by losing angular momentum. The Angular momentum measures an object's tendency to continue to spin. And angular momentum is equal to the vector product among mass, velocity and distance (from point object is spinning or orbiting around)[click] Both the mixed-layer solution and the Ekman spiral solution indicate that in the planetary boundary layer the horizontal wind has a component directed toward lower pressure. As the rad arrows shown in Fig. 5.6, [click]this implies mass convergence in a cyclonic circulation and mass divergence in an anticyclonic circulation, which by mass continuity requires vertical motion out of and into the boundary layer, respectively. [click] In order to estimate the magnitude of this induced vertical motion, we note that if vg = 0 so that ug is independent of x, the cross isobaric mass transport per unit area at any level in the boundary layer is given by ρ0v. For the Ekman spiral, it is given by the equation five point thirty five, where De is equal to πdivide byγ, and is the Ekman layer depth defined in Section 5.3.4.[click]Integrating the mean continuity equation (5.13) through the depth of the boundary layer gives equation five point thirty six. Assuming that w(0) = 0, and vg = 0 , and comparing with (5.35) that the mass transport at the top of the Ekman layer is given by equation five point thirty

seven.

[click]Noting that minus partial ug divide by partial y is just the geostrophic vorticity in this case. we find equation five point thirty eight after substituting into (5.36), where we have neglected the variation of density with height in the boundary layer and have assumed that one plus e to minus π is approximately equal to 1. Hence, we obtain the important result that the vertical velocity at the top of the boundary layer is proportional to the geostrophic vorticity. In this way the effect of boundary layer fluxes is communicated directly to the free atmosphere through a forced secondary circulation. This process is often referred to as boundary layer pumping. It only occurs in rotating fluids and is one of the fundamental distinctions between rotating and nonrotating flow. For a typical synoptic-scale system , the vertical velocity given by (5.38) is of the order of a few millimeters per second. [click]

An analogous boundary layer pumping is responsible for the decay of the circulation created when a cup of tea is stirre. Radial inflow takes place near the bottom of the cup. By continuity of mass, the radial inflow in the bottom boundary layer requires upward motion and a slow compensating outward radial flow throughout the remaining depth of the tea. This slow outward radial flow approximately conserves angular momentum, and by replacing high angular momentum fluid by low

angular momentum fluid serves to spin down the vorticity in the cup far more rapidly than could mere diffusion. [click]

For synoptic-scale motions the barotropic vorticity equation (4.24) can be written approximately as equation four point twenty four. where we have neglected ζg compared to f in the divergence term and have also neglected the latitudinal variation of f .Recalling that the geostrophic vorticity is independent of height in a barotropic atmosphere, (5.39) can be integrated easily from the top of the Ekman layer (z = De) to the tropopause (z = H) to give equation five point forty. Substituting for w(De) from (5.38), assuming that w(H) = 0 and that H is much lager than De, the equation (5.40) may be written as equation five point forty one. This equation may be integrated in time to give equation five point forty two.where ζg(0) is the value of the geostrophic vorticity at time t = 0, and τe is the time that it takes the vorticity to decrease to e ?1 of its original value. [click]

This e-folding time scale is referred to as the barotropic spin-down time. Taking typical values of the parameters as follows, we find that τe approximates 4 days. Thus, for nidlatitude synoptic-scale disturbances in a barotropic atmosphere, the spin-down time is a few days. This decay time scale should be compared to the time scale for ordinary viscous diffusion. For viscous diffusion the time scale can be estimated from scale analysis of the diffusion equation five point forty three.So that for the above value of H and Km, the diffusion time scale is thus about 100 days. Hence, in the absence of convective clouds the spin-down process is a far more effective mechanism for destroying vorticity in a rotating atmosphere than eddy diffusion. [click]

Physically the spindown process in the atmospheric case is primarily the Coriolis force that balances the pressure gradient force away from the boundary. The Coriolis force for the outward-flowing fluid is directed clockwise, and this force thus exerts a torque opposite to the direction of the circulation of the vortex. In the case of boundary layer, viscosity is responsible for the presence of the secondary circulation. In a stably stratified baroclinic atmosphere, the buoyancy force will act to suppress vertical motion. So the interior secondary circulation will decrease with altitude, shown in Fig. five point eight. This flow will rather quickly spin down the vorticity at the top of Ekman layer that is enough to bring ζg to zero. The result is a baroclinic vortex with a vertical shear of the azimuthal velocity that is very strong. [click]This vertical shear of the geostrophic wind requires a radial temperature gradient that is in fact produced during the spin-down phase by adiabatic cooling of the air forced out of the Ekman layer.[click]

Finally, let us see my summary. By the physical constraints of the system, a secondary circulation will occur. It only occurs in rotating fluids. And in boundary layer, viscosity is responsible for the presence of it. The secondary circulation can spin down an atmosphere vortex. The e-folding time is referred to as the barotropic spin-down time. The secondary circulation in the baroclinic atmosphere serves two purposes: it changes the azimuthal velocity field of the vortex through the action of the Coriolis force and it changes the temperature distribution so that a thermal wind balance is always maintained between the vertical shear of the azimuthal velocity and the radial temperature gradient. That’s all.

Thanks four your attention, thank you!

 

第二篇：presentation(根据英语经典电影对白ppt的演讲稿2)(ppt在我的文辑里)

A：First of all,I want to thank my front teammates.You really told me many movie's classical dialogs and the character’s postive attitude to life.

B:Yeal,I agree with you.I learnt a lot too.And now,I want to introduce a movie which has a great influence on me for everyone.

A:What?

B:That is The Shawshank Redemption.It is about {AndyDefresne,a successful young banker who is sentenced to life imprisonment for the murder of his wife and her lover. Andy is sent to Shawshank prison. He make a friend named "Red" who has already spent many years inside. Over years in the prison, Andy gained the respect of the most of the other prisons and guards. More importantly, he never loses hope for the future---a hope he tries to pass on to the other prisoners.}

A: Enen,{Shawshank is a story about hope. It’s about justice. It’s about the struggle to find the meaning of life, to find redemption, and the triumph of the human spirit. This film teaches us, as has been conveniently turned into a tagline, that " Fear " can hold you prisoner, but hope can set you free” .}

B:Today I will perform a part of a scene in the movie.It show us although in the prison ,Andy never give up his hope.What about you play the role of Andy,and I Red?

A:Ok

{

ANDY :My wife used to say I‘m a hard man to know. Like a closed book（高深莫测的东西）. Complained about it all the time. (pause) She was beautiful. I loved her. But I guess I couldn’t show it enough. (softly) I killed her, Red. I didn‘t pull the trigger(枪). But I drove her away. That's why she died. Because of me, the way I am.

RED :That don‘t make you a murderer. Bad husband, maybe. Feel bad about it if you want. But you didn't pull the trigger.

ANDY :No. I didn't. Someone else did, and I wound up here. Bad luck, I guess.

RED :Bad luck? Jesus.

ANDY :It floats around. Has to land on somebody. It was my turn, that's all. I was in the path of the tornado. I just had no idea the storm would go on as long as it has. I just didn’t expect the storm would last as long as it has. Think you'll ever get out of here?

RED: Sure. When I got a long white beard(胡子) and about three marbles left rolling around upstairs（形容神志不清）

Andy：tell you where I’d go .Zihuatanejo.

}

A:In my opinion,movie is the life.It can affect human’s mind,set us thinking,jolt our menmory.

B:What’s more, The movie is also a language school, through watching

movie, we can expand us our Vocabulary , improve our spoken language B: So let us enter the palace of movie.That is all.Thank you.