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مدیر وبلاگ : mehdi doostkamian
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Coriolis Force

The Coriolis force is an apparent force resulting from the fact that the earth is constantly rotating. The Coriolis force acts at a 90 degree angle to the right of the motion of a parcel of air (in the northern hemisphere and 90 degrees to the left in the southern hemisphere). This means that a northbound parcel (wind from the south) would have a coriolis force acting toward the east.

So why does the Earth's rotation matter?

If you were to move one parcel from Latitude 1 to Latitude 2 it would be effected by the Earth's rotation because it's distance from the center of rotation would be changing. Or in other words, the radius for the center of rotation would change. Therefore, its angular momentum would be changing. For our purposes the

Angular Momentum(اندازه حرکت زاویه ای) = Vradial + Vrelative

AM is conserved if the northbound parcel moves eastward or if a southbound parcel moves westward. The magnitude of the coriolis force is proportional to the speed of the wind. Coriolis force acts on all parcels that are in motion that are not at the equator. If you have a slow moving parcel you will have a small AM, VRelative will be small so AM will be small and the latitude displacement will not greatly effect your parcel.

The magnitude of the coriolis force is also dependent on the cosine of the latitude. Generally, the farther way from the equator you go, the stronger the coriolis force is.

Geostrophic Wind

So now we know about two important atmsopheric forces, the pressure gradien force and the Coriolis force. As it turns out, the magnitude of the PGF and Coriolis force are very similar. In fact, these two force commonly balance each other:

Geostrophic Balance is the balance between the pressure gradient force and the Coriolis force. So that the PGF is equal and opposit the Coriolis force. This balance will tell us the magnitude and direction of the Geostrophic Wind.

This figure is in the X-Y plane, along the 500mb surface.

Remember that the Coriolis force acts 90° to the left of the wind in the northern hemisphere.

The example above is looking at the 500 mb heights. How do we interpret the pressure gradient based on heights?

Well lets look at the specific points in which the forces are drawn on the 500mb surface. We see that the PGF is points to the left, why? The PGF is pointed from high to low pressure. Below the 500mb surface, the pressures are greater than 500m. Above the 500mb surface, pressure are less than 500mb. Therefore to get the PGF, we want to draw a horizontal arrow from high to low pressure just like the red arrow here.

Since the wind is in geostrophic balance, the Coriolis force will point in the exact opposit direction. Based on this balance Which way will the wind go? (into or out of the figure)

Frictional Force

The frictional force is dependent on the roughness of the surface, the height above the surface, and the speed of the wind.

The roughness of the surface plays a large role in friction. For example, the friction over the ocean will be less than the friction over the mountains.

As you go heigher in the atmosphere, friction has less of an effect. The surface (and therefore friction) only effects the lower levels of the atmosphere

The speed of the wind also determines how large or small the frictional force will be. Large winds - large effect from friction, small winds - small effect from friction

Friction is in the direction exactly opposit the direction of the wind.

Surface Wind

At the surface, the effects of friction are very important. Friction destroys geostrophic balance. As the speed of the wind increases from zero, friction begins to do it's work.

Since friction is directed opposite of the wind, it slows the wind. When it slows the wind, the magnitude of the Coriolis force is effected and the Coriolis force no longer balances the PGF. Remember, COR is always 90° right of the wind in the northern hemisphere. As a result the PGF is the dominate force driving the wind and the wind turns in the direction of the PGF. This allows the wind to cross the isobars toward low pressure.






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