|MadSci Network: Earth Sciences|
The analogy of a squirting garden hose isn't bad, but it isn't perfect either.
I will begin my discussion by discussing a simpler problem, that of water flowing along in a channel which encounters a bump in the bottom of the channel. I'll then use ideas from this picture to discuss what happens in the atmosphere. In everything that I'll describe, I'm assuming a steady flow: that is, the velocity at every point is constant in time.
Consider a smooth flow of water from left to right with velocity U through a shallow channel whose depth is h. Let h vary with distance x along the channel -- specifically, let the depth be uniform everywhere except for a hill at x=0. What happens when the water goes over the bump? I won't show you the mathematics, but there are two possibilities:
The key difference between the two cases is that for subcritical flow, a surface wave can travel forward ahead of the bump to "warn" the water that the bump is coming. But for supercritical flow, the wave can't go upstream faster than it's carried downstream by the current, so the water hits the bump without "warning" and piles up there.
Putting your thumb over a garden hose is a good example of a subcritical flow: the pressure in the hose is higher than the pressure right where your thumb is, so the water accelerates through the constriction and out of the hose. The ratio of wave speed to flow speed is still important, although this time, the waves in question are sound waves travelling through the water in the hose. Supercritical flows in this case are supersonic. An airplane travelling at supersonic speed through the air piles up air in front of it just as the bump piles up water in the figure above.
The parallel between supersonic air flows and supercritical flows over bumps in a channel goes even farther. Have you ever watched water flow over a rock in a stream, and seen it form a foaming, turbulent wave that hangs behind the rock? That's a sudden transition between supercritical and subcritical flow, which is similar to the sudden transition between supersonic and subsonic flow which occurs in the shock wave of a supersonic airplane!
Anyway, on to the second example, which should answer your question. Now we consider flows of air over mountains. While the atmosphere doesn't have an upper surface like a water-filled channel does, it does have layers of heavier and lighter air. If you took air from high in the atmosphere down to ground level in a rubber balloon, you would find it was much lighter than the ground-level air. It acts a lot like a mixture of oil and water: if you play with a bottle of oil-and-vinegar salad dressing, you can make waves which slosh around on the interface between the two fluids. These are much like water waves, but move more slowly. The atmosphere has similar "internal waves", but the tricky bit is that there are lots of layers (we call it a "continuously stratified fluid").
So, since the atmosphere has "internal waves" which act a lot like water
waves, we might expect the behavior to change greatly depending on whether
the wind speed is subcritical or supercritical with respect to the internal
waves. And, indeed, it does. My second figure shows a sketch of the
mathematical result for a series of sinusoidal hills on the ground (which
is the simplest case to handle.)
The blue lines are "streamlines", which show the path of individual particles within the fluid. When streamlines are closer together, the flow speed is higher. The letters "H" and "L" correspond to high and low atmospheric pressure. In the top panel, we see what happens when the wind speed is very high. We get low pressure and accelerated flow over the tops of the hills. In this case, the atmosphere's response to the hill's presence dies out as we go up. As a result, flow speed must be higher over the hils, and so there must be low pressure at the hillcrests to accelerate the winds. The bottom panel shows what happens when the wind is slower -- the flow is subcritical. Internal waves are able to propagate upstream and vertically upwards, carrying the pressure signal forward -- you can see them in the figure. Now, wind speeds are largest (streamlines are closest together) down the backside of the hills, and the air pressure is lowest there.
Whether you get supercritical or subcritical flow in this situation depends on the value of (2 pi U/NL), where U is the wind speed, L is the wavelength of the hill, and N is a measure of how rapidly the air changes density with height. If this value is greater than one, the flow is supercritical; if it is less than one, the flow is subcritical. A typical value of N is 0.01 1/seconds. So if the wind speed is 10 meters per second, flow over a hill 1 kilometer across would be supercritical (and thus look like the upper panel) while flow over a mountain 10 kilometers across would be subcritical (and thus look like the lower panel).
So you see that the pinched-hose analogy is both helpful and misleading. In all cases, the pressure is lowest where the velocity is highest. In the pinched hose and in the supercritical atmospheric flow, the flow accelerates because it must pass the same volume per unit time through a smaller aperture, but the pinched hose is subcritical, unlike the atmospheric case it resembles. In the garden hose, the flow has low pressure at the location of maximum constriction, while in the atmosphere the maximum velocity for subcritical flows (which are very common) is found behind the hills rather than on top of them, and the pressure is lowest behind the hills.
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