Wx Watch: Zooming through jet cores

How high-speed, high-altitude winds create lows at the surface

March 1, 2011

Imagine you’re a lecturer in Aviation Weather 101. Want to know a dangerous topic? The Coriolis effect. Explanations are often ignored because to many,
it’s a rarified concept that deserves attention only with regard to passing a test. But the Coriolis force plays a big part in generating the atmospheric motions that direct upper-air flows, create storm systems, and even affect ocean currents. All pilots should understand weather, and you can’t do that until you understand Coriolis force.

How it works

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We’ve all learned that air flows from high to low pressure. But without Coriolis force air would rush directly to low pressure. If you could see the air, it would look like the spokes of a wheel. But because of the Coriolis force, air moving to low pressure is deflected to the right. Yes, air moving to low pressure will eventually get there (and produce rising motions that create clouds, storms, and other adverse weather), but it will curve around the low as it does—all because of Coriolis force. There’s evidence of this on surface analysis charts, in the form of the isobars (lines of equal atmospheric pressure) that encircle low- and high-pressure centers.

At the heart of the matter is the battle between two strong forces. One—the pressure gradient force—is the force that defines the difference in air pressure. This is the force that would like to draw air directly to a low pressure center. But the Coriolis force prevents this by deflecting air to the right. So the two forces are always at work whenever air moves.

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The faster the wind speed, the stronger the Coriolis force. In the surface boundary layer (say, below 5,000 feet agl) friction with terrain slows airflows, and so the pressure gradient force is stronger. The Coriolis force is still present, which explains why the wind barbs on a surface chart show air moving to low pressure at an angle, but it’s the pressure gradient force that’s mainly in control. Aloft, as depicted on constant pressure charts, surface friction isn’t a factor, air pressures are reduced, and wind speeds are faster—because both pressure gradient and Coriolis forces are greater.

Coriolis force is zero at the equator, because this is the latitude that’s farthest from the center of the Earth. Tropical waves and hurricanes won’t form south of approximately 15 degrees of latitude, because the nearer the equator, the more straight-line the winds. At higher latitudes, where the Earth’s axis is closer to the surface, the Earth has more power to impart greater rotation (“spin,” or vorticity) to the atmosphere. At the poles, Coriolis force is at its maximum.

Flow chart

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Do pressure gradient and Coriolis forces ever balance each other out? You bet. This happens when isobars or height contours run parallel to each other. The resultant wind runs parallel to those isobars or height contours. That’s what meteorologists call a geostrophic flow.

Is there a way that air parcels can ever cross those parallel lines? Yes again. This is called an ageostrophic flow, and though this fact may never reach the pages of most aviation weather texts, it’s an important, dynamic element of activity in the cores of the strongest winds at high altitudes. Any time you see a core of strong jet-stream winds on a constant pressure chart, you can bet that there are ageostrophic flows at work.

Motions in jet cores

A quick look at the 300-millibar (which corresponds to approximately 30,000 feet msl) constant pressure chart will immediately show any large-scale low-pressure troughs or high-pressure ridges in the upper atmosphere. It’s the troughs aloft that concern us here.

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Troughs aloft are southward projections of cold air. The cold air will run into warmer air to the south, creating a maximum differential in atmospheric thicknesses (cold air is denser than warm, so the height contours on constant pressure charts are lower in colder air, and higher in warmer air). This translates into tight spacing between height contours, which in turn means the strongest winds aloft. These strong jet-core winds are typically downstream from the trough’s apex.

Coriolis force is strongest in strong winds, and weaker when wind speeds drop. That’s important to remember as we follow an imaginary air parcel as it makes its way west to east, around the trough’s southernmost projection, and on to the northeast. To the west, upon entering the jet-core maximum winds, height contours become suddenly compressed. For a time, the pressure gradient force is stronger than the Coriolis force—and air parcels are drawn to the north, toward the area of low pressure. It’s an ageostrophic motion because these slugs of air cross the orientation of the height contours.

Once in the middle of the jet core, the Coriolis force has a chance to reassert itself, balance with the pressure gradient force, and—for a time—the parcel flow is geostrophic. But not for long. Leaving the jet core, the height contours fan out a bit, meaning that wind speeds decrease. Now it’s time for the Coriolis force to have its say, drawing air parcels exiting the jet core to the right. This counts as another ageostrophic flow.

Vertical motions, lows, and fronts

As air parcels enter and exit a jet core, they create zones of convergence and divergence aloft. To help illustrate this, we use what’s called the four-cell jet max concept. This concept divides jet cores into quadrants, and then traces the paths of the ageostrophic flows. What we’re looking for are zones where air is diverging—because if it’s diverging aloft, it’s converging near the surface, creating the lifting motions that generate low pressure, fronts, clouds, and maybe even severe thunderstorms. And that’s the connection between jet cores and adverse weather.

Viewed from above, the four-cell quadrant specifies that “left” is the pole-ward direction, “right” is equator-ward, “rear” is upstream of the jet core, and “front” is downstream.

Which jet quadrants have divergent air? The right rear and left front. Those are the zones where the height contours fan out slightly, where the pressure gradient and Coriolis forces are momentarily out of balance, and where rising air fills the gap created by this imbalance. On the other hand, the left rear and right front zones are areas of convergent air aloft—and divergent air below. This sinking, divergent air is associated with higher atmospheric pressures and more benign weather.

Small wonder, then, that meteorologists pay close attention to 300-millibar constant pressure charts. If there’s a mighty jet core, then a surface low can be expected to form or intensify beneath the left rear quadrant.

Still awake out there? Good, because you’ve just learned some central facts about low-pressure formation, learned how to interpret the meaning of jet cores on constant pressure charts, and may even be able to predict where the lousiest weather will crop up—well before the sigmets begin to rear their heads.

E-mail the author at tom.horne@aopa.org.