Coupled Jet Streams

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The synoptic-scale set-up for major outbreaks of tornadoes often includes speed maxima in upper-tropospheric and low-level jet streams. In this context, the upper-tropospheric jet stream refers to either the mid-latitude jet stream or the "subtropical jet stream," which is simply a high-altitude band of relatively strong winds located around 30 degrees latitude (which you will study, or may have already studied, in METEO 241).

Lest you get the impression that the intrusion of a newly formed low-level jet stream beneath the left-exit region of an upper-level jet streak is somehow just a huge coincidence, you're about to learn that speed maxima (jet streaks) traveling in the upper-level jet stream can encourage the formation of a low-level jet stream. In such situations, meteorologists say that the upper-tropospheric and low-level jet streams are coupled.

The development of a low-level jet stream in the vicinity of an upper-level jet streak can be a big deal. That's because the presence of a low-level jet stream increases the vertical wind shear in the lowest kilometer of the troposphere. Research has shown that such strong, low-level vertical wind shear heightens the risk of tornadogenesis (assuming the environment is favorable for supercells to erupt), so there's a pretty good big-picture reason for studying the coupling of upper- and low-level jet streams. Let's investigate.

Meteorology of Coupled Jet Streams

The 500-mb height analysis and associated wind barbs at 12Z on November 7, 2004.

Credit: Penn State University

To understand the concept of coupled upper-level and low-level jet streams, I have to first introduce a new concept: the "ageostrophic wind". As you learned previously, the overall wind flow on the synoptic-scale in the middle and upper troposphere tends to be geostrophic, which means that the pressure-gradient force and Coriolis force are balanced. In this balanced (and idealized) state, the wind blows parallel to local height lines. As you can plainly see on this 500-mb image of the northeastern quarter of the nation at 12Z on November 7, 2004 (on the right), the wind seems to be nearly everywhere parallel to the height contours. I emphasize the word "nearly' because the wind is never perfectly geostrophic. Relatively small imbalances between the pressure-gradient and Coriolis forces result in small accelerations characteristic of most synoptic-scale weather systems.

As original course author, Lee Grenci, likes to say, "You can't fly a kite in the geostrophic wind." The geostrophic wind isn't real--it's idealized. The observed wind always departs from the geostrophic wind, by at least a little bit. This departure from the geostrophic wind is the ageostrophic wind. Thus, we can express the "total wind" (the observed wind) as a vector sum of the geostrophic and ageostrophic components, and believe it or not, you've seen the ageostrophic wind in action before. Jet streaks are a perfect example. The ageostrophic wind is the basis for the divergence / convergence patterns associated with the four-quadrant model of straight jet streaks.

To see what I mean, let's start by assuming that an air parcel is pretty darn close to geostrophic balance as it approaches a jet streak. As it enters the jet streak (remember that air parcels move through jet streaks), the parcel finds itself "subgeostrophic." That's because the parcel's speed, and thus Coriolis force acting on it, no longer match the greater pressure-gradient force (height-gradient force) that is the hallmark of jet streaks. Given this imbalance of forces, the parcel accelerates northward (essentially "downhill") to lower 300-mb heights. At this point, the parcel's velocity has two components, the geostrophic component, which blows from the west, and the ageostrophic component, which blows from the south (see image on the left below). In this region of the jet streak, the northward ageostrophic component of the wind produces the classic mass convergence in the left entrance of a straight jet streak, and divergence in the right entrance.

(Left) For a parcel in the entrance region of a west-to-east, straight jet streak, the ageostrophic wind blows from the south (perpendicular to the major axis of the jet streak). (Right) For a parcel in the exit region of a west-to-east, straight jet streak, the ageostrophic wind blows from the north (perpendicular to the major axis of the jet streak).

Credit: David Babb

As the parcel moves toward the core of the jet streak, the ageostrophic wind increases (the parcel continues to accelerate in response to the increasing height gradient). Upon reaching the core of the jet streak, the parcel attains a state of fleeting geostrophy (the parcel's speed and, thus, the Coriolis force acting on it finally are able to offset the height-gradient force). The key word here is "fleeting," because after leaving the core, the parcel quickly finds itself "supergeostrophic." That's because the parcel's breakneck eastward speed now exceeds the geostrophic threshold dictated by the now slightly weaker height-gradient force (the magnitude of the Coriolis force exceeds the magnitude of the height-gradient force). Again, given this imbalance of forces, there must be an acceleration. Indeed, the parcel slows and swerves southward (essentially "uphill") to higher heights. Now the ageostrophic wind blows from the north (see the image on the right above), paving the way for divergence in the left exit and convergence in the right exit.

The key takeaway here is that that the observed wind always has two components: geostrophic and ageostrophic. The geostrophic component is the idealized component (from a "perfect world" that lacks horizontal accelerations), while the ageostrophic component (although sometimes very, very small), accounts for real-life departures from the state of geostrophy. Now with this background out of the way, we can better understand how jet streams can become coupled.

Focus your attention on the idealized schematic below, which shows a 300-mb jet streak (the thin, green lines are 300-mb isotachs). For each wind vector within the jet streak, there is a geostrophic component and an ageostrophic component. The black streamlines pointing southward from the left-exit region indicate the ageostrophic components of the 300-mb winds in the exit region of the jet streak. It's important for you to keep in mind that in reality, the ageostrophic components are small compared to the geostrophic components. On a 300-mb chart, you would only observe westerly winds (not northerly winds) in this scenario, but on very close inspection, these westerly winds would have a slight southward deviation. This slight southward swerve would be the footprint of the ageostrophic wind. The bottom line here is that you would never observe streamlines like the black ones below, on a standard 300-mb analysis. For sake of this presentation, we artificially removed the geostrophic components so you can better see the otherwise very subtle contributions of the ageostrophic wind.

An idealized schematic showing the transverse circulation (black arrows) in the exit region of a 300-mb jet streak. In response to low-level pressure falls beneath the area of upper-level divergence in the left-exit region, a southerly ageostrophic component develops. In turn, the associated horizontal acceleration can help to generate a low-level jet stream (thick orange arrow). In this context, the upper-level and low-level jet streams are coupled.

Credit: David Babb

Any way you slice it, the ageostrophic flow of air in the exit region produces upper-level divergence in the left-exit region. In response, a region of negative pressure tendencies develops in the lower troposphere beneath the area of upper-level divergence. In turn, this pocket of pressure falls causes low-level southerly winds to accelerate, which often paves the way for a low-level jet stream.

With the idea of coupled jet streams in mind, a low-level, southeasterly jet stream can develop over California during winter in concert with an arriving 300-mb jet streak. Such a low-level jet stream rapidly transports moisture northward and increases the low-level vertical wind shear (and thus heightens the risk of California tornadoes). Yes, tornadoes form in California, mostly in the wintertime. Let's investigate.

Severe Weather in California

The Storm Prediction Center occasionally issues severe-thunderstorm and tornado watches during the cold season for California's Central Valley when conditions are favorable for supercells to form. As a general rule, California supercells erupt behind cold fronts associated with strong, occluded mid-latitude cyclones. In such situations, mid-level lapse rates steepen as the trailing, cold 500-mb low starts to move inland. Vertical wind shear in the lowest six kilometers increases, and a strong 300-mb jet streak typically induces a low-level jet stream. The schematic below shows the classic synoptic set-up for severe thunderstorms in the Central Valley during the cold season.

The synoptic set-up for an outbreak of severe thunderstorms in California during the cold season. A 300-mb jet streak approaching California is coupled with a low-level jet stream over the Central Valley.

Credit: David Babb

Let's examine this pattern favorable for severe weather more closely. When a strong low-pressure system approaches the California Coast during winter, the cold front can sweep inland relatively far east of the longitude of the surface low. The west-southwesterly flow behind the advancing cold front then paves the way for a lee trough to form in California's Central Valley. Meanwhile, with the surface low still lingering offshore (northwest of San Francisco, for example), California's Central Valley channels the low-level flow of air, causing the expected southwesterly, post-frontal winds to blow from the southeast instead. These south-easterlies can become a bona fide low-level jet stream in response to upper-level divergence in the left-exit region of a 300-mb jet streak. Meanwhile, the trailing 500-mb trough often deepens (intensifies), producing robust southwesterly or westerly winds in the middle troposphere (which, of course, enhances vertical wind shear in the first six kilometers of the troposphere).

Such patterns are conducive to small supercells erupting in the Central Valley. The bottom line here is that most of California's infrequent but recognizable regional severe weather events typically occur in concert with low-level southeasterly (and post frontal) winds during the cold season. But, there's a slightly different "twist" to this general pattern whenever cold-season tornadoes develop in the the Los Angeles metropolitan area. Tornadoes in Los Angeles? No, this isn't the set up for a horribly cheesy sci-fi movie. On occasion, tornadoes really do happen near Los Angeles! To see what I mean, check out the Case Study below.