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Polar-front Jet Stream Relationships

Researchers have associated many meteoro-logical features with the jet streams of the mid-latitudes in order to further our under-standing of these high-speed winds and their effect on weather. Obviously, they’re related to polar fronts, but they are also related to migratory low-pressure systems, various weather conditions, the tropopause, clouds, and turbulence. Lets begin with the jet’s relationship to fronts.

FRONTAL RELATIONSHIP.— The jet stream’s location relative to its front is dependent on the height the front attains as it slopes up from the surface.

1. If the front reaches (intersects) the 500-mb level over a station, the jet core lies almost directly above the station.

2. If the front fails to reach the 500-mb level over a station, the jet core lies north of the station.

3, If the front intersects a level above the 500-mb level over a station, the jet core lies south of the station.

Frontal intersections with constant-pressure levels show up as isotherm packing at each level the front intersects. If more than one zone of packing exists on a chart, more than one front extends to that level. The strongest jet is the one associated with the zone of greatest packing (strongest temperature gradient) at the 500-mb level or below. This packing may be as great as 10°C in 45 miles, but usually ranges between 10°C in 90 miles and 10°C in 150 miles. The packing of isotherms is weaker through ridges and stronger through troughs, and the width of the packing, as represented on the 500-mb chart, equates to the width of the jet core above it.

POLAR FRONT AND DEVELOPING LOWS.— A polar-front jet stream is associated with surface low-pressure systems in a very simple way. As a surface frontal wave develops, a surface low is formed. The jet moves south-ward, pushing cold air into the west side of the low, and the low intensifies. When the polar front occludes, the jet moves south of the low center and crosses the polar front at the apex (triple point) of the occlusion. In other words, it parallels the polar front and remains north of the migratory surface lows until after the polar front occludes. The following is a list of jet stream relationships to fronts and surface lows:

1. The jet stream remains north of un-occluded lows.

2. In a series of migratory lows (cyclone family), each low is associated with a jet maximum. But remember, every jet maximum is not necessarily associated with a surface low.

The two most common positions for these lows and their jet maxima are given in figure 8-3-9. NOTE: As the upstream low deepens, a jet maximum develops along the axis to the west of the low. This cycle of development continues with each developing frontal wave.

Figure 8-3-9.—Usual position of surface lows in relation to moving jet maxima.

3. The jet stream parallels the warm-sector isobars of surface lows.

4. The jet stream is found south of occluded lows, near the point of occlusion.

5. The jet stream is perpendicular to occlu-sions.

6. The jet stream roughly parallels the isobars south of a cold, slow-moving surface low.

HIGHS.— The polar-front jet is also related to surface high-pressure systems.

1. The jet stream roughly parallels the isobars north of a warm, slow-moving high.

2. When a cold, migratory polar high stag-nates and begins to warm up, the jet stream configuration upstream can change drastically. If a cold-core high becomes warm-cored, the original jet dissipates and a new jet forms to the north.

WEATHER.— The weather associated with polar-front jet streams varies. However, bad weather is more often associated with them than good, especially in winter. Extensive bad weather is normally found between the surface warm front of a developing or mature surface low and the jet axis to the north. Also, when a cold front has a very shallow slope and the jet is positioned well back in the cold air (usually 600 miles), the cold front will be overrun by a warm southwest flow of air. This overrunning produces extensive cloudiness and continuous precipitation. The occurrence of precipitation associated with the jet stream is controlled primarily by the distribution of wind shear and curvature along the jet stream.

There is general agreement that the highest incidence of precipitation almost straddles the jet axis, with a slight bias toward the cold-air side. Severe frontal thunderstorms are also thought to be related to the jet stream. These thunder-storms form in regions having strong vertical wind shears, which are commonly found beneath a jet. The energy of the jet is transferred from the large-scale upper-level circulation to the smaller scale cyclonic circulation at the surface (the surface low). From here, the energy is transferred to an even smaller scale circulation, the thunder-storm. If a tornado was to develop from the thunderstorm, the energy transfer would have been carried one step further. This type of energy transfer is known as CONSERVATION OF ANGULAR MOMENTUM.

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