WESTERLIES AND THE JET STREAM

Objectives:

At the planetary (global) scale in the middle and upper troposphere, the prevailing upper-air westerlies encircle middle latitudes in a wave-like pattern. These winds are important components of day-to-day weather in that they steer storm systems from one place to another and are ultimately responsible for the movement of air masses. Surveying the basic characteristics of these upper-air tropospheric westerlies is key to understanding the variability of midlatitude weather.

Relatively narrow “rivers” of strong winds, called jet streams, exist at middle and upper tropospheric levels at different times within the westerlies. Jet streams that occur over the polar front and near the tropopause have important influences on the weather of middle latitudes. These so-called polar-front jet streams exist where relatively cold air at higher latitudes comes in contact with warm air from lower latitudes. In addition, these jet streams provide upper-air support for the development of surface low-pressure systems.

After completing this investigation, you should be able to:

• Describe the wave patterns exhibited by the meandering upper-air westerlies.
• Determine the location of the polar-front jet stream on an upper-air weather map.
• Explain the general relationships between the jet stream in the upper-air westerlies and the paths air masses and storms take.
• Describe how atmospheric temperature patterns are associated with the upper-air circulation and the jet stream.

Introduction:

The upper-air westerlies flow generally from west-to-east around the planet in a wave-like pattern of ridges and troughs as shown below. Ridges are topographic crests and troughs are elongated depressions on constant-pressure surfaces. (Refer to Investigation 8B to review features on upper-air maps, including ridges and troughs.) In Figure 1 below, “H” locates a ridge and “L” locates troughs.

Figure 1. Wave pattern in upper-air westerlies.

1. The ridges of the Northern Hemisphere’s upper-air westerlies exhibit clockwise (anticyclonic) curvature as seen from above. As shown in Figure 1, a line can be drawn that divides a ridge into two roughly symmetrical sectors. The line is known as a ridge line. Note that west of the ridge line, winds are from the southwest (a warm weather direction) and east of the ridge line, winds are from the northwest (a cold weather direction). We can conclude that winds to the west of a ridge line favor [(warm)(cold)] air advection while winds to the east of a ridge line favor cold air advection.

2. The troughs of upper-air westerlies curve counterclockwise (cyclonic). As shown in Figure 1, a line can be drawn that divides a trough into two roughly symmetrical sectors. The line is known as a trough line. Note that west of the trough line, winds are from the northwest (a cold weather direction) and east of the trough line, winds are from the southwest (a warm weather direction). We conclude that winds to the west of a trough line favor [(warm)(cold)] air advection while winds to the east of a trough line favor warm air advection.

Ridges and troughs usually migrate from west to east over time so that as a ridge line shifts eastward, a location that had been experiencing cold air advection then experiences warm air advection, and a location that had been experiencing warm air advection then experiences cold air advection.

3. Upper-air winds steer low-pressure systems as well as air masses. A surface Low that is centered to the east of a trough line and west of a ridge line will be expected to move toward the [(southeast)(northeast)].

The wavy pattern of the upper-air westerlies consists of ridges alternating with troughs. The distance between successive ridge lines or, equivalently, between successive trough lines is the wavelength. At any one time, usually between 2 and 5 such waves encircle the Earth in the middle latitudes.

With time, the wave pattern of the upper-air westerlies changes. These changes may involve a change in the number of waves, the wavelength, or the amplitude of the wave. At one extreme, shown in Figure 2a below, upper-air westerlies blow almost directly from west to east with little sign of ridges or troughs. This westerly flow pattern is described as zonal. At the other extreme, shown in Figure 2b below, upper-air westerlies blow in huge north/south loops with high amplitude ridges and troughs. This westerly flow pattern is described as meridional. The circulation patterns displayed in Figure 2a and 2b are opposite extremes of many possible patterns commonly exhibited by middle latitude upper-air westerly waves.

Figure 2. (a) Upper-air wave pattern with little north-south variation (zonal), and (b) pattern with great north-south excursions (meridional).

4. When the upper-air westerly flow pattern is zonal, the source region for much of the air over the coterminous U.S. is the Pacific Ocean. On the other hand, when the upper-air westerly flow pattern is meridional, the source regions for air masses over the lower 48 states are Canada (where winds are from the northwest) or Mexico or the Gulf of Mexico (where winds are from the southwest). Hence, from west to east across the lower 48 states, temperatures are likely to be more variable with a [(meridional)(zonal)] flow pattern.

5. Fundamental to the formation of the polar-front jet stream within the westerlies is the physical property that warm air is less dense than cold air when both are at the same pressure. Air pressure drops [(more)(less)] rapidly with increasing altitude in cold air than in warm air.

The polar front marks the lower-atmosphere boundary between higher latitude cold air and lower latitude warm air. This temperature contrast extends from Earth’s surface up to the altitude of the polar-front jet stream. As demonstrated in Investigation 5B, the effect of temperature on air density means that the air pressure at any given altitude above the surface is higher in the warm air column than in the cold air column. Hence, a horizontal pressure gradient is directed across the front from the warm side toward the cold side. In response, the horizontal wind initially blows from warm air toward cold air but is soon deflected to the right by the Coriolis Effect. Consequently, the wind blows parallel to the polar front with the cold air to the left when facing in the direction towards which the air is flowing (in the Northern Hemisphere). Furthermore, where cold and warm air reside side by side, the magnitude of the horizontal pressure gradient increases with increasing altitude. This causes the horizontal wind to strengthen with altitude and reach its maximum speed in the polar-front jet stream.

6. In the Northern Hemisphere, when the polar-front jet stream is south of a locality, the weather at that location is relatively [(cold)(warm)].

7. As a component of the planetary-scale upper-air westerlies and similar to the winds at 500 mb, the polar-front jet stream steers low pressure systems. Hence, middle latitude storms generally move from [(west to east)(east to west)].

Examine Figure 3, the upper-air map for 250 mb at 12Z 20 NOV 2013 (average 250-mb height is about 10.4 km above sea-level). These maps are provided by NOAA’s Storm Prediction Center (SPC) archive. Dark blue shading areas enclose stations that have wind speeds of 75 knots or higher (triangular pennant and two long feathers) across the U.S. Within those shaded areas are medium blue shades for 100 kts or higher. Highlight the overall flow pattern by drawing a continuous dark, heavy, smooth, curved arrow across the map approximately through the middle of the discontinuous band of highest wind speeds across the southern U.S. Add an arrowhead to represent wind direction. The large arrow you drew on your map approximates the location of the polar-front jet stream across the coterminous 48 states and Canada at that time. (SPC uses 75 knots as the threshold of jet stream winds for ease of identification in shading.) The pattern of wind flow at this level on this date is basically west to east across the U.S. with the highest speeds in a band across the southern portion of the country. Another area of high speed flow is seen crossing southern Canada.

Figure 3. 250-mb map for 12Z 20 NOV 2013.

Examine Figure 4, the upper-air map for 250 mb at 00Z on 27 NOV 2013, about a week later. In addition to the shading noted above, within the medium blue shades, there is light blue for 125 kts or more and purple for 150 kts or greater. Using a pencil, again draw a continuous dark, heavy, smooth, curved arrow across the map through the middle of the band of highest wind speeds. Have your arrow continue from the band in south-central Canada and the northern U.S. Plains States to that of the eastern U.S. The wind flow pattern at this time was quite different from that of Figure 3 with varying directions and highest speeds in more north to south bands. At this time, there was a strong storm system along the East Coast.

Figure 4. 250-mb map for 00Z on 27 NOV 2013.

8. The pattern of winds in Figure 4 indicates a [(ridge)(trough)] over the central U.S. states. The flow pattern across the country is meridional.

9. On average, one would expect temperatures at similar latitudes to be similar. Zonal wind patterns, as in Figure 3, exhibit this relationship. However, given the wind pattern of Figure 4, at 00Z on 27 NOV 2013, surface air temperatures in the Midwest states of Minnesota and Iowa are likely to be [(lower)(higher)] than surface temperatures over New England.

10. Knowledge of the location of the jet stream and upper-air winds in general is very important for commercial aviation and can result in fuel savings and shorter flight times. At Figure 4 map time, an airline flight from New Orleans, Louisiana to Boston, Massachusetts would take [(less)(more)] time than a flight along the same route from Boston to New Orleans.

The past weekend saw a vigorous weather pattern develop that may be the harbinger of more active autumn storms. At the end of last week an intense storm system crossed the center of the country and was followed in the middle of the weekend by a storm that came ashore and affected the Pacific Northwest. Details of these storms’ passages are in the Monday, 4 November 2013, Daily Weather Summary.

Figure 5 is the surface weather map for 00Z 03 NOV 2013, Saturday evening. At map time both storm systems were visible, one with a low-pressure center marked off the New England coast and one along the Washington State-British Columbia border. Between the storm systems was an elongated area of high pressure stretching across the southern Plains States through Ontario Province, Canada.

Figure 5. Surface weather map for 00Z 03 NOV 2013.

11. Wind directions at station models near isobar lines across the country were generally seen to flow [(parallel to surface isobars)(at angles across isobars)].

12. Reported surface wind speeds across the coterminous U.S. were generally less than those at Spokane, Washington, which was three full feathers or [(10)(20)(30)] knots.

13. Figure 6 is the 500-mb constant-pressure map for Saturday evening, 00Z 03 NOV 2013, the same time as the Figure 5 surface map. The mid-tropospheric flow pattern shown by the contour lines on the 500-mb map, displayed [(a trough in the northwestern U.S.)(a ridge centered in the west-central U.S.)(a broad trough over the eastern U.S.)(all of these features)].

Figure 6. 500-mb constant-pressure map for 00Z 03 NOV 2013.

14. Recall that the heights at which the radiosondes detect 500-mb of pressure are reported in the upper right position of the upper-air station models on the map. Also, the heights are plotted in tens of meters, so that a “0” needs to be added to the digits for the actual height. The height of the 500-mb pressure level plotted at Denver, in north-central Colorado, was [(5320)(5510)(5750)] m above sea level.

15. Compare the heights of the 500-mb pressure surface from south to north on the map. As latitude increases (one moves poleward), the height of the 500-mb surface generally [(increases)(remains constant)(decreases)].

16. The highest wind speeds plotted on the 500-mb map were at Wallops Island, VA, Chatham, MA, and Yarmouth, Nova Scotia, where there were speeds of [(30)(60)(110)] kts generally from the southwest, much higher than surface wind speeds at the same locations.

17. Figure 7 is the 300-mb constant-pressure map for 00Z 03 NOV 2013, the same time as the surface and 500-mb maps. The 300-mb station model heights are also plotted in tens of meters. The heights of the undulating 300-mb pressure surface were within several hundred meters of [(5500)(9200)(12,500)] meters. The 300-mb level occurs in the upper troposphere.

Figure 7. 300-mb constant-pressure map for 00Z 03 NOV 2013.

18. The general 300-mb contour pattern at 00Z on 03 NOV exhibited an upper air flow with [(a trough in the northwestern U.S.)(a ridge centered in the west-central U.S.)(a broad trough over the eastern U.S.)(all of these features)].

19. Comparing the heights of the 300-mb pressure surface from south to north on the map, as latitude increases (one moves poleward), the height of the 300-mb surface generally [(increases)(remains constant)(decreases)]. This pattern is similar to that found at 500 mb.

20. From the 300-mb station models, the highest wind speed plotted on the Figure 7 map was at Caribou, Maine. The speed was about [(95)(155)(205)] knots.

21. Using an arbitrary wind speed threshold of 70 knots to define the existence of a jet stream, we can therefore conclude that there [(was)(was not)] evidence of a jet stream on the Figure 7, 300-mb constant-pressure map curving across the U.S.

22. Now compare the wind speeds plotted on the 00Z 03 NOV 2013, 500-mb constant-pressure map (Figure 6) with those of the corresponding areas on the Figure 7, 300-mb map. The comparison shows that wind speeds typically [(decrease)(remain the same)(increase)] as altitude increases (upward to lower pressures) in the troposphere.

23. As was seen at 500 mb, higher wind speeds at 300 mb are also located where the spacings of height contours are generally relatively [(far apart)(close together)]. Furthermore, the relationship is consistent with that on surface weather maps between wind speeds and the spacings of isobars.

24. Compare the 500-mb and 300-mb wind flows. The pattern of wind directions also shows that the winds generally flow [(“parallel” to the contour lines)(across contours at large angles)]. This directional relationship is particularly evident at higher wind speeds.

25. On the Figure 7, 300-mb map, mark the location of the low-pressure center off the New England coast with a bold “L”. As is typical with developing and rapidly moving storm systems, the center of the surface circulation was found generally to the [(east)(west)] of the upper air trough line. This “tilting” of the weather system results from the temperature profile in the air column indicating the location of the coldest air in the system. The relationship of the surface Washington State-British Columbia border low pressure center to the 300-mb trough line in the Northwest is similar but more closely aligned because this system is in its occluded stage.

The dashed brown (on-screen) lines on the 300-mb map are lines of equal wind speed, isotachs, generally drawn at 20-knot intervals for wind speeds of 30 knots and higher. An isotach surrounds an area where wind speeds of the isotach value or higher are found. For example, look for the highest wind speeds shown by the half circle of the 150-kt isotach in the northern tip of Maine at the map edge. Also look for other locally high speed areas: the 110-kts or greater oval over eastern Iowa and Illinois and the 90-kts or higher oval from eastern Oregon to western Montana. Shade these areas. Such high speed regions within the overall jet stream flow are called jet streaks. In this example, the eastern and western jet streaks are associated with the movement and development of those surface low-pressure centers.

Maps of upper atmospheric conditions provide information on the development and movement of broad-scale weather systems. In later investigations we will consider how the upper level conditions also influence the development of thunderstorms and severe weather.

Suggestions for further activities: You might try shading areas of highest wind speeds on 300-mb charts to identify jet streaks. Jet streaks are regions of accelerated wind speed along the axis of a jet stream. See if you can spot relationships between jet streaks and the location of associated surface low-pressure systems. Also, for developing storm systems in the central U.S., you might see if the positions of the low pressure/low height centers are successively more westerly with height (surface to 700 mb to 500 mb to 300 mb), as is expected with cold air being advected southward on the west side of storm centers. You might fit this with the earlier investigation of warm and cold air columns and their relationship to heights of pressure levels.