Upper wind conditions

It is often observed that clouds at different levels move in different directions. The wind speeds at these levels may also differ markedly, although this is not so evident to the casual observer. The gradient of wind velocity with height is referred to as the (vertical) wind shear, and in the free air, above the friction level, the amount of shear depends upon the vertical temperature profile. This important relationship is illustrated in Figure 7.6. The diagram shows hypothetical contours of the 1000 and 500 mb pressure surfaces. As discussed in A.1 above, the thickness of the 1000 to 500 mb layer is proportional to its mean temperature: low thickness values correspond to cold air, high thickness values to warm air. This relationship is shown in Figure 7.1. The theoretical wind vector (VT) blowing parallel to the thickness lines, with a velocity proportional to their gradient, is termed the thermal wind. The geostrophic wind velocity at 500 mb (G500) is the vector sum of the 1000 mb geostrophic wind (G1000) and the thermal wind (VT), as shown in Figure 7.6.

The thermal wind component blows with cold air (low thickness) to the left in the northern hemisphere when viewed downwind; hence the poleward decrease of temperature in the troposphere is associated with

Contours of 1000-mb surface Figure 7 6 Schematic map of super

imposed contours of isobaric height

0 60 120 180

and thickness of the 1000 to 500-mb v ' \ ' \ ' \ ' layer (in metres). G|000 is the geostrophic

\ / \ / \ / \ i velocity at 1000 mb,G„nthat at 500 mb;

-^-nj-^-- 5640 VTis the resultant 'thermal wind' blowing v / \ / ^ ' * parallel to the thickness lines.

5760

' \ ' \ ' \ ' \ ' \ / > / \ ' \ / \ v \ \ / \ / x / \ * \

5700 5640 5580 5520

1000-500-mb thickness

Diy Box Joint Router Jig Plans

Figure 7.7 Structure of the mid-latitude frontal zone and associated jet stream showing generalized distribution of temperature, pressure and wind velocity.

Source: After Riley and Spolton (1981).

Figure 7.7 Structure of the mid-latitude frontal zone and associated jet stream showing generalized distribution of temperature, pressure and wind velocity.

Source: After Riley and Spolton (1981).

a large westerly component in the upper winds. Furthermore, the zonal westerlies are strongest when the meridional temperature gradient is at a maximum (winter in the northern hemisphere).

The total result of the above influences is that in both hemispheres the mean upper geostrophic winds are dominantly westerly between the subtropical high-pressure cells (centred aloft at about 15° latitude) and the polar low-pressure centre aloft. Between the subtropical high-pressure cells and the equator the winds are easterly. The dominant westerly circulation reaches maximum speeds of 45 to 65 m s-1, which even increase to 135 m s-1 in winter. These maximum speeds are concentrated in a narrow band, often situated at about 30° latitude between 9000 and 15000 m, called the jet stream (see Note 2 and Box 7.2). Plate 14 shows bands of cirrus cloud that may have been related to jet-stream systems.

The jet stream is essentially a fast-moving ribbon of air, connected with the zone of maximum slope, folding or fragmentation of the tropopause; this in turn coincides with the latitude of maximum poleward temperature gradient, or frontal zone, shown schematically in Figure 7.7. The thermal wind, as described above, is a major component of the jet stream, but the basic reason for the concentration of the meridional temperature gradient in a narrow zone (or zones) is dynamical. In essence, the temperature gradient becomes accentuated when the upper wind pattern is confluent (see Chapter 6B.1).

Figure 7.8 shows a north-south cross-section with three westerly jet streams in the northern hemisphere. The more northerly ones, termed the polar front and

Continue reading here: The Discovery Of Jet Streams

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