Zones Of Wave Development And Frontogenesis

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Fronts and associated depressions tend to develop in well-defined areas. The major zones of frontal-wave development are areas that are most frequently baro-clinic as a result of airstream confluence (Figure 9.18). This is the case off East Asia and eastern North America, especially in winter, when there is a sharp temperature gradient between the snow-covered land and warm offshore currents. These zones are referred to as the Pacific polar and Atlantic polar fronts, respectively (Figure 9.19). Their position is quite variable, but they are displaced equatorward in winter, when the Atlantic frontal zone may extend into the Gulf of Mexico. Here there is convergence of airmasses of different stability between adjacent subtropical high-pressure cells. Depressions developing here commonly move northeastward, sometimes following or amalgamating with others of the northern part of the polar front proper or of the Canadian Arctic front. Frontal frequency remains high across the North Atlantic, but it decreases eastward in the North Pacific, perhaps owing to a less pronounced gradient of sea-surface temperature. Frontal activity is most common in the central North Pacific when the subtropical high is split into two cells with converging airflows between them.

Another section of the polar front, often referred to as the Mediterranean front, is located over the Mediterranean-Caspian Sea areas in winter. At intervals, fresh Atlantic mP air, or cool cP air from southeast Europe, converges with warmer airmasses of North African origin over the Mediterranean Basin and

Airmasses Europe

Figure 9.18 Mean pressure (mb) and surface winds for the world in January and July. The major frontal and convergence zones are shown as follows: intertropical convergence zone (ITCZ), South Pacific convergence zone (SPCZ), monsoon trough (MT), Zaire air boundary (ZAB), Mediterranean front (MF), northern and southern hemisphere polar fronts (PF), Arctic fronts (AF) and Antarctic fronts (AAF).

Figure 9.18 Mean pressure (mb) and surface winds for the world in January and July. The major frontal and convergence zones are shown as follows: intertropical convergence zone (ITCZ), South Pacific convergence zone (SPCZ), monsoon trough (MT), Zaire air boundary (ZAB), Mediterranean front (MF), northern and southern hemisphere polar fronts (PF), Arctic fronts (AF) and Antarctic fronts (AAF).

Source: Partly from Liljequist (1970).

Mediterranean Polar Front

initiates frontogenesis. In summer, the Azores subtropical anticyclone influences the area, and the frontal zone is absent.

The summer locations of the polar front over the western Atlantic and Pacific are some 10° further north than in winter (see Figure 9.19), although the summer frontal zone is rather weak. There is a frontal zone over Eurasia and a corresponding one over middle North America. These reflect the general meridional temperature gradient and also the large-scale influence of orography on the general circulation (see G, this chapter).

In the southern hemisphere, the polar front is on average about 45°S in January (summer), with branches spiralling poleward towards it from about 32 °S off eastern South America and from 30°S, 150°W in the South Pacific (Figure 9.20). In July (winter), there are two polar frontal zones spiralling towards Antarctica from about 20°S; one starts over South America and the other at 170°W. They terminate some 4 to 5° latitude further poleward than in summer. It is noteworthy that the southern hemisphere has more cyclonic activity in summer than does the northern hemisphere in its summer. This appears to be related to the seasonal strengthening of the meridional temperature gradient (see p. 133).

The second major frontal zone is the Arctic front, associated with the snow and ice margins of high latitudes (see Figure 9.19). In summer, this zone is developed at the land-sea boundary in Siberia and North America. In winter over North America, it is formed between cA (or cP) air and Pacific maritime air modified by crossing the coast ranges and the Rocky Mountains (see Plate 18). There is also a less pronounced Arctic frontal zone in the North Atlantic-Norwegian Sea area, extending along the Siberian coast. A similar weak frontal zone is found in winter in the southern hemisphere. It is located at 65 to 70°S near the edge of the Antarctic pack-ice in the Pacific sector (see Figure 9.20), although few cyclones form there. Zones of airstream confluence in the southern hemisphere (cf. Figures 9.2B and 9.4B) are fewer and more persistent, particularly in coastal regions, than in the northern hemisphere.

The principal tracks of depressions in the northern hemisphere in January are shown in Figure 9.21. The major tracks reflect the primary frontal zones discussed above. In summer, the Mediterranean route is absent and lows move across Siberia; the other tracks are similar, although more zonal and located in higher latitudes (around 60°N).

Between the two hemispherical belts of subtropical high pressure there is a further major convergence zone, the intertropical convergence zone (ITCZ). This was formerly designated as the intertropical front (ITF), but airmass contrasts are not typical. The ITCZ moves seasonally away from the equator, as the subtropical high-pressure cell activity alternates in opposite hemispheres. The contrast between the converging airmasses

Itcz Full Form

180'

Figure 9.20 The major southern hemisphere frontal zones in winter (Wi) and summer (Su).

180'

Figure 9.20 The major southern hemisphere frontal zones in winter (Wi) and summer (Su).

Subtropical High Pressure Cell

Figure 9.21 The principal northern hemisphere depression tracks in January. The full lines show major tracks, the dashed lines secondary tracks that are less frequent and less well defined. The frequency of lows is a local maximum where arrowheads end. An area of frequent cyclogenesis is indicated where a secondary track changes to a primary track or where two secondary tracks merge to form a primary.

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Figure 9.21 The principal northern hemisphere depression tracks in January. The full lines show major tracks, the dashed lines secondary tracks that are less frequent and less well defined. The frequency of lows is a local maximum where arrowheads end. An area of frequent cyclogenesis is indicated where a secondary track changes to a primary track or where two secondary tracks merge to form a primary.

Source: After Klein (1957).

obviously increases with the distance of the ITCZ from the equator, and the degree of difference in their characteristics is associated with considerable variation in weather activity along the convergence zone. Activity is most intense in June to July over South Asia and West Africa, when the contrast between the humid maritime and dry continental airmasses is at a maximum. In these sectors, the term intertropical front is applicable, although this does not imply that it behaves like a mid-latitude frontal zone. The nature and significance of the ITCZ are discussed in Chapter 11.

G SURFACE/UPPER-AIR RELATIONSHIPS AND THE FORMATION OF FRONTAL CYCLONES

We have noted that a wave depression is associated with airmass convergence, yet the barometric pressure at the centre of the low may decrease by 10 to 20 mb in twelve to twenty-four hours as the system intensifies. This is possible because upper-air divergence removes rising air more quickly than convergence at lower levels replaces it (see Figure 6.7). The superimposition of a region of upper divergence over a frontal zone is the prime motivating force of cyclogenesis (i.e. depression formation).

The long (or Rossby) waves in the middle and upper troposphere, discussed in Chapter 7A.2, are particularly important in this respect. The latitudinal circumference limits the circumpolar westerly flow to between three and six major Rossby waves, and these affect the formation and movement of surface depressions. Two primary stationary waves tend to be located about 70°W and 150°E in response to the influence on the atmospheric circulation of orographic barriers, such as the Rocky Mountains and the Tibetan plateau, and of heat sources. On the eastern limb of troughs in the upper westerlies of the northern hemisphere the flow is normally divergent, since the gradient wind is subgeostrophic in the trough but supergeostrophic in the ridge (see Chapter 6A.4). Thus, the sector ahead of an upper trough is a very favourable location for a surface depression to form or deepen (see Figure 9.22). It will be noted that the mean upper troughs are significantly positioned just west of the Atlantic and Pacific polar front zones in winter.

With these ideas in mind, we can examine the three-dimensional nature of depression development and the links existing between upper and lower tropospheric

Frontogenesis Cyclones

Figure 9.22 Schematic representation of the relationship between surface pressure (H and L), airflow and frontal systems, on the one hand, and the location of troughs and ridges in the Rossby waves at the 300-mb level. The locations of maximum (cyclonic) and minimum (anticyclonic) relative vorticity are shown, as are those of negative (anticyclonic) and positive (cyclonic) vorticity advection.

Sources: Mostly after Musk (1988), with additions from Uccellini (1990).

Figure 9.22 Schematic representation of the relationship between surface pressure (H and L), airflow and frontal systems, on the one hand, and the location of troughs and ridges in the Rossby waves at the 300-mb level. The locations of maximum (cyclonic) and minimum (anticyclonic) relative vorticity are shown, as are those of negative (anticyclonic) and positive (cyclonic) vorticity advection.

Sources: Mostly after Musk (1988), with additions from Uccellini (1990).

Advection Potential Absolute Vorticity

flow. The basic theory relates to the vorticity equation, which states that, for frictionless horizontal motion, the rate of change of the vertical component of absolute vorticity (dQ/dt or df + Z)/dt) is proportional to airmass convergence (-D, i.e. negative divergence):

dQ 1 dQ

dt Q dt

The relationship implies that a converging (diverging) air column has increasing (decreasing) absolute vor-ticity. The conservation of vorticity equation, discussed above, is in fact a special case of this relationship.

In the sector ahead of an upper trough, the decreasing cyclonic vorticity causes divergence (i.e. D positive), since the change in Z outweighs that in f, thereby favouring surface convergence and low-level cyclonic vorticity (see Figure 9.23). Once the surface cyclonic circulation has become established, vorticity production is increased through the effects of thermal advection. Poleward transport of warm air in the warm sector and the eastward advance of the cold upper trough act to sharpen the baroclinic zone, strengthening the upper jet stream through the thermal wind mechanism (see p. 131). The vertical relationship between jet stream and front has already been shown (see Figure 7.8); a model depression sequence is demonstrated in Figure 9.23. The actual relationship may depart from this idealized case, although the jet is commonly located in the cold air (Plate 18). Velocity maxima (core zones) occur along the jet stream and the distribution of vertical motion upstream and downstream of these cores is known to be quite different. In the area of the jet entrance (i.e.

Sept Jet Stream Location

Figure 9.24 The relations between surface fronts and isobars, surface precipitation (< 25 mm vertical hatching; >25 mm cross-hatching), and jet streams (wind speeds in excess of about 45 m s-1 shown by stipple) over the United States on 20 September 1958 and 21 September 1958. This illustrates how the surface precipitation area is related more to the position of the jets than to that of the surface fronts. The air over the south-central United States was close to saturation, whereas that associated with the northern jet and the maritime front was much less moist.

Figure 9.24 The relations between surface fronts and isobars, surface precipitation (< 25 mm vertical hatching; >25 mm cross-hatching), and jet streams (wind speeds in excess of about 45 m s-1 shown by stipple) over the United States on 20 September 1958 and 21 September 1958. This illustrates how the surface precipitation area is related more to the position of the jets than to that of the surface fronts. The air over the south-central United States was close to saturation, whereas that associated with the northern jet and the maritime front was much less moist.

Source: After Richter and Dahl (1958), by permission of the American Meteorological Society.

upstream of the core), divergence causes lower-level air to rise on the equatorward (i.e. right) side of the jet, whereas in the exit zone (downstream of the core) ascent is on the poleward side. Figure 9.24 shows how precipitation is related more often to the position of the jet stream than to that of surface fronts; maximum precipitation areas are in the right entrance sector of the jet core. This vertical motion pattern is also of basic importance in the initial deepening stage of the depression. If the upper-air pattern is unfavourable (e.g. beneath left entrance and right exit zones, where there is convergence) the depression will fill.

The development of a depression can also be considered in terms of energy transfers. A cyclone requires the conversion of potential into kinetic energy. The upward (and poleward) motion of warm air achieves this. The vertical wind shear and the superimposition of upper tropospheric divergence drive the rising warm air over a baroclinic zone. Intensification of this zone further strengthens the upper winds. The upper divergence allows surface convergence and pressure fall to occur simultaneously. Modern theory relegates the fronts to a subordinate role. They develop within depressions as narrow zones of intensified ascent, probably through the effects of cloud formation.

Recent research has identified a category of mid-latitude cyclones that develop and intensify rapidly, acquiring characteristics that resemble tropical hurricanes. These have been termed 'bombs' in view of their explosive rate of deepening; pressure falls of at least 24 mb/24 hr are observed. For example, the 'QEII storm', which battered the ocean liner Queen Elizabeth II off New York on 10 September 1978, developed a central pressure below 950 mb with hurricane-force winds and an eye-like storm centre within twenty-four hours (see Chapter 11C.2). These systems are observed mainly during the cold season off the east coast of the United States, off Japan, and over parts of the central and northeastern North Pacific, in association with major baroclinic zones and close to strong gradients of sea-surface temperature. Explosive cyclogenesis is favoured by an unstable lower troposphere and is often located downstream of a travelling 500-mb-level trough. Bombs are characterized by strong vertical motion, associated with a sharply defined level of non-divergence near 500 mb, and large-scale release of latent heat. Wind maxim a in the upper troposphere, organized as jet streaks, serve to amplify the lower-level instability and upward motion. Studies reveal that average cyclonic deepening rates over the North Atlantic and North Pacific are about

10 mb/24 hr, or three times greater than over the continental United States (3 mb/24 hr). Hence, it is suggested that explosive cyclogenesis represents a more intense version of typical maritime cyclone development.

The movement of depressions is determined essentially by the upper westerlies and, as a rule of thumb, a depression centre travels at about 70 per cent of the surface geostrophic wind speed in the warm sector. Records for the United States indicate that the average speed of depressions is 32 km hr-1 in summer and 48 km hr-1 in winter. The higher speed in winter reflects the stronger westerly circulation. Shallow depressions are mainly steered by the direction of the thermal wind in the warm sector and hence their path closely follows that of the upper jet stream (see Chapter 7A.3). Deep depressions may greatly distort the thermal pattern, however, as a result of the northward transport of warm air and the southward transport of cold air. In such cases, the depression usually becomes slow moving. The movement of a depression may in addition be guided by energy sources such as a warm sea surface that generates cyclonic vorticity, or by mountain barriers. The depression may cross obstacles such as the Rocky Mountains or the Greenland ice sheet as an upper low or trough, and subsequently redevelop, aided by the lee effects of the barrier or by fresh injections of contrasting airmasses.

Ocean-surface temperatures can crucially influence the location and intensity of storm tracks. Figure 9.25B indicates that an extensive relatively warm surface in the north-central Pacific in the winter of 1971 to 1972 caused a northward displacement of the westerly jet stream together with a compensating southward displacement over the western United States, bringing in cold air there. This pattern contrasts with that observed during the 1960s (see Figure 9.25A), when a persistent cold anomaly in the central Pacific, with warmer water to the east, led to frequent storm development in the intervening zone of strong temperature gradient. The associated upper airflow produced a ridge over western North America with warm winters in California and Oregon. Models of the global atmospheric circulation support the view that persistent anomalies of sea-surface temperature exert an important control on local and large-scale weather conditions.

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