The cold front

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The weather conditions observed at cold fronts are equally variable, depending upon the stability of the warm sector air and the vertical motion relative to the frontal zone. The classical cold front model is of the ana-type, and the cloud is usually cumulonimbus. Figure 9.15 illustrates the warm conveyor belt associated with such a frontal zone and the line convection. Over the British Isles, air in the warm sector is rarely unstable, so that nimbostratus occurs more frequently at the cold front (see Figure 9.11A). With the kata-cold front the cloud is generally stratocumulus (see Figure 9.11B) and precipitation is light. With ana-cold fronts there are usually brief, heavy downpours, sometimes accompanied by thunder. The steep slope of the cold front (approximately 2°) means that the bad weather is of shorter duration than at the warm front. With the passage of the cold front, the wind veers sharply, pressure begins to rise and temperature falls. The sky may clear abruptly, even before the passage of the surface cold front in some cases, although with kata-cold fronts the changes are more gradual. Forward-tilting cold fronts are sometimes observed, either due to surface friction slowing the low-level motion of the front, or as a result of a cold front aloft (see Figure 9.10).

• WATER PARTICLES • ICE PARTICLES [ ] ICE PARTICLE CONCENTRATION EMBEDDED CONVECTION CELLS ( > CLOUD LIQUID WATER CONTENT igm

(PER LITRE}

• WATER PARTICLES • ICE PARTICLES [ ] ICE PARTICLE CONCENTRATION EMBEDDED CONVECTION CELLS ( > CLOUD LIQUID WATER CONTENT igm

(PER LITRE}

Figure 9.14 Cross-section along the line X-Y in Figure 9.13 showing cloud structures and rain bands. The vertical hatching represents rainfall location and intensity. Raindrop and ice particle regions are shown, as are ice particle concentrations and cloud liquid water content. Numbered belts refer to those shown in Figure 9.13. Scales are approximate.

Source: After Hobbs and Matejka et al.; from Houze and Hobbs (1982), copyright © Academic Press. Reproduced by permission.

Cumulonimbus Embedded Stratiform

Figure 9.15 Schematic diagrams showing airflows, relative to the moving frontal system, at an ana-cold front. A warm conveyor belt (stippled) ascends above the front with cold air (dashed arrows) descending beneath it. (A) Plan view. (B) Vertical section along the line X-Y, showing rates of vertical motion.

Figure 9.15 Schematic diagrams showing airflows, relative to the moving frontal system, at an ana-cold front. A warm conveyor belt (stippled) ascends above the front with cold air (dashed arrows) descending beneath it. (A) Plan view. (B) Vertical section along the line X-Y, showing rates of vertical motion.

Source: Browning (1990), by permission of the American Meteorological Society.

3 The occlusion

The cold front moves faster relative to the warm front, eventually catching up with it, leading to occlusion, where the warm sector air is lifted off the ground. Occlusions are classified as either cold or warm, depending on the relative states of the airmasses lying in front and to the rear of the warm sector (Figure 9.16). If airmass 2 is colder than airmass 1, then the occlusion is warm, but if the reverse is so it is termed a cold occlusion. The air in advance of the depression is likely to be coldest when depressions occlude over Europe in winter and very cold cP air is affecting the continent. Recent work suggests that most occlusions are warm and that the thermal definition is often misleading. A new definition is proposed: a cold (warm) occlusion forms when the air that is more statically stable lies behind the cold front (ahead of the warm front) (Figure 9.16).

The line of the warm air wedge aloft is associated with a zone of layered cloud (similar to that found with a warm front) and often of precipitation. Hence its position is indicated separately on some weather maps and it is referred to by Canadian meteorologists as a trowal (trough of warm air aloft). The passage of an occluded front and trowal brings a change back to polar airmass weather.

A different process occurs when there is interaction between the cloud bands within a polar trough and the main polar front, giving rise to an instant occlusion. A warm conveyor belt on the polar front ascends as an upper tropospheric jet, forming a stratiform cloud band (Figure 9.17), while a low-level polar trough conveyor belt at right angles to it produces a convective cloud band and precipitation area poleward of the main polar front on the leading edge of the cold pool.

Frontolysis (frontal decay) represents the final phase of a front's existence although it is not necessarily linked with occlusion. Decay occurs when differences no longer exist between adjacent airmasses. This may arise in four ways: (1) through their mutual stagnation over a similar surface; (2) as a result of both airmasses moving on parallel tracks at the same speed; (3) through their movement in succession along the same track at the same speed, or (4) by the system entraining air of the same temperature.

4 Frontal-wave families

Observations show that frontal waves over the oceans, at least, do not generally occur as separate units but in families of three or four (see Figure 9.9; Plate 18). The depressions that succeed the original one form as

Weather Frontal Depression Plan View

Figure 9.16 Schematic illustration of a warm and a cold occlusion in the classical model. Plan view of synoptic pattern (above) and cross-sections along lines A-B and C-D. Colder air is shaded darker. The bottom panel illustrates proposed criteria for identifying warm and cold occlusions based on static stability.

Figure 9.16 Schematic illustration of a warm and a cold occlusion in the classical model. Plan view of synoptic pattern (above) and cross-sections along lines A-B and C-D. Colder air is shaded darker. The bottom panel illustrates proposed criteria for identifying warm and cold occlusions based on static stability.

Source: Above and centre from Stoelinga et al. (2002, p. 710, fig. 1), by permission of the American Meteorological Society. The bottom panel is based on their new definition.

secondary lows along the trailing edge of an extended cold front. Each new member follows a course that is south of its progenitor as the polar air pushes further south to the rear of each depression in the series. Eventually, the front trails far to the south and the cold polar air forms an extensive meridional wedge of high pressure, terminating the sequence.

Another pattern of development may take place on the warm front, particularly at the point of occlusion, as a separate wave forms and runs ahead of the parent depression. This type of secondary is more likely with very cold (cA, mA or cP) air ahead of the warm front, and it tends to form when mountains bar the eastward movement of the occlusion. This situation often occurs

Instant Occlusion

Figure 9.17 Schematic illustrations of vortex developments in satellite imagery. The sequences run from bottom to top. Left. Comma cloud (c) developing in a polar airstream. Centre. Instant occlusion from the interaction of a polar trough with a wave on the polar front. Right. The classical frontal wave with cold and warm conveyor belts (CCB, WCB). C = enhanced convection; D = decaying cloud band; cloud cover stippled.

Source: After Browning (1990) by permission of the American Meteorological Society.

Figure 9.17 Schematic illustrations of vortex developments in satellite imagery. The sequences run from bottom to top. Left. Comma cloud (c) developing in a polar airstream. Centre. Instant occlusion from the interaction of a polar trough with a wave on the polar front. Right. The classical frontal wave with cold and warm conveyor belts (CCB, WCB). C = enhanced convection; D = decaying cloud band; cloud cover stippled.

Source: After Browning (1990) by permission of the American Meteorological Society.

when a primary depression is situated in the Davis Strait and a breakaway wave forms south of Cape Farewell (the southern tip of Greenland), moving away eastward. Analogous developments take place in the Skagerrak-Kattegat area when the occlusion is held up by the Scandinavian mountains.

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Responses

  • Paulina Dickson
    How do weather depressions occur?
    9 years ago
  • cino zetticci
    How frontal depression form?
    8 years ago
  • Is a Polar Front and a Cold Front the same?
    4 years ago
  • MELVIN
    What is kata front and ana front in climatalogy?
    4 years ago
  • camelia
    Do cold or warm fornts affect climate change?
    4 months ago
  • jan-erik
    Is a cold front weather or climate?
    3 months ago
  • omero conti
    How does cold front influence climate change?
    3 months ago
  • Leonie
    How do cold fronts effect climate?
    2 months ago
  • igor
    Where do the passage of depression of rainfall get its energy from?
    24 days ago

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