The frontalwave depression

A depression (also termed a low or cyclone) (see Note 2) is an area of relatively low pressure, with a more or less circular isobaric pattern. It covers an area 1500 to 3000 km in diameter and usually has a life span of four to seven days. Systems with these characteristics, which are prominent on daily weather maps, are referred to as synoptic-scale features. The mid-latitude depression is usually associated with the convergence of contrasting

Initial Stage Mid Latitude Cyclones

Early stage

Early stage

Open Stage Cyclone

Figure 9.7 Four stages in the typical development of a mid-latitude depression. Satellite views of the cloud systems corresponding to these stages are shown in Figure 9.8.

Notes: C = cold air; W = warm air. Source: Mostly after Strahler (1965), modified after Beckinsale.

Open stage

Open stage

Figure 9.7 Four stages in the typical development of a mid-latitude depression. Satellite views of the cloud systems corresponding to these stages are shown in Figure 9.8.

Notes: C = cold air; W = warm air. Source: Mostly after Strahler (1965), modified after Beckinsale.

Mid Latitude Depression

Occluded stage

Occluded stage

Mid Latitude Cyclone Dissolving Stage

Dissolving stage

Dissolving stage aiimasses. According to the 'Norwegian cyclone model' (see Figure 9.7), the interface between these airmasses develops into a wave form with its apex located at the centre of the low-pressure area. The wave encloses a mass of warm air between modified cold air in front and fresh cold air in the rear. The formation ofthe wave also creates a distinction between the two sections of the original airmass discontinuity for, although each section still marks the boundary between cold and warm air, the weather characteristics found within each section are very different. The two sections of the frontal surface are distinguished by the names warm front for the leading edge of the wave and the warm air and coldfront for that of the cold air to the rear (see Figure 9.7B).

The boundary between two adjacent airmasses is marked by a strongly baroclinic zone of large temperature gradient, 100 to 200 km wide (see C, this chapter, and Figure 9.1). Sharp discontinuities of temperature, moisture and wind properties at fronts, especially the warm front, are rather uncommon. Such discontinuities are usually the result of a pronounced surge of fresh, cold air in the rear sector of a depression, but in the middle and upper troposphere they are often caused by subsidence and may not coincide with the location of the baroclinic zone. In meteorological analysis centres, numerous criteria are used to locate frontal boundaries: 1000 to 500-mb thickness gradients, 850-mb wet-bulb potential temperature, cloud and precipitation bands, and wind shifts. However, a forecaster may have to use judgement when some of these criteria disagree.

On satellite imagery, active cold fronts in a strong baroclinic zone commonly show marked spiral cloud bands, formed as a result of the thermal advection (Figure 9.8B, C). A cirrus shield, however, typically covers warm fronts. As Figure 9.23 shows, an upper tropospheric jet stream is closely associated with the baroclinic zone, blowing roughly parallel to the line of the upper front (see Plate 17). This relationship is examined below.

Air behind the cold front, away from the low centre, commonly has an anticyclonic trajectory and hence moves at a greater than geostrophic speed (see Chapter 6A.4), impelling the cold front to acquire a super-geostrophic speed also. The wedge of warm air is pinched out at the surface and lifted bodily off the ground. This stage of occlusion eliminates the wave form at the surface (see Figure 9.7). The depression usually achieves its maximum intensity twelve to

Stages Depressions Weather
Figure 9.8 Schematic patterns of cloud cover (white) observed from satellites, in relation to surface fronts and generalized isobars. A, B, C and D correspond to the four stages in Figure 9.7.

Source: After Boucher and Newcomb (1962), by permission of the American Meteorological Society.

twenty-four hours after the beginning of occlusion. The occlusion gradually works outward from the centre of the depression along the warm front. Sometimes, the cold air wedge advances so rapidly that, in the friction layer close to the surface, cold air overruns the warm air and generates a squall line (see Chapter 9I).

By no means all frontal lows follow the idealized life cycle discussed above (cf. the caption for Plate 18). It is generally characteristic of oceanic cyclogenesis, although the evolution of those systems has been reexamined using aircraft observations collected during North Atlantic meteorological field programmes in the 1980s. These suggest a different evolution of maritime frontal cyclones (Figure 9.9). Four stages are identified: (I) cyclone inception features a broad (400 km) continuous frontal zone; (II) frontal fracture occurs near the centre of the low with tighter frontal gradients; (III) a T-bone structure and bent-back warm front develop, and (IV) the mature cyclone shows seclusion of the warm core within the polar airstream behind the cold front.

Over central North America, cyclones forming in winter and spring depart considerably from the Norwegian model. They often feature an outflow of cold Arctic air east of the Rocky Mountains forming an

Arctic front, a lee trough with dry air descending from the mountains, and a warm, moist, southerly flow from the Gulf of Mexico (Figure 9.10). The trough superposes dry air over warm, moist air, generating instability and a rain band analogous to a warm front. The Arctic air moves southward west of the low centre, causing lifting of warmer, dry air but giving little precipitation. There may also be an upper cold front advancing over the trough that forms a rain band along its leading edge. Such a system is thought to have caused a record rainstorm at Holt, Missouri, on 22 June 1947, when 305 mm fell in just forty-two minutes!

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Responses

  • Brooke
    Where is the centre of a depression geography?
    9 years ago
  • Asmeret Russom
    What is the dissolving stage cyclone?
    9 years ago
  • kasey
    What is weather associated with frontal depression?
    9 years ago
  • kevin
    What is a wave depressions weather?
    8 years ago
  • Hamid Abrha
    What type of cloud is found on the leading edge of a distant frontal depression?
    6 years ago
  • danielle
    What kind of weather is prominent in depression in the mid latitude?
    3 years ago
  • lillie
    What are wave deprissions weather?
    12 months ago

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