Frontal Characteristics

The character of frontal weather depends upon the vertical motion in the airmasses. If the air in the warm sector is rising relative to the frontal zone the fronts are usually very active and are termed ana-fronts, whereas sinking of the warm air relative to the cold airmasses gives rise to less intense kata-fronts (Figure 9.11).

Frontal Depression Unstable

Figure 9.9 Stages in the life cycle of a marine extratropical depression showing: (I) Incipient frontal depression; (II) Frontal fracture; (III) Bent-back warm front and frontal T-bone; (IV) Warm-core seclusion. (A) Schematic isobars of sea-level pressure, fronts and cloud cover (stippled). (B) Isotherms and flows of cold air (solid arrows) and warm air (dashed arrows).

Figure 9.9 Stages in the life cycle of a marine extratropical depression showing: (I) Incipient frontal depression; (II) Frontal fracture; (III) Bent-back warm front and frontal T-bone; (IV) Warm-core seclusion. (A) Schematic isobars of sea-level pressure, fronts and cloud cover (stippled). (B) Isotherms and flows of cold air (solid arrows) and warm air (dashed arrows).

Source: After Shapiro and Keyser (1990), by permission of the American Meteorological Society.

1 The warm front

The warm front represents the leading edge of the warm sector in the wave. The frontal boundary has a very gentle slope of the order of 0.5-1°, so the cloud systems associated with the upper portion of the front herald its approach some twelve hours or more before the arrival of the surface front (see Plate 17). The ana-warm front, with rising warm air, has multi-layered cloud that steadily thickens and lowers towards the surface position of the front. The first clouds are thin, wispy cirrus, followed by sheets of cirrus and cirrostratus, and altostratus (Figure 9.11A). The sun is obscured as the altostratus layer thickens and drizzle or rain begins to fall. The cloud often extends through most of the troposphere and, with continuous precipitation occurring, is generally designated as nimbostratus. Patches of fracto-stratus may also form in the cold air as rain falling through this air undergoes evaporation and quickly saturates it.

The descending warm air of the kata-warm front greatly restricts the development of medium- and highlevel clouds. The frontal cloud is mainly stratocumulus,

POTENTIALLY

POTENTIALLY

Warm Kata Front

Figure 9.10 Schematic model of a dry trough and frontogenesis east of the Rocky Mountains. (A) Warm, dry air with low equivalent potential temperature (8 ) from the Rockies overrides warm, moist, high 8e air from the Gulf of Mexico, forming a potentially unstable zone east of the dry trough. (B) Upward motion associated with the cold front aloft (CFA). (C) Location of the CFA rain band at the surface. (Equivalent potential temperature is the potential temperature of an air parcel that is expanded adiabatically until all water vapour is condensed and the latent heat released then compressed adiabatically to 1000 mb pressure.)

Figure 9.10 Schematic model of a dry trough and frontogenesis east of the Rocky Mountains. (A) Warm, dry air with low equivalent potential temperature (8 ) from the Rockies overrides warm, moist, high 8e air from the Gulf of Mexico, forming a potentially unstable zone east of the dry trough. (B) Upward motion associated with the cold front aloft (CFA). (C) Location of the CFA rain band at the surface. (Equivalent potential temperature is the potential temperature of an air parcel that is expanded adiabatically until all water vapour is condensed and the latent heat released then compressed adiabatically to 1000 mb pressure.)

Source: After Locatelli et al. (1995), by permission of the American Meteorological Society.

with a limited depth as a result of the subsidence inversions in both airmasses (see Figure 9.11B). Precipitation is usually light rain or drizzle formed by coalescence.

At the passage of the warm front the wind veers, the temperature rises and the fall of pressure is checked. The rain becomes intermittent or ceases in the warm air and the thin stratocumulus cloud sheet may break up.

Forecasting the extent of rain belts associated with the warm front is complicated by the fact that most fronts are not ana- or kata-fronts throughout their length or even at all levels in the troposphere. For this reason, radar is being used increasingly to map the precise extent of rain belts and to detect differences in rainfall intensity. Such studies show that most of the production and distribution of precipitation is controlled by a broad airflow a few hundred kilometres across and several kilometres deep, which flows parallel to and ahead of the surface cold front (see Figure 9.12). Just ahead of the cold front, the flow occurs as a low-level jet with winds of up to 25-30 m s-1 at about 1 km above the surface. The air, which is warm and moist, rises over the warm front and turns southeastward ahead of the front, merging with the mid-tropospheric flow (Figure

9.12). This flow is termed a 'conveyor belt (for large-scale heat and momentum transfer in mid-latitudes). Broad-scale convective (potential) instability is generated by the overrunning of this low-level flow by potentially colder, drier air in the middle troposphere. Instability is released mainly in small-scale convection cells that are organized into clusters, known as mesoscale precipitation areas (MPAs). These MPAs are further arranged in bands 50 to 100 km wide (Figure

9.13). Ahead of the warm front, the bands are broadly parallel to the airflow in the rising section of the conveyor belt, whereas in the warm sector they parallel the cold front and the low-level jet. In some cases, cells and clusters are further arranged in bands within the warm sector and ahead of the warm front (see Figures 9.13 and 9.14). Precipitation from warm front rain bands often involves 'seeding' by ice particles falling from the upper cloud layers. It has been estimated that 20 to 35 per cent of the precipitation originates in the 'seeder' zone and the remainder in the lower clouds (see also Figure 5.14). Orographic effects set up some ofthe cells and clusters, and these may travel downwind when the atmosphere is unstable.

Frontal Zone Schematic Cross Section

Figure 9.11 (A) Cross-sectional model of a depression with ana-fronts, where the air is rising relative to each frontal surface. Note that an anawarm front may occur with a kata-cold front and vice versa. JU and JL show the locations of the upper and lower jet streams, respectively. (B) Model of a depression with kata-fronts, where the air is sinking relative to each frontal surface.

Sources: After Pedgley (1962), and Bennetts et al. (1988). Crown copyright ©, reproduced by permission of the Controller of Her Majesty's Stationery Office.

Extratropical Cyclone Conveyor

Figure 9.12 Model of the large-scale flow and mesoscale precipitation structure of a partially occluded depression typical of those affecting the British Isles. It shows the 'conveyor belt' (A) rising from 900 mb ahead of the cold front over the warm front. This is overlaid by a mid-tropospheric flow (B) of potentially colder air from behind the cold front. (C) indicates air subsiding ahead of the occluded front. Most of the precipitation occurs in the well-defined region shown, within which it exhibits a cellular and banded structure.

Source: After Harrold (1973), by permission of the Royal Meteorological Society.

Rainbands Midlatitude Cyclones
Figure 9.13 Fronts and associated rain bands typical of a mature depression. The broken line X-Y shows the location of the cross-section given in Figure 9.14.

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

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  • Stefanie
    How wide are rain belts along warm front?
    1 year ago

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