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12.3: Surface Fronts

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  • Surface fronts mark the boundaries between airmasses at the Earth’s surface. They usually have the following attributes:

    • strong horizontal temperature gradient
    • strong horizontal moisture gradient
    • strong horizontal wind gradient
    • strong vertical shear of the horizontal wind
    • relative minimum of pressure
    • high vorticity
    • confluence (air converging horizontally)
    • clouds and precipitation
    • high static stability
    • kinks in isopleths on weather maps

    In spite of this long list of attributes, fronts are usually labeled by the surface temperature change associated with frontal passage.

    Some weather features exhibit only a subset of attributes, and are not labeled as fronts. For example, a trough (pronounced like “trof”) is a line of low pressure, high vorticity, clouds and possible precipitation, wind shift, and confluence. However, it often does not possess the strong horizontal temperature and moisture gradients characteristic of fronts.

    Another example of an airmass boundary that is often not a complete front is the dryline. It is discussed later in this chapter.

    Recall from the Weather Reports and Map Analysis chapter that fronts are always drawn on the warm side of the frontal zone. The frontal symbols (Fig. 12.10) are drawn on the side of the frontal line toward which the front is moving. For a stationary front, the symbols on both sides of the frontal line indicate what type of front it would be if it were to start moving in the direction the symbols point.

    Fronts are three dimensional. To help picture their structure, we next look at horizontal and vertical cross sections through fronts.

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    Figure 12.10 (at right) Glyphs for fronts, other airmass boundaries, and axes (copied from the Weather Reports and Map Analysis chapter). The suffix “genesis” implies a forming or intensifying front, while “lysis” implies a weakening or dying front. A quasi-stationary front is a frontal boundary that doesn’t move very much. Occluded fronts and drylines will be explained later.

    12.3.1. Horizontal Structure Cold Fronts (Fig. 12.11)

    Figure 12.11 (both columns) Cold front horizontal structure (maps can be overlain).

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    In central N. America, winds ahead of cold fronts typically have a southerly component, and can form strong low-level jets at night and possibly during day. Warm, humid, hazy air advects from the south.

    Sometimes a squall line of thunderstorms will form in advance of the front, in the warm air. These squall lines can be triggered by wind shear and by the kinematics (advection) near fronts. They can also consist of thunderstorms that were initially formed on the cold front, but progressed faster than the front.

    Along the front are narrow bands of towering cumuliform clouds with possible thunderstorms and scattered showers. Along the front the winds are stronger and gusty, and pressure reaches a relative minimum. Thunderstorm anvils often spread hundreds of kilometers ahead of the surface front.

    Winds shift to a northerly direction behind the front, advecting colder air from the north. This air is often clean with excellent visibilities and clear blue skies during daytime. If sufficient moisture is present, scattered cumulus or broken stratocumulus clouds can form within the cold airmass.

    As this airmass consists of cold air advecting over warmer ground, it is statically unstable, convective, and very turbulent. However, at the top of the airmass is a very strong stable layer along the frontal inversion that acts like a lid to the convection. Sometimes over ocean surfaces the warm moist ocean leads to considerable post-frontal deep convection.

    The idealized picture presented in Fig. 12.11 can differ considerably in the mountains. Warm Fronts (Fig. 12.12)

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    Figure 12.12 (both columns) Warm front structure (maps can be overlain).

    In central N. America, southeasterly winds ahead of the front bring in cool, humid air from the Atlantic Ocean, or bring in mild, humid air from the Gulf of Mexico.

    An extensive deck of stratiform clouds (called a cloud shield) can occur hundreds of kilometers ahead of the surface front. In the cirrostratus clouds at the leading edge of this cloud shield, you can sometimes see halos, sundogs, and other optical phenomena. The cloud shield often wraps around the poleward side of the low center.

    Along the frontal zone can be extensive areas of low clouds and fog, creating hazardous travel conditions. Nimbostratus clouds cause large areas of drizzle and light continuous rain. Moderate rain can form in multiple rain bands parallel to the front. The pressure reaches a relative minimum at the front.

    Winds shift to a more southerly direction behind the warm front, advecting in warm, humid, hazy air. Although heating of air by the surface might not be strong, any clouds and convection that do form can often rise to relatively high altitudes because of weak static stabilities throughout the warm airmass.

    Sample Application

    Given the plotted surface weather data below, analyze it for temperature (50°F and every 5°F above and below) and pressure (101.2 kPa and every 0.4 kPa above and below). Identify high- and low-pressure centers and fronts. Discuss how the winds, clouds and weather compare to the descriptions in Figs. 12.11 & 12.

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    Find the Answer

    First, using methods shown in the Weather Reports & Map Analysis chapter, draw the isotherms (°F):

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    Next, analyze it for pressure. Recall that the plotted pressure is abbreviated. We need to prefix 9 or 10 to the left of the pressure code, and insert a decimal point two places from the right (to get kPa). Choose between 9 and 10 based on which one results in a pressure closest to standard sea-level pressure 101.3 kPa.

    For the pressure data in this Sample Application, every prefix is 10. For example, the plotted pressure code 097 means 100.97 kPa. Similarly, 208 means 102.08 kPa. By analyzing pressures, we get the following map of isobars (kPa):

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    Next, overlay the isobars and isotherms, and find the frontal zones (drawn with the thick black and grey lines) and fronts (thick black line).

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    Check: Looks reasonable.

    Exposition: Winds are generally circulating counterclockwise around the low center (except the very light winds, which can be sporadic). Overcast skies cover most of the region, with some thunderstorms and rain north of the warm front, and snow further northwest of the low where the air is colder.

    12.3.2. Vertical Structure

    Suppose that radiosonde observations (RAOBs) are used to probe the lower troposphere, providing temperature profiles such as those in Fig. 12.13a. To locate fronts by their vertical cross section, first convert the temperatures into potential temperatures θ (Fig. 12.13b). Then, draw lines of equal potential temperature (isentropes). Fig. 12.13c shows isentropes re-drawn at 5°C intervals. Often isentropes are labeled in Kelvin.

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    Figure 12.13 Analysis of soundings to locate fronts in a vertical cross section. Frontal zone / frontal inversion is shaded in bottom figure, and is located where the isentropes are tightly packed (close to each other).

    A frontal inversion is where the isentropes are packed closely together (shaded in Fig. 12.13c). This concept applies to both warm and cold fronts.

    In the absence of diabatic processes such as latent heating, radiative heating, or turbulent mixing, air parcels follow isentropes when they move adiabatically. For example, consider the θ = 35°C parcel that is circled in Fig. 12.13b above weather station B. Suppose this parcel starts to move westward toward C.

    If the parcel were to be either below or above the 35°C isentrope at its new location above point C, buoyant forces would tend to move it vertically to the 35°C isentrope. Such forces happen continuously while the parcel moves, constantly adjusting the altitude of the parcel so it rides on the isentrope.

    The net movement is westward and upward along the 35°C isentrope. Air parcels that are forced to rise along isentropic surfaces can form clouds and precipitation, given sufficient moisture. Similarly, air blowing eastward would move downward along the sloping isentrope.

    In three dimensions, you can picture isentropic surfaces separating warmer θ aloft from colder θ below. Analysis of the flow along these surfaces provides a clue to the weather associated with the front. Air parcels moving adiabatically must follow the “topography” of the isentropic surface. This is illustrated in the Extratropical Cyclones chapter.

    At the Earth’s surface, the boundary between cold and warm air is the surface frontal zone. This is the region where isentropes are packed relatively close together (Figs. 12.13b & c). The top of the cold air is called the frontal inversion (Fig. 12.13c). The frontal inversion is also evident at weather stations C and D in Fig. 12.13a, where the temperature increases with height. Frontal inversions of warm and cold fronts are gentle and of similar temperature change.

    Within about 200 m of the surface, there are appreciable differences in frontal slope. The cold front has a steeper nose (slope ≈ 1 : 100) than the warm front (slope ≈ 1 : 300), although wide ranges of slopes have been observed.

    Fronts are defined by their temperature structure, although many other quantities change across the front. Advancing cold air at the surface defines the cold front, where the front moves toward the warm airmass (Fig. 12.14a). Retreating cold air defines the warm front, where the front moves toward the cold airmass (Fig. 12.14b).

    Above the frontal inversion, if the warm air flows down the frontal surface, it is called a katafront, while warm air flowing up the frontal surface is an anafront (Fig. 12.15). It is possible to have cold katafronts, cold anafronts, warm katafronts, and warm anafronts.

    Frequently in central N. America, the cold fronts are katafronts, as sketched in Fig. 12.16a. For this situation, warm air is converging on both sides of the frontal zone, forcing the narrow band of cumuliform clouds that is typical along the front. It is also common that warm fronts are anafronts, which leads to a wide region of stratiform clouds caused by the warm air advecting up the isentropic surfaces (Fig. 12.16b).

    A stationary front is like an anafront where the cold air neither advances nor retreats.

    Sample Application

    What weather would you expect with a warm katafront?

    Find the Answer & Exposition

    Cumuliform clouds and showery precipitation would probably be similar to those in Fig. 12.16a, except that the bad weather would move in the direction of the warm air at the surface, which is the direction the surface front is moving.

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    Figure 12.14 Vertical structure of fronts, based on cold air movement.
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    Figure 12.15 Vertical structure of fronts, based on overlying-air movement.
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    Figure 12.16 Typical vertical structure of fronts in central N. America.