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13.7: Cyclone Self Development

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  • Up to this point, cyclogenesis has been treated as a response to various external imposed forcings. However, some positive feedbacks allow the cyclone to enhance its own intensification. This is often called self development.

    13.7.1. Condensation

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    Figure 13.48 50 kPa chart showing cloud and precipitation region in the upper troposphere, causing latent heating and a westward shift of the ridge axis.

    13.7.1. Condensation

    As discussed in the quasi-geostrophic vorticity subsection, divergence of the upper-level winds east of the Rossby-wave trough (Fig. 13.48) causes a broad region of upward motion there. Rising air forms clouds and possibly precipitation if sufficient moisture is present. Such a cloud region is sometimes called an upper-level disturbance by broadcast meteorologists, because the bad weather is not yet associated with a strong surface low.

    Latent heating of the air due to condensation enhances buoyancy and increases upward motion. The resulting stretching enhances spin-up of the vorticity, and the upward motion withdraws some of the air away from the surface, leaving lower pressure. Namely, a surface low forms.

    Diabatic heating also increases the average temperature of the air column, which pushes the 50 kPa pressure surface upward (i.e., increasing its height), according to the hypsometric relationship. This builds or strengthens a ridge in the upper-level Rossby wave west of the initial ridge axis.

    The result is a shortening of the wavelength between trough and ridge in the 50 kPa flow, causing tighter turning of the winds and greater vorticity (Fig. 13.48). Vorticity advection also increases.

    As the surface low strengthens due to these factors (i.e., divergence aloft, vorticity advection, precipitation, etc.), more precipitation and latent heating can occur. This positive feedback shifts the upper-level ridge further west, which enhances the vorticity and the vorticity advection. The net result is rapid strengthening of the surface cyclone.

    13.7.2. Temperature Advection

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    Figure 13.49 50 kPa chart showing a temperature field (shaded) that is 1/4 wavelength west of the wave in the height contours (solid lines).

    Cyclone intensification can also occur when warm air exists slightly west of the Rossby-wave ridge axis, as sketched in Fig. 13.49. For this situation, warm air advects into the region just west of the upper-level ridge, causing ridge heights to increase. Also cold air advects under the upper-level trough, causing heights to fall there.

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    Figure 13.50 50 kPa chart showing the westward shift and intensification of north-south wave amplitude caused by differential temperature advection.

    The net result is intensification of the Rossbywave amplitude (Fig. 13.50) by deepening the trough and strengthening the ridge. Stronger wave amplitude can cause stronger surface lows due to enhanced upper-level divergence.

    13.7.3. Propagation of Cyclones

    For a train of cyclones and anticyclones along the mid-latitude baroclinic zone (Fig. 13.51a), a Q-vector analysis (Fig. 13.51b) suggests convergence of Q-vectors east of the low, and divergence of Q-vectors west of the low (Fig. 13.51c). But convergence regions imply updrafts with the associated clouds and surface-pressure decrease — conditions associated with cyclogenesis. Thus, the cyclone (L) in Fig. 13.51c would tend to move toward the updraft region indicated by the Q-vector convergence.

    The net result is that cyclones tend to propagate in the direction of the thermal wind, i.e., parallel to the thickness contours.

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    Figure 13.51 Using Q-vectors to estimate cyclone propagation. Grey dashed lines are isotherms (or thickness contours). Thin black lines are isobars (or height contours). (a) Airplane is flying in the direction of the thermal wind. Black arrows are geostrophic wind vectors encountered by the airplane on either side of the cyclone (L). (b) Vector difference (black dashed line) between starting and ending geostrophic-wind vectors (drawn displaced to the right a bit so you can see it) for the portion of aircraft flight across the low. Q-vector (grey thick arrow) is 90° to the right of the dashed vector. (c) Q-vector from (b) is copied back to the cyclone. Similar analyses can be done to find the Q-vectors for the anticyclones (H). Convergence of Q-vectors (grey shaded box with dotted outline) indicates region of updraft. Divergence of Q-vectors (white box with dotted outline) indicates region of downdraft. (for N. Hemisphere.)

    13.7.4. Creation of Baroclinic Zones

    Cyclones and anticyclones tend to create or strengthen baroclinic zones such as fronts. This process is described below.

    Consider a train of lows and highs in a region with uniform temperature gradient, as shown in Fig. 13.52a. The rotation around the lows and highs tend to distort the center isotherms into a wave, by moving cold air equatorward on the west side of the lows and moving warm air poleward on the east side.

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    Figure 13.52 Rotational and divergent wind components (thin black arrows) and isotherms (dashed grey lines) in the lower troposphere. (a) Initial train of highs (H) and lows (L) in a uniform temperature gradient in the N. Hemisphere. (b) Later evolution of the isotherms into frontal zones (shaded rectangle is a baroclinic zone).

    In addition, convergence into lows pulls the isotherms closer together, while divergence around highs tends to push isotherms further apart. The combination of rotation and convergence/divergence tends to pack the isotherms into frontal zones near lows, and spread isotherms into somewhat homogeneous airmasses at highs (Fig. 13.52b).

    Much of the first part of this chapter showed how cyclones can develop over existing baroclinic regions. Here we find that cyclones can help create those baroclinic zones — resulting in a positive feedback where cyclones modify their environment to support further cyclogenesis. Thus, cyclogenesis and frontogenesis often occur simultaneously.

    13.7.5. Propagation of Cold Fronts

    Recall from Fig. 13.8 that the circulation around a cyclone can include a deformation and diffluence region of cold air behind the cold front. If the diffluent winds in this baroclinic zone are roughly geostrophic, then you can use Q-vectors to analyze the ageostrophic behavior near the front.

    Screen Shot 2020-03-13 at 9.09.18 PM.pngScreen Shot 2020-03-13 at 9.09.43 PM.pngScreen Shot 2020-03-13 at 9.10.06 PM.png
    Figure 13.53 Using Q-vectors to locate regions of upward and downward motion due to diffluence of air behind a cold front. (a) Thin black lines are wind direction. Dashed grey lines are isotherms, along which an imaginary airplane flies. Thick black arrows show geostrophic wind vectors, with “o” representing zero wind. (b) Estimation of Q-vectors between points 1 and 2, and also between points 2 and 3. (c) The Q-vectors from (b) plotted in the baroclinic zone, with near-zero Q-vectors (grey “o”) elsewhere. Q-vector convergence in grey shaded region suggests updrafts.

    Fig. 13.53a is zoomed into the diffluence region, and shows the isotherms and geostrophic wind vectors. By flying an imaginary airplane along the isotherms and noting the change in geostrophic wind vector, you can estimate the Q-vectors at the front as drawn in Fig. 13.53b. Further from the front, the Q-vectors are near zero.

    Thus, Q-vector convergence is along the leading edge of the cold front (Fig. 13.53c), where warm air is indeed rising over the front. Behind the cold front is Q-vector divergence, associated with downward air motion of cool dry air from higher in the troposphere (Fig. 13.8). This agrees with observations of typical cold fronts, which have a line of cumuliform clouds along the front, and clearing skies behind the front.

    A SCIENTIFIC PERSPECTIVE • Uncertainty and Truth in Science

    The Supreme Court of the USA ruled that: “There are no certainties in science. Scientists do not assert that they know what is immutably ‘true’ — they are committed to searching for new, temporary theories to explain, as best they can, phenomena.” They ruled that science is “a process for proposing and refining theoretical explanations about the world that are subject to further testing and refinement.”

    [CAUTION: The definition, role, and activities of science cannot be defined or constrained by a legal court or religious inquisition. Instead, science is a philosophy that has gradually developed and has been refined by scientists. The “A SCIENTIFIC PERSPECTIVE” boxes in this book can help you to refine your own philosophy of science.]