• Evolution of the frontal cyclone discussion centers around Fig. 10.4.

• 5 stages are described, of which 4 have accompanying diagrams with this information plotted:
  Lower level diagrams (a, c, e, g) have: SLP contours, scalloped line for cloudmass boundary, shading for precipitation, and arrows showing vertical motion.
  Upper level diagrams (b, d, f, h) have: Z at 500mb contours, (absolute?) vorticity dashed contours, the jet streak (arrow), and relative location of the surface low (L).

• Stages (number) and panel in Fig. 10.4 (letter:)
  1. a: cloud band has uniform width, vertical motion mainly from PVA. SLP pattern weak, so little T advection, front is stationary.
     b: Vort max & jet streak on SW side. Trof axis has SW-NE tilt. Still, some PVA over sfc low will initiate cyclone development.

  2. c: SLP contours develop, which initiates WAA and CAA; the latter opposes some of the upper PVA. Warm & cold fronts begin to move. Though not shown, the cloud has developed the "leaf" shape - wider at developing warm front, narrower above the cold front.
     Note: this change along a uniform cloud band is often a first clue that a system is forming out in the Pacific. When that happens the motion of the cloud band can be stalled.
     d: PVA above sfc low (plus the WAA) explain intensifying sfc low. Vort max & jet streak still on SW side of upper trough, though they are closer to the sfc low than before. Trough more symmetric, i.e. north-south axis.

  3. e: Cloud developing comma shape, even more differnce between warm and cold front cloud bands. NO Occluded front yet. Comma head is where cloud "wraps around" to the N and W of the sfc low position. Upper level PVA leads to the rising motion where the arrows point "up" W and NW of the surface low. Further W and SW of the surface low, there is cloud even though arrows point "down" (and CAA is ocurring). This shallow, low-level, non-precipitating cloud results from a shallow layer of rising motion caused by frictional convergence at the surface. Above this shallow cloud the CAA causes sinking and so the arrows W and SW of the low in the figure point "down". (For details, consult Figs. 9.3 and 9.5.)
     f: jet streak & vort max have rotated around to SE side of the upper trough; they are still South of the sfc low position. Trof axis is now NW-SE oriented. Coldest air now to west of vort max, from BEBVE expect growth at all levels. Note how max value of vort has now increased whereas it was constant the two earlier times.

  4. g: Even more prominent spiral in comma head of cloud band. An occluded front has formed, as the sfc low has migrated into the cold air whilst continuing to deepen. Dry tongue where there is sinking has formed. Precipitation well back from the fronts now and will be discussed further Chap 12 (as a "cold conveyor belt"). Thickness contours closer together due to the CAA and WAA.
     See supplemental notes for connection to a "valley" in the 3-D frontal surface. In the valley is warm sector air that has displaced cooler, denser air. The result is the "valley" lies above the occluded front because there is less mass in a column of air in the troposphere (forming a trough in SLP) and also consistent with a ridge in the thickness field. Hence the empirical rule linking the occlusion and these two items.
     h: jet streak still on SE side of the trough. stronger winds are cross over the warm/cold front intersection (no longer above the sfc low). Trough has pinched off cold air and a closed low in Z field has formed. The vort max continues to increase.

  5. No diagrams are shown of this stage. T advections will diminish as low center moves away from the h gradient. The close proximity of the cold air to the surface low leads to less tilt with height of the cyclone.
     Decay of the system occurs by several means:
     1. slowing down of winds by surface friction
     2. negative baroclinic conversion: cold air rising in cyclone center. The rising may arise from friction-driven sfc convergence.
     3. latent heat release in cyclone (warming up the cold air, thus reducing the T contrasts on which the cyclone feeds)
     4. barotropic energy conversion (one would need a special horizontal tilt to develop in the trough)

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