1. Cloud shape changes:
   • Prior to mature stage: cloud "leaf" as in fig 10.5a: upstream (southern) end narrower than than downstream (northern) end.
   • As warm front develops, the downstream end becomes wider.
   • Eventually a "comma" cloud shape is formed from some higher clouds with possibly a more extensive lower stratus cloud deck.
   • a Dry tongue may form

2. The Dry Tongue has:
   • strong cold advection along its length
   • some upper level PVA @ its northern end
   • some upper level NVA @ its southern end
   • sinking air from middle to higher levels along most of its length, except possibly:
   • Note: on the north end, there may be PVA and weak cold advection. So, the air may be rising there, but it has come from high elevation with low T_d so it often remains cloud-free.

3. Satellite analysis:
   • 500 mb low center near the "cloud cusp" of the UPPER cloud. (NOT the arrow tip in fig 10.5b)
   • surface low center near the point where the "comma" cloud "head" joins the "tail". See fig. 10.5a (In terms of later jargon in Chapter 12: near where the west edge of the warm conveyor belt meets the south edge of the cold conveyor belt.)

4. Occlusions: 3 views:
   • Introductory textbook view:
     • "the cold front overtakes the warm front". (not really, way too simple!)
     • empirical rule: front motion proportional to the speed of the air perpendicular to the front on the cold air side.
       • winds behind a cold front often perpendicular to the front, winds ahead of the warm front often parallel to the front.
       • winds behind cold front driven by high speed upper level air that sank to lower levels, so those winds often stronger than winds ahead of a warm front
       • So, the cold fronts often do mover faster.
   • Isentropic motion of air masses:
     1. See 3-D depiction of warm sector air close to a mature low being drawn to the north, then westward around the low. (much better)
     2. A "tongue" of warm air curls around the low creating a ridge in the thickness because warmer air is displacing colder air.
     3. The "tongue" of warm air also has less mass than the cold air it displaced. Since surface pressure is the sum of the mass of all the air above, a surface trough is created as well.
     4. An Empirical rule follows from items 2 & 3: The occlusion is analyzed where the thickness ridge and surface trough line up.
   • Divergence and Convergence fields:
     • divergent circulations refine some details
     • _a max is offset from the 500 mb min: L₅ in fig. 10.5b. Implies absolute PVA nicely centered over the upper comma cloud (cross-hatched).
     • The NVA behind cannot explain the dry tongue sinking because there is little vorticity advection there. Hence cold air advection (CAA) causes that sinking.
     • Note: at the start of Chap 14, we consider jet streaks (the jet stream max indicated by the double arrow). The jet max has a pattern of divergence and convergence which reinforces both the comma cloud and the dry tongue sinking.

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