• Notes:
  • a frontal zone is a region of stronger horizontal temperature gradient.
  • a front is drawn on the warm air side of a frontal zone.
  • Choose coordinate "y" to be perpendicular to the axis of the front.
  • Choose coordinate "x" to be parallel to the axis of the front.
  • Carlson places the origin y =0 or y' = 0 in the middle of the frontal zone, at least at the the start of the frontogenesis.

• Derivation:
• Take "y" derivative, of adiabatic eqn: D/Dt = "Q dot" Then use chain rule.
• Multiply result by "-1" so that positive tendency means the front is getting stronger (i.e. the gradient across the front is increasing). Result is eqn (13.2)
• Note: the winds here are total winds; which include both ageostrophic and geostrophic components.

• Processes that affect front strength:
• Tendency on LHS of eqn (13.2) is a total derivative so RHS of eqn has things that change the front strength from a frame of reference moving with the front
• 4 terms appear on RHS

  1. Shear (see fig. 13.5)
     • flow parallel to front (= u) reverses direction across the front
     • Result: if shear in u bends isentropes at the front to be more parallel with front, the spacing between isentropes is reduced. This is frontogenesis.

  2. Confluence (see fig. 13.7)
     • Jargon: "axis of dilatation" this is the line that the winds are approaching from both sides of the frontal zone. (It is horizontal in in fig. 13.7, and corresponds to the x' axis.)
     • if there is flow perpendicular to the front (= v) and that flow advects isentropes towards the front, then:
     • Result: spacing between isentropes reduced. i.e. frontogenesis
     • Essentially, there is cold advection on cold side, warm advection on warm side

  3. Tilting term (see fig. 13.8)
     • similar to shear term, but in a vertical plane oriented perpendicular to the front:
     • As with term S, think of how isentropes are "moved" by the flow, in this case vertical motion.
     • Result for downward motion on the warm air side, upward motion on cold air side is frontogenesis.
     • Result for upward motion on the warm air side, downward motion on cold air side is frontolysis.

  4. Diabatic heating distribution (see fig. 13.9)
     • diabatic processes include:
       1. absorption of sunlight,
       2. infrared radiation (usually a cooling),
       3. latent heat release.
     • diabatic processes change front intensity simply by how heating or cooling is distributed across the front.
     • if heating same on both sides of front, front strength won't change. Formula agrees, y derivative of "Q dot" will be zero.
     • greater heating on the warm air side of the front, and/or greater cooling on the cold air side of the front will increase the temperature change across the front: i.e. are frontogenetic
     • opposite to above reduces the temperature gradient and hence is frontolytic
     • examples in fig 13.9 use a stratus cloud to "shade" a region; "Q dot" is given solely by absorbed solar radiation where sky is clear in this example.

Back to ATM 111 homepage