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du/dt>0

geopotential heights/geostrophic streamlines. V ag <0. z. V ag >0. LEFT. divergence. 300 hPa. y. convergence. du/dt<0. x. JET EXIT. JET ENTRANCE. du/dt>0. convergence. divergence. rising. sinking. RIGHT. thermally direct. thermally indirect. 900 hPa. V ag >0. sinking.

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du/dt>0

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  1. geopotential heights/geostrophic streamlines Vag<0 z Vag>0 LEFT divergence 300 hPa y convergence du/dt<0 x JET EXIT JET ENTRANCE du/dt>0 convergence divergence rising sinking RIGHT thermally direct thermally indirect 900 hPa Vag>0 sinking COOL SIDE OF JET Vag<0 rising WARM SIDE OF JET LOW LEVEL JET ENHANCEMENT/FORMATION

  2. Maintaining Thermal Wind Balance Assume that the height of the 1000 hPa surface is = 0 here (i.e., 500 hPa hgts are equivalent to the thickness field!) > 0 > 0 If the wind shear decreases, the temperature gradient should also decrease!

  3. How does it do this? CONV/sinking DIV/rising

  4. Thus two things happen in tandem here to ensure thermal wind balance: • Under the influence of the Coriolis force the southerly ageostrophic flow aloft will deflect eastward (i.e., in the direction of the zonal flow) – and the northerly ageostrophic below will deflect westward (against the westerly flow) thereby increasing the vertical wind shear! • 2. The jet entrance is marked by a thermally direct circulation (i.e., warm air rising, cold/sinking). This acts to weaken the temperature gradient.

  5. How does Holton simplify his discussion? • The vorticity advection is weak (or non-exstent) at the base of 500 hPatrof/ridge axes (eliminates boxed terms) • 2. Temperature advection is weak over a surface trof/ridge (eliminates circled terms)

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