• Purpose: to make further progress in understanding the properties of frontal cyclones beyond what is easily seen using isobaric coordinates.

• Terminology:
  • potential temperature symbol:
  • isentrope: line of constant
  • isentropic flow = adiabatic flow

• Advantages: (list from Carlson)
  1. motion is approximately adiabatic. Main exception: where precipitation is forming.
  2. corollary: flow is approximately 2-D on constant surfaces
  3. vertical motion explicitly seen when parcels travel up and down constant surface.
  4. isentropic flow gives better picture of 3-D motion than do wind barbs on an isobaric chart.

• Disadvantages: (list from Carlson)
  1. atmosphere not completely adiabatic, esp in:
     • boundary layer
     • "vicinity of strong vertical mixing"
     • where latent heat is released: precipitating clouds
  2. isentropic surfaces intersect the ground, and the location varies in time.
  3. a particular surface, especially near a front, may extend from very low to very high elevation, hard to visualize the horizontal distribution of weather features.
  4. not in common usage

• T and curves are like "mirror images":
  • Note first: both T and must decrease with increasing latitude.
  • increases with increasing height;
    T decreases with increasing height.
  • surfaces arc up and over a blob of cold air;
    T surfaces arc up and over a blob of warm air.
  • surfaces roughly horizontal in tropics and polar regions compared to midlatitudes: where they cross each other, hence like mirror image w.r.t. mid latitudes
  • vertical spacing of isentropes () is larger in troposphere, than in stratosphere;
    vertical spacing of isotherms (T) is smaller in troposphere than in stratosphere.

• Other properties:
  • knowing that parcel moves on an isentropic surface does not tell you which direction; need another conserved property.
  • dry static energy (DSE = gZ + C_PT) is conserved. DSE in coordinates is called Montgomery Streamfunction (M).
  • M is to winds on surface like geopotential is to geostrophic winds on isobaric (P constant) surface. See eqns (12.6)
  • example: fig. 12.1. Note sinking motion deduced in NE Nevada; rising motion deduced along Colorado/Utah border.

• Pseudo-adiabatic motion
  • above an LCL, further rising releases latent heat of condensation.
  • release of latent heat causes parcel to move to a higher value of
  • Example figs. 12.3 and 12.4; see also "3-D perspective" schematic in the supplementary notes.
    North and eastern part of the area of precip: motion on isentropic surface is rising.
    southwestern part of the area of precip: motion on isentropic surface seems to be sinking. However, a parcel may still actually be rising if it moves to a higher isentropic surface faster than the individual surfaces slope downward. See schematic in supplementary notes.

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