• Purpose: to illustrate how following the motion on constant surfaces of or _W explains cloud patterns, precipitation, and other features that may be hard to deduce from a conventional isobaric analysis.

• The different examples illustrate somewhat different aspects of motion for cloud bands formed in relatively warmer air.

• First example: a cloud band, such as the "cloud leaf" stage (fig. 12.7a)
  1. from thermal wind relation, jet tends to lie above a strong horizontal T gradient. Such a gradient would form at the boundary between 2 distinctly different airmasses.
  2. sharp cloud edge near axis of the max upper level winds
  3. air in the cloud band has a subtropical or tropical origin
  4. clear air to the west has a polar origin.
  5. Note: long thin arrows are streamlines: trajectories would not necessarily cross the cloud bands because
     • the whole band is also moving towards the east.
     • these are geostrophic streamlines, not actual trajectories based on the total wind
     • there are sinking and rising motions not included in the streamlines shown.
  6. cloud band called a "warm conveyor belt" (WCB)
  7. sharp edge to the cloud lines up well with a "limiting streamline" (LSW) for the WCB.

• Second example: fig. 12.7b is similar to figure 12.7a except that the cloud band has an west-east orientation.
  Such bands are common in the North Pacific and often provide an indicator of a jet streak on a satellite image.

• Third example: fig. 12.8
  • WCB with poleward then eastward streamlines
  • dashed contours () and even more so (_W) indicate that parcels moving through the WCB must rise considerably. (In going from point A, near New Orleans, to B, to C; a parcel goes from the lower to the upper troposphere.)
  • note how clouds seen in this visible image become more glaciated (appear fuzzy due to composition as ice crystals) the further north on the cloud band.

• Fourth example: fig. 12.9
  • compares conventional analysis (fig. 12.9a) with isentropic views (esp. 12.9b).
  • where there is strong T advection, there is likely to be the biggest differences in the analysis. (Isentropic depiction will follow the vertical motion implied by T advection, isobaric depiction will not.)
  • fig. 12.9c: relatively sharp cloud edge matches LSW well.
  • fig. 12.9e: shows
    • layer of high humidity air rising dramatically as head N (hatched). This is WCB.
    • gradient in _W below and N of WCB suggests transition to colder, drier air mass
    • arrows cross lines consistent with latent heating, though _W is conserved.
    • _W increases with height at most stations (not south end) indicates air is potentially stable.
    • fig. 12.9d shows how you might spot this on a single sounding (Pittsburg).

• Fifth example: fig. 12.10 (emphasize 12.10c)
  • fig 12.10c shows a cross section perpendicular to the WCB
  • surfaces bend up and over the cold air
  • tropopause is lowered ("x" symbols) where the upper trough is located
  • strong contrast in RH (dashed contours) reveals air masses of contrasting properties

• Sixth example, fig. 12.12:
  • precipitation present far away from location of the surface front (fig 12.12a).
    However, since the 500 mb winds strongest further north (fig. 12.12b big arrow), the T gradient at 700 mb (say) may be strongest well north of the surface front. So, there might be some midlevel WAA that might not be easily deduced from a conventional SLP and h map. I mention this because eqn analysis must still apply, the WAA might not be obvious.
  • there is a broad region of PVA over much of the Great Lakes and extending to the east coast (fig. 12.12b).
  • however, the precipitation has a well defined western edge over Ohio and Kentucky.
  • relative wind analysis explains these features, specifically the sharp edge of the precipitation contained within the region of PVA. The edge lies on another LSW (fig. 12.12c).

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