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Author
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Topic: 500hPa High vs. Surface High
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HG Lux
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posted November 20, 2019 01:32 PM
Reading and rereading pp99/100 of modern marine weather:
the book describes upper level lows as “low pressure systems at higher elevations”. it writes of “troughs of low pressure” in the winds aloft.
Figure 4.4.2 illustrates a cutoff low formation, where effectively a lobe of cold air gets cut off from the main Rossby wave marked as an “L” at the 5640 m contour, surrounded by 5760 or so.
Isn’t that “Low” a cut-of-low contour trough on the 500mb levels? Isn’t that effectively a cut-off high-pressure, cold air trough or dimple?
Having the inset maps in tiny print at a different scale and with an offset doesn’t help quick understanding much. But anyways, after getting a magnifying glass, it seems to me, that what is being described has nothing to do with an upper level low pressure system. Isn’t it just that there is a divergence at the typical spot aloft, just after when the wind gets around the southern tip of the lobe, which gets more pronounced the second day and stays stationary the third?
The entire topic is hobbled by a homonym problem, it seems to me? So the question is, am I not getting it or are these not low pressure systems aloft, but strong divergences that have the same effect, even though they are caused by a high pressure centre of the winds aloft?
Would really appreciate a clarification.
Many thanks
From: Switzerland
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David Burch
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posted November 20, 2019 09:57 PM
Thanks for your comments. We will look into this and get back to you shortly.
Please clarify which words are the homonyms you refer to.
From: Starpath, Seattle, WA
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Dave Wilkinson
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posted November 21, 2019 12:51 PM
On your first question regarding Fig 4.4-2, yes the “Low” refers to a low contour of height on the 500 mb chart. All pressures on the 500 mb are just that, 500 mb. On the surface chart, a “Low” refers to low pressure. This may be the homonym issue you speak of.
The insets on the three charts in Fig 4.4-2 show the characteristic relationship of surface lows and cut-off lows on the 500 mb charts. That is that the surface low tends to be positioned very close to directly under the cut-off low aloft and tend to stay there for some time.
In a practical sense, the value of the 500 mb chart is less with a cut-off low pattern because it is not as reliably forecast and the general guidance regarding surface lows doesn’t tend to apply. Some of those guidelines for surface lows being they:
1) develop under and downwind of the 500 mb trough
2) move downwind in a direction roughly parallel with the 500 mb contours
3) intensify downwind of the 500 mb trough
4) move at a speed roughly ¼ to ½ the speed of the 500 mb winds.
One editorial note is that on p. 100 of the text, there is a reference to Section 7.8. It seems that should be Section 7.6 which is on p. 171.
From: Starpath, Port Townsend WA
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