Monday, April 4, 2011

Updating the Norwegian Cyclone Model

Although it remains a cornerstone of most introductory level meteorology textbooks, the Norwegian Cyclone Model, as first proposed by Bjerknes and Solberg (1922), fails to properly explain some aspects of frontal-cyclone evolution and is difficult to apply to some cyclones.
The Norwegian cyclone model (e.g., Bjerknes and Solberg 1922)
as presented by Schultz and Vaughan (2011).
A great example is provided by the 1200 UTC 4 April GFS forecast, which produces a powerful cyclone with a central pressure of ~940 hPa over the north Pacific at 0300 UTC 7 April.

In particular, note the following:

  • T-bone frontal structure: In this cyclone, the cold front does not overtake the warm front as occurs in the Norwegian cyclone model.  Instead, the cold front and warm fronts remain oriented normal to each other, resulting above in the so-called frontal T-bone as proposed by Shapiro and Keyser (1990).  The process of occlusion, as envisioned by Bjerknes and Solberg (1922), never occurs.
  • Frontal fracture: There is a loss of cold-frontal baroclinity near the warm front.  This was observed by Bergen School Meteorologists (see Godske et al. 1957), but was more strongly emphasized by Shapiro and Keyser (1990).  
  • Warm-Core Seclusion: As proposed by Bjerknes and Solberg, the pocket of locally warm air near the low center, known as the warm-core seclusion, formed as the cold front overtook the warm front south of the low center, cutting off a pocket of warm-sector air.  Instead, in most cyclones like the one above, the warm-core seclusion forms from rotation and deformation in the cold sector, with polar air encircling the low center, cutting off a pocket of warmer air.  
  • Bent-Back Front: An intense pressure gradient exists along this feature, which represents an extension of baroclinity that formed along the warm front into the polar airstream behind the low.  Bergen School meteorologists call this feature the bent-back occlusion, but that isn't appropriate here because this cyclone never occludes.  Shapiro and Keyser (1990) called it the bent-back warm front, but that name has been a source of confusion, so I prefer the more generic bent-back front.  
Although this case fits the Shapiro and Keyser (1990) conceptual model quite well, there remain cyclones that are clearly better described by the Norwegian Cyclone Model.   Work is needed to provide a more robust conceptual model describing cyclone evolution.  With regards to occlusion, a good effort to do this is provided by Schultz and Vaughan (2011).

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