Rethinking the Occluded Front and Occlusion Process

There is a paper forthcoming in the Bulletin of the American Meteorological Society that that provides a much needed new perspective on occluded fronts and the occlusion process (Schultz and Vaughan 2011).  Lead author David Schultz has done much to advance our knowledge of occluded fronts and I believe the paper will help reconcile some (but probably not all) of the concerns that have been raised about the  classical model of occlusion as introduced by the Bergen School beginning in the 1920s.

As discussed in the paper, conventional wisdom and textbook discussions of occluded fronts and the occlusion process are based primarily on the view that occluded fronts form and lengthen as the cold front overtakes the warm front.  Schultz and Vaughan (2011) call this the catch-up model.

Conceptual model of a Norwegian cyclone from
Schultz and Vaughan (2011)

Concerns about the catch up model were raised probably moments after it was envisioned by meteorologist Tor Bergeron.  Schultz and Mass (1993) summarize many of these concerns, which include:

• Structures resembling an occlusion (i.e., a narrow tongue of warm air with back-to-back zones of warm and cold advection) can develop without a cold front overtaking a warm front.  
• Often, there is little evidence of a warm tongue between the low center and the peak of the warm sector and analysts simply add an occluded front between the two for convenience.
• Occluded fronts often appear to form as the low center separates from the junction of the warm and cold fronts and then deepens into the polar airstream.  In this case, the catch-up process does not occur and the occlusion is essentially a new front.

Further, more recent cyclone conceptual models completely omit the occlusion process.  For example, in the Shapiro and Keyser (1990) conceptual model, the warm and cold fronts remain oriented normal to one another, resulting in the formation of a frontal T-bone.

Life cycle of the marine extratropical cyclone
from Shapiro and Keyser (1990)

Schultz and Vaughan attempt to reconcile the concerns above and produce a new paradigm for occluded fronts and occluded cyclones.  This new paradigm views the occlusion process as a "wrap up" of the thermal wave and deformation of the warm-air tongue, rather than the "catch-up" of the cold front with the warm front.  

I like the paradigm they present and encourage readers of this blog to have a look at the paper.  We will almost certainly be examining the paper as part of Atmos 5210/6210 this semester.