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The Process of Occlusion Objections to the NCM Occlusion Objections To 1/8/18 Objections to the NCM Occlusion The Process of Occlusion • Structures resembling an occlusion can be present without a pre-existing warm sector Jim Steenburgh • The so-called “occluded front” is often drawn from the University of Utah peak of the warm sector to the low center without [email protected] evidence that the process of occlusion occurred • “The occlusion is added as an accretion” Supplemental Reading: Schultz and Mass (1993), Schultz and Vaughan (2011) • There is often little or no temperature gradient across an occlusion at the surface • The largest gradients are often aloft Source: Schultz and Mass (1993) Objections to the NCM Occlusion The Instant Occlusion • There are few if any well documented cases of cold fronts overtaking warm fronts to form occlusions • Occluded fronts often appear to form when the low center separates from the peak of the warm sector and deepens back into the cold air • Occurs when a comma cloud/PVA aloft • The occluded front is essentially a new front approaches and merges with a frontal wave • Satellite imagery suggests that occluded-like • Structure that forms looks like an occlusion, but structures form in non-classical ways without the history of catch up Source: Schultz and Mass (1993) Source: Reed (1979), Schultz and Mass (1993), Djuric (1994) Outstanding Questions Evolution of Ideal Occlusion • Do cyclones ever occlude in a classical manner in which the cold front catches up to the warm L L front? L • A: Yes! • Can occlude structures form from non-classical mechanisms Pre-existing warm & cold fronts • A: Yes! L Warm sector narrows as cold L front overtakes warm front • Might there be a better way to conceptualize the Occlusion lengthens as warm & occlusion process cold fronts “zipper” • A: Yes! Source: Schultz and Mass (1993) 1 1/8/18 Vertical Structure Vertical Structure • HR 18 • HR 30 – Distinct warm and cold – Cold & warm fronts fronts meet, forming warm- – Intervening warm sector type occlusion – Cold & upper level fronts somewhat distinct • HR 33 – Upper-level cold front • HR 24 continues to move – Warm sector narrows downstream, forming elevated cold front of – Cold & upper-level fronts the occlusion merge Source: Schultz and Mass (1993) Source: Schultz and Mass (1993) Vertical Structure Airflow • Sfc trajectories show confluence of air across • Key points occlusion – Surface-based cold front • Air within warm tongue aloft originates in warm catches warm front in sector (40, 42, 47, 56) classical manner • Warm sector air removed from SFC – Cold front does not “ride up” over warm front – Upper-level frontal zone provides elevated cold- front of the occlusion Source: Schultz and Mass (1993) Source: Schultz and Mass (1993) Non-Classical Occlusion Non-Classical Occlusion • Deeply occluded system with warm-core seclusion • • Front forms from “occludogenesis” rather than catch Confluence of trajectories along occlusion “ ” up • Crossing of post-occlusion trajectories – #5 starts to south in warmer air than 13. #5 ends in seclusion. • Wind fields generate the warm tongue & seclusion • Seclusion forms from the “wrapping” of cold air originating in warm/occluded frontal zone Source: Kuo et al. (1992) Source: Kuo et al. (1992) 2 1/8/18 A New Perspective Conventional Wisdom • The occluded front forms and lengthens as the cold front overtakes the warm front OCCLUDED FRONTS AND THE OCCLUSION PROCESS A Fresh Look at Conventional Wisdom • The occlusion may be warm or cold type depending on the BY DAVID M. SCHULTZ AND GERAINT VAUGHAN temperatures behind the cold front and ahead of the warm Comparing the 90-yr-old Norwegian cyclone model to recent research results demonstrates that descriptions of the occlusion process in textbooks need to be rewritten. front he Norwegian cyclone model is the foundation of model is a testament to its ingenuity, simplicity, and observational synoptic meteorology. In the early basic accuracy. Elements of the Norwegian cyclone T twentieth century, the scientists at the Geophysical model continue to be presented in meteorological Institute in Bergen, Norway, drew upon previous re- textbooks, whether for meteorologists (e.g., Palmén search and a mesoscale observing network to create and Newton 1969; Wallace and Hobbs 1977, 2006; a conceptual model for the structure and evolution of Carlson 1991; Bluestein 1993; Gordon et al. 1998; extratropical cyclones and their attendant fronts (e.g., Barry and Carleton 2001; Martin 2006), introductory • Bjerknes 1919; Bjerknes and Solberg 1921, 1922). Even classes (e.g., Gedzelman 1980; Moran and Morgan The formation of an occlusion signifies an end to the 90 yr hence, the success of the Norwegian cyclone 1997; Lutgens and Tarbuck 2001; Aguado and Burt 2001; Ackerman and Knox 2007; Ahrens 2008; Barry and Chorley 2010), or the general public (e.g., Kimble cyclone deepening phase AFFILIATIONS: SCHULTZ—Division of Atmospheric Sciences, 1951; Williams 1997, 2009; Lehr et al. 2001). Figure 1 Department of Physics, University of Helsinki, and Finnish provides a typical example of how the Norwegian Meteorological Institute, Helsinki, Finland, and Centre for cyclone model is illustrated in one of the most recent Atmospheric Science, School of Earth, Atmospheric and books (Williams 2009). Environmental Sciences, University of Manchester, Manchester, Despite the durability and longevity of conceptual United Kingdom; VAUGHAN—National Centre for Atmospheric Science, University of Manchester, Manchester, United Kingdom models such as the Norwegian cyclone model, their CORRESPONDING AUTHOR: Dr. David M. Schultz, Centre applicability needs to be continually reassessed. The for Atmospheric Science, School of Earth, Atmospheric and body of conventional wisdom, or common knowledge Environmental Sciences, University of Manchester, Simon Building, shared among the members of the scientific commu- • The occluded front features the pre-frontal weather of a Oxford Road, Manchester M13 9PL United Kingdom nity, is called a paradigm (Kuhn 1970). Weaknesses, E-mail: [email protected] inconsistencies, contradictions of existing theory, The abstract for this article can be found in this issue, following the table and observations that do not fit the paradigm [called warm front (widespread clouds and precipitaiton) and the of contents. anomalies by Kuhn (1970)] are occasionally revealed. DOI:10.1175/2010BAMS3057.1 As a growing number of such anomalies accumulate, A supplement to this article is available online (10.1175/2010BAMS3057.2) eventually a new conceptual model that explains the anomalies arises. This new conceptual model In final form 13 November 2010 post-frontal weather of a cold front (clear skies and drying) ©2011 American Meteorological Society replaces the old, and a paradigm shift occurs. Science advances. AMERICAN METEOROLOGICAL SOCIETY APRIL 2011 | 443 Source: Schultz and Vaughan (2011) Catch-Up Process Deformation and Rotation • Question – Is the catch-up process an explanation of occlusion formation or a consequence of the underlying physical process? • Alternative Answer – Lengthening and narrowing of warm tongue, and catch-up of cold and warm fronts better explained by differential deformation and rotation around low center Advantage: Explains “occludogenesis”, strengthening of instant occlusion, lengthening of occlusion as low deepens into cold air, catchup process, etc. Source: Schultz and Vaughan (2011) Source: Schultz et al. (1998), Schultz and Vaughan (2011) Reconciling conceptual models Alternative S-K Model • Key physical process operating in traditional occlusion and Shapiro-Keyser model is “wrap-up” of the thermal wave by differential rotation and deformation • In the NCM, the narrowing of the warm sector and “catch-up” of the cold front to the warm front is a consequence of this process • In the S-K model, the separation of the low center from the warm sector, development of the intervening warm front, and formation of bent-back front are a consequence of this process Eliminates pesky use of “warm front” and ”bent-back warm front” Source: Schultz and Vaughan (2011) Source: Schultz and Vaughan (2011) 3 1/8/18 Warm and Cold Type Occlusion Occlusion Ends Cyclone Deepening • Temperature “rule” for occlusion formation • Norwegian Model doesn’t work because fronts are not zero order – Occlusion indicates end of deepening phase because discontinuities in temperature cyclone no longer has access to potential energy stored in warm sector • Static stability is a better discriminator for • “After the occlusion, the cyclone soon begins to fill up” occlusion type (low static stability goes over high) (Bjerknes and Solberg 1922) – Explains why warm-type is more common • Cold front is usually less stable • Reality – Suggests that cold-type occlusions may be more – Cyclones often deepen after occlusion formation common in areas where warm fronts tend to be weak (e.g., California) – Cyclogenesis better viewed from QG or PV perspective Source: Stoelinga et al. (2002), Schultz and Vaughan (2011) Source: Schultz and Vaughan (2011) Occlusion Weather and Precip Occlusion Weather and Precip • Norwegian Model/CW – Occlusions are associated with widespread clouds/precip followed by clearing after surface frontal passage • Reality – Occlusions are associated with a variety of The Dry Airstream Gradients in “weather” not necessarily collocated with surface front cloud/precip patterns, including dry slots and Don’t use back edge of cirrostratus to locate surface front! banded precipitation Source: Schultz and Vaughan
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