This page is referenced in two research papers that appear in the Monthly Weather Review in the February 1998 issue. Animation of various data sets are displayed here. The animations are presented in FLI format. If you need information about FLI format or need to acquire an FLI player to view the animations, click here. Although the papers can stand on their own without reference to the page, it is not certain that the converse is true. For a thorough understanding of what is being shown here, the browser is referred to the two manuscripts;

The structure and evolution of a continental winter cyclone. Part I: Frontal structure and the occlusion process


The structure and evolution of a continental winter cyclone. Part II: Frontal forcing of an extreme snow event

by Jonathan E. Martin,
University of Wisconsin-Madison

In Part I, the development of a warm occluded structure that was observed over the central United States on January 19, 1995 is examined. It is shown that the occluded structure formed first at middle tropospheric levels and that it was a result of the classical occlusion process. The animation of the 309 K Theta-E surface demonstrates that this moist isentrope (located on the warm edge of both the cold and warm frontal zones) folded over itself during the cyclone's evolution. Such a folding over is interpreted as the development of the warm occluded structure.

Animation of the Plan-View of the 309 K Theta-E isosurface (4.4 M)

An alternate view of this evolution is provided by looking at this moist isentropic surface from a northerly, and slightly elevated perspective. Such a perspective allows for a fuller appreciation of the 3-D structure of this surface as the cyclone evolves. Notice the sloping intersection between the steeply sloped cold frontal portion of the surface and the more shallowly sloped (ramp-like) portion of the surface which is the warm frontal zone. This sloping intersection has been called the "trowal" by scientists at the Canadian Meteorological Service since the 1950's (Galloway 1951, 1958; 1960; Penner, 1955). The trowal is an important feature in precipitation producing, occluded cyclones.

Animation of the 309 K Theta-E isosurface from an elevated, northern perspective (3.3 M)

As stated earlier, the trowal is an important feature in precipitation producing occluded cyclones. The UW-NMS output was used to trace hundreds of air parcel trajectories in the diagnosis of this storm. One of the coherent airstreams that this analysis identified is shown by a representative trajectory in the next animation. This airstream was characterized by high theta-e, moist flow off of the Gulf of Mexico and it ascended through the trowal portion of the warm occluded structure. This ascent was responsible for the production of heavy snow in Iowa and Wisconsin as described in detail in Part II.

Animation of the 309 K Theta-E isosurface from an elevated, northern perspective with an air parcel trajectory showing flow through the TROWAL (4.4 M)

In order to get a sense of the vertical motions involved in this airstream, an eastern view of this trowal trajectory is given next. Notice that the ascent is rapid over the heavy snow area. Again, it should be noted that this one trajectory is representative of a coherent airstream within this cyclone.

Animation of an eastern perspective of the TROWAL trajectory (104 K)

The software used to animate the UW-NMS model output on this page is called VIS-5D. For more information on VIS-5D, and how you might get a version, click here.

Please send me your comments.

Jonathan Martin (jon@marrella.meteor.wisc.edu)

This research was supported by the National Science Foundation, Division of Atmospheric Sciences, Grant Number ATM-9505849.

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