This page contains animated output from the University of Wisconsin - Nonhydrostatic Modeling System (UW-NMS) 24h simulation of the April Fool's Day Blizzard that occurred in southern New England on 31 March-1 April 1997. The specific purpose of these animations is to demonstrate the 3-D structure of the occluded quadrant of this cyclone, paying particular attention to the TRough Of Warm air ALoft (TROWAL) structure. The animations are given in FLI format, (click here for more information).


The trowal is a 3-D, sloping line that connects the crests of the thermal wave as successive heights in a warm occluded system. It can be thought of as the intersection of the upper cold front with the warm frontal zone in the classical warm occlusion model as shown in FIGURE 1. As such, the trowal only appears in the occluded quadrant of cyclones. One way of approximately locating the trowal is to place it in the horizontal thermal ridge on an isobaric or geometric height surface. Such a thermal ridge may manifest itself as a ridge of 1000-500 hPa thickness, and axis of maximum theta, or an axis of maximum theta-e. FIGURE 2 shows the 3 km theta-e at 0600 UTC 1 April 1997. Note the axis of maximum theta-e that runs into southern New England. A vertical cross-section of theta-e along line A-A` in Fig. 2 is shown in FIGURE 3. The highest theta-e at any level in the plane of Fig. 3 is located in the trowal. The 312 K theta-e isentrope (in red in Fig. 3) lies near the warm edges of both the cold and warm frontal portions of the warm occluded structure. Thus, 312 K theta-e serves as a convenient moist isentropic surface for investigation of the trowal structure.

A planview of the 312 K theta-e surface is given in the following animation. Notice the "notch" that develops in this surface during the evolution of the storm. This "notch" is a physical manifestation of the presence of a trough of warm air aloft and so depicts the full 3-D trowal structure. The animation first shows the surface as opaque and then as transparent so that geographical boundaries can be seen.

Animation of the 312 K Theta-E surface (10.1 Mb)

A different prespective on the evolution of the 312 K Theta-E surface is offered in the next animation. Here the view is from the NNE and allows one to see the development of the sloping intersection between the cold frontal and warm frontal zones that characterizes the trowal. The reader is encouraged to stop the animation at Hour 19 to see the instantaneous trowal structure at 0600 UTC 1 April.

Animation of the 312 K Theta-E surface viewed from the north-northeast (1.9 Mb)

The motivation for the trowal conceptual model, developed by scientists at the Canadian Meteorlogical Service in the 1950's, was that its position usually bore a closer correspondence to the clouds and precipitation in the occluded quadrant of cyclones than did the often weak surface warm occluded front. In fact, these scientists (Crocker et al. 1947, Godson 1951, Penner 1955, Galloway 1958, 1960) contended that the essential feature of the warm occlusion was the trowal, not the surface occluded front. The April Fool's Day Blizzard was accompanied by record snowfall totals in southern New England. The accumulated snowfall totals forecasted by the UW-NMS simulation are depicted in the next animation along with the transparent version of the 312 K Theta-E planview. (Note that the accumulated totals are set to ZERO every 12 hours). The important relationship between the trowal and the sensible weather is well demonstrated by this animation.

Animation of the accumulated snowfall forecast of the UW-NMS with a planview of the 312 K Theta-E surface (2.7 Mb)

The significant precipitation that accompanied the April Fool's Day Blizzard was, as all precipitation is, a direct result of ascent of air. The fact that the heavy snow fell in the vicinity of the trowal (as depicted in the 312 K Theta-E surface) suggests that significant ascent occurred in the occluded quadrant of the cyclone. The following animation shows what has been termed the "trowal airstream" by Martin (1998a,b). It is a coherent flow of air that originates in the warm sector boundary layer and ascends cyclonically through the trowal. The airstream is color coded with the colors indicating the pressure level at which it is located (red and orange - 1000 to 900 hPa; blue ~ 400 hPa) and it is shown along with the 312 K Theta-E surface. The entire animation is from a northeastern perspective.

Animation of the 312 K Theta-e isosurface and the Trowal Airstream from an elevated Northern perspective (1.96 Mb)

In the classical occlusion model, the warm sector surface air is "squeezed"aloft as a result of the encroachment, and subsequent ascent, of the cold frontal zone on the warm frontal zone. This next animation shows that the air that is lifted aloft in the trowal airstream originates in the warm sector and is squeezed aloft during the occlusion process. Note that all of the air parcels comprising the trowal airstream originate to the east of the cold frontal trough in the sea-level pressure and equatorward of the warm frontal trough. The rapid development of this disturbance is also made evident by the color coded sea-level pressure.

Animation of the sea-level isobars and the trowal airstream from a planview (4.32 Mb)

For a description of the quasi-geostrophic forcing of the vertical motion manifest in the trowal airstream, the reader is directed to the manuscript entitled;

Quasi-geostrophic Forcing of Ascent in the Occluded Quadrant of Cyclones and the Trowal Airstream

by J. E. Martin

Submitted to Monthly Weather Review

A World Wide Web page consisting of the abstract for that paper as well as animations of other cases of the trowal airstream can be accessed at the following address;

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 (

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

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