Identification and Interrogation of the Trowal Structure in Three Different
Winter Cyclones
Laboratory Exercise for Significant Winter Weather Lectures
by
Jonathan E. Martin
University of Wisconsin-Madison
for
MSC/COMET Winter Weather Course
In this rather lengthy laboratory exercise, I want to acquaint the student with a
variety of means of identifying the trowal structure in winter cyclones. I would also like
to point out some salient characteristics of that structure and its role in the production and
distribution of precipitation in these cyclones.
We will be undertaking this exercise using output from the University of
Wisconsin-Nonhydrostatic Modeling System (UW-NMS). The UW-NMS simulated the
three cyclones and the output has been converted into both GEMPAK and VIS-5D
formats. The cyclones in question occurred on
22-23 October 1996
7 November 1996
31 March-1 April 1997
Data sets for the three cases are as follows;
oct22.gem
GEMPAK file for 22-23 October 1996 case
GRID2.OCT22.v5d VIS-5D file for 22-23 October 1996 case
nov7.gem
GEMPAK file for 7 November 1996 case
GRID2.NOV7.v5d
VIS-5D file for 7 November 1996 case
apr1.gem
GEMPAK file for 31 March-1 April 1997 case
GRID2.apr1.v5d
VIS-5D file for 31 March-1 April 1997 case
The basic assignment in this laboratory exercise is to identify and interrogate the
occluded thermal structure in each of these cyclones. This can be done in a number of
steps that should be repeated for each cyclone’s data set. Therefore, by the end of the lab
you should have answers (analyses) to the following questions (assignments) for each of
the three storms. It is best if you use a separate notebook for your answer/note sheet. I
encourage you to work together and to continue to take notes during the discussion we
will have at the
end of the laboratory period.
1) Identify the occluded thermal ridge in each cyclone. You might start by
considering such a structure at forecast hour 18 (f18) in the October case, f12 in the
November case, and f18 in the April case. There are a number of ways to do this
including;
• Considering the column average  or e
• Considering the  or e at a single layer, etc.
Find what works best for you in the context of the rest of the assignment.
Once you find the occluded thermal ridge, you should observe its evolution
through the forecast data. Does it lengthen over time? By superimposing the sea-level
pressure (SLP) forecast, do you notice if the lengthening of the occluded thermal ridge
occurs as the SLP minimum becomes further removed from the peak of the original warm
sector?
2) Construct a GEMPAK cross-section of e through the occluded thermal ridge.
It is important that you align your cross-section so that it is nearly perpendicular to the
axis of the thermal ridge. Examine the following characteristics of the cross-section;
• What evidence is there of a classical occluded structure? What type of
occluded structure is portrayed? Defend your choice.
• What is the static stability like in and around the occluded thermal
structure, especially compared to its surroundings?
• What is  like in the vicinity of the occluded thermal structure? What is
its maximum magnitude?
3) Identify the e surface that lies nearest the warm edge of both the warm front
and the cold front in the cross-section depiction above. Does it exhibit temporal
continuity as the e surface near the warm edges of the fronts?
4) Construct an isobaric contour map of that surface at the central analysis time.
(requires a script called surface.script).
• Note the location of the trowal at the central analysis time (T=0) based
upon this analysis.
• By constructing such a map at T=-12h, T=-6h, and T=6h during the
forecast, comment upon the evolution of the trowal structure during the
forecast.
• What relationship does the trowal position have to the radar reflectivity
at
the requested times (if available), RH forecasts at 700 hPa, and
accumulated
precipitation forecasts?
VIS-5D EXERCISES
We now move to VIS-5D to demonstrate alternative ways of visualizing the
trowal structure and the important relationship it has with the sensible weather in
occluded winter cyclones. If you have questions about VIS-5D, make sure that you ask
me as I am walking around the room.
1) Using the VIS-5D data sets from the three cases, use the ISOSFC command to
construct isosurfaces of e that best illustrate the trowal structure for each case.
• Manipulate the view to interrogate the 3-D structure of each trowal.
• Animate the isosurface from a variety of perspectives to further
understand the evolution of the occluded structures in each case.
• Highlight the relationship between the trowal position and the
accumulated
precipitation in the model forecast. Animation is very powerful
here.
2) Using the TRAJECT mode, find and initialize trajectories that represent the
flow
of air up the trowal, the trowal airstream.
• In what direction does this airstream curve as it ascends?
• Over what period of time does this air experience the vast majority of its
ascent? How high does it rise during that time?
• Are there differences in the intensity of ascent from case to case?
Comment on these differences if they exist.
3) Using the PROBE mode, examine some salient characteristics of the air parcels
in the trowal airstream in each case.
• Do the parcels conserve e reasonably well?
4) Highlight the relationship between the trowal airstream and the accumulated
precipitation in the model forecast.
• Are there portion of the total precipitation shield that seem inextricably
tied
to the trowal airstream?
• Where does most of this type of precipitation fall in the cyclone?
5) Identify the warm conveyor belt (WCB) airstream, a relative of the trowal
airstream, in each cyclone.
• What is the major characteristic of the WCB that differentiates it from
the
trowal airstream?
6) Overlay the 9 km potential vorticity (PV) onto the e surface you identified for
each cyclone. Examine the relationship between the two. Comment on the importance of
the “treble clef” structure in the upper-tropospheric PV.
• Comment on the relationship between the trowal airstream and the notch
in the PV treble clef?
SATELLITE IDENTIFICATION OF THE TROWAL POSITION
We now move on to use of the 6.7 m water vapor imagery to help identify the
trowal position. This exercise depends on establishing a relationship between the
characteristic dark bands in this imagery and the vorticity (potential vorticity) at upper
tropospheric levels. We will restrict our attention to the 23 October and 1 April cases as
we have water vapor imagery for both. These data will be examined using the GEMPAK
Analysis and Rendering Program (GARP).
1) Overlay the 500 hPa vorticity on top of the water vapor imagery at T=0 for both
the 23 October and 1 April cases. What type of relationship do you see?
2) Overlay the 200-300 hPa column averaged PV on top of the water vapor
imagery at T=0 for both the 23 October and 1 April cases. What type of relationship do
you see in this instance?
3) Describe what type of signature in the 6.7 m water vapor imagery might be a
faithful indicator of the trowal position in an occluded cyclone? Why might this be true?
We ended the lab with a discussion section in which we covered all the “answers”
to the questions and exercises posed. Animations of the various characteristic features
that I asked you to uncover are all available for inspection on the web pages that I have
referenced in the lecture notes. If you have follow-up questions after the lectures and lab,
please do not hesitate to call or write me in Wisconsin at
Prof. Jonathan E. Martin
1225 W. Dayton Street
Madison, WI 53706
(608) 262-9845
jon@meteor.wisc.edu