Case 14: 17 - 18 October 1996

Case 14: 17 - 18 October 1996

Storm Development -- Traditional Perspective

Objectives

Understand the relationships between surface and upper level perturbations from initial to mature states of an extratropical cyclone.
Examine the frontal structure in horizontal and vertical planes
Explore cloud and precipitation structure in this cyclone.

General Structure
Garp Instructions

Start the oct96 case.
Using the button "model plan projection" you will create a loop of the 500 hPa heights, sea level pressure and 1000 - 500 hPa thicknesses.
Select the Eta model and the time period 00 UTC 17 October - 00 UTC 18 October using the F000 and F006 periods (5 times total).
Suggestions for "nicer" plots: zoom in on the area of interest (continental US and southern Canada), plot thickness first using the color fill option, then plot the other fields.

Questions

1. Compare your loop of heights, sea level pressure and thicknesses to the idealized figure (figure 1 at end of this write-up). Does the October 1996 case resemble the idealized picture? Which time periods most closely resemble each stage in the figure?

2. How does the relative position of the surface low pressure center and the upper level trough change as the cyclone develops? Are these relationships similar to or much different from those shown in the figure from Wallace and Hobbs?

Frontal Structure
Garp Instructions

Clear screen.
Make a loop of sea level pressure, 1000 hPa temperature, and 1000 hPa winds using the Eta model for the period 00 UTC 17 Oct through 00 UTC 18 Oct using the F000 and F006 fields.
You may want to zoom in closer to the cyclone and use color shading for the temperature field. I prefer a 2.5C interval for temperature.

Questions

3. Sketch the position of the "model" fronts on the plots (Note: I have the students make hard copies of these plots, but just do it on the screen for today. I also have the students compare their hand analysis to the numerical depiction of the fronts and in this case there are differences).

4. Comment on the relative strengths of the fronts as measured by the strengths of the temperature gradients: Is the cold front stronger or weaker than the warm front? Do either front strengthen or weaken overtime? When is there evidence of an occluded front?

Garp Instructions -- I am deferring the plot of a cross-section through a cold front to the lab on jetstreams
Cloud and Precipitation Structure
Garp Instructions

Make a loop of the GOES-8 IR satellite imagery every 6 hours from 00 UTC 17 October through 00 UTC 18 October.
Zoom in on the area of interest and overlay the 500 hPa heights and sea level pressure from the Eta model for the same times using the F000 and F006 times.

Questions

5. Do the cloud patterns evident in these images relate to the upper level flow and surface features in a similar fashion as the "ideal" relationships shown in figure 2 at the end of this write-up?

Garp Instructions

Clear screen and reload the IR imagery every hour for the same 24 hour period.
Overlay the surface observations for the same times.

Questions

6. Identify which cloud band(s) are associated with the surface cold front throughout the loop. (More advanced students could further investigate why the cold front suddenly becomes a line of strong thunderstorms by contrasting the environments where the thunderstorms broke out vs. the areas that did not experience thunderstorms).

Main Points

During the developing stages of a storm, the trough tilts westward with height. When a storm matures and begins to decay, the surface low and upper level trough are vertically stacked.
Fronts are characterized by windshifts, pressure troughs and they lie on the warm side of a temperature gradient.
Not all storms are "ideal".

Back to Oct 96 Overview Page