Case 23: 8 - 11 November 1998
Lab Exercises Using Garp to Explore General Structure
Note: I plan to enhance these labs with example imagery pertaining
to each set of questions
Garp Instructions
- Start the November 1998 case using garp.
- 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-104 model and the time period 12 UTC 9 Nov - 00 UTC 11 Nov using the initial and 6 hour forecasts (i.e. F000 and F006).
- Plot the 500 hPa heights, sea level pressure and 1000 - 500 mb thickness.
- Suggestions for "nicer" plots -- zoom into the area of interest, which should be from the west coast to most of the east coast and from southern Texas to southern Canada, and plot thickness first using the color fill option, then plot the other fields.
Questions
1. Compare your loop of heights, slp and thicknesses to the
idealized figure (e.g. figure 3.22 in Wallace and Hobbs). Does the November 1998 case closely resemble the idealized picture? Which time periods of the November case 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 different from the ideal picture?
Garp Instructions
- Clear Screen (clear not reset, so you can keep the same area)
- Now plot 500 mb heights, 500 mb absolute vorticity (AvorObs_s-1), and sea level pressure for the same time period.
Questions
3. Identify the period of most rapid development (i.e. period when the surface low deepened most rapidly). Describe the relative position of the surface low to the upper level vorticity maximum and/or the positive vorticity advection maximum throughout this period of rapid development
4. At the end of the loop (18 UTC 10 Nov and 00 UTC 11 Nov) the surface low and the upper level low are co-located. Describe the relative position of the surface low to the upper level vorticity maximum and/or the positive vorticity advection maximum at these times. Does the surface low continue to deepen? Why or why not?
Garp Instructions
- Clear the screen.
- Now plot 300 mb heights, 300 mb isotachs (in knots, and color fill helps readibility), and sea level pressure.
Questions
5. Describe the position of the surface low with respect to the upper level jet. Is the surface low deepening in a region of preferred development?
Main Points
- During the developing stages, the trough tilts westward with height. When the storm matures and begins to decay, the surface low and upper level trough are vertically stacked.
- In the traditional q-g framework (i.e. Chapter 6 of Holton), positive absolute vorticity advection forces upward motion and spin-up of a surface cyclone. A deepening surface low increases thermal advections which then increase the amplitude of an upper-level wave. When the surface cyclone and upper level trough are no longer in favorable position, the cyclone stops developing and begins to decay.
- Secondary circulation in association with an upper level jetstreak
forces upward motion and spin-up of cyclones in the right-entrance and
left-exit regions of a jetstreak (i.e. Uccellini and Johnson, 1985 or
Sanders and Hoskins, 1990).
Back to Nov 98 Overview Page