Case 23: 8 - 11 November 1998
Laboratory Exercises using Garp to Explore Frontal Structure
Note: I plan to enhance these labs with example imagery pertaining
to each set of questions
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 10 Nov through 00 UTC 11 Nov 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
1. Sketch the position of the "model" fronts on the plots. (Note: I have the students make hard copies of these plots, but today, just look at the on the screen and see if you can distinguish where the model positions the fronts. I also have the students compare their hand analysis to the numerical depiction of the fronts and comment on the accuracy of the model).
2. Comment on the relative strengths of the fronts as the storm develops as measured by the strengths of the temperature gradients, i.e.: 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
- Clear Screen
- You will make a vertical cross-section of potential temperature, geostrophic windspeed normal to the cross-section and geostrophic wind barbs across the cold front at 12 UTC 10 Nov.
- Select the cross section end points as: 37;-100>37;-85.
- Select the 12 UTC 10 Nov time, F000.
- Select the parameter Theta_k and in the more window use a contour interval of 2.5.
- Select the parameter Geowind and in the more window change the symbol type to barbs.
- Select the parameter Scalar_normal _wind and in the box below change obs to geo.
Questions
3. Indicate regions of significant cold and/or warm air advections on the cross section using the turning of the winds with height. Sketch the location of the cold front on the cross section up to about 700 mb (again, I usually have students make a hard copy of this.)
4. How does the jet centered in the cross section relate to the horizontal temperature gradient evident in the cross-section?
Garp Instructions (vertical cross-section through a warm front -- optional)
- Clear Screen
- Now make a cross section through the warm front at 06 UTC 10 November.
- Select the end points: 35;-92>47:92.
- Plot the same three parameters as above except use ObsWind instead of Geowind for the windbarbs.
Questions
5. Locate the warm front in the cross section and indicate the regions of significant cold and warm air advections on the cross section.
Main Points
- Fronts are characterized by windshifts, a pressure trough and they lie on the warm side of a temperature gradient.
- In a vertical cross section, the frontal boundary is located at the top of the shear and at the top of a stable layer. Evidence of cold and warm advection are given by the turning of the wind with height.
- Thermal wind predicts that the geostrophic wind will increase with
height in a layer in proportion to the horizontal temperature gradient
within that layer. Therefore strong jets occur over strong horizontal
gradients of temperature.
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