Case 23: 8 - 11 November 1998

Case 23: 8 - 11 November 1998

Lab Exercises using Garp exploring the Upper Level Front of Nov 1998

Note: I plan to enhance these labs with example imagery pertaining to each set of questions

Objectives

Become familiar with the structure of an upper level front
Explore possible mechanisms for its formation and maintenance.

Background

Upper level fronts are a zone of quasi-horizontal strong temperature gradient and high static stability extending down from the tropopause. They are also known as tropopause folds. Within the frontal zone, the air has stratospheric properties such as high potential vorticity and high ozone. They often form in an environment of cold advection and can be initiated by differential vertical motion with the maximum downward motion within the frontal zone. Excellent review articles on upper level fronts are by Keyser and Shapiro (1986) and Shapiro and Keyser (1989).

Laboratory Exercise
Garp Instructions -- Horizontal View

Start garp for the Nov 98 case.
Select the model plan projection icon, and the time 981110/0000F000
Make a map of the 500 mb flow, the sea level pressure and the 250 mb isotachs (it looks nice to use color fill for this parameter).

Questions

1. You will be creating a cross section along 106ûW from 40ûN to 28ûN which intersects New Mexico. Use the cursor or add a lat/lon grid (select options and choose lat/lon grid) to see exactly where this cross section will be. Describe briefly where this cross section intersects the baroclinic system (i.e. through the low?, through a upper level jet?, through a cold/warm front, etc.). Use the map you created and the surface map provided to help answer this question.

Garp Instructions -- Cross Section

Select the model cross-section icon. From the pop-up window, select the "more" pop-up window. You will be making changes in both these windows to create your cross-section.
In the upper left corner of the model cross-section window, input the end points of the cross section exactly as follows: 40;-106>28;-106.
Select the time 981110/0000F000
Select the parameter Theta_K and in the more window, change the interval from 5 to 2.5. Then select display back in the first window.
Select the parameter PotVort_Ks-1P-1 (potential vorticity) and in the box below change the variable from TMPK to THTA and in the more window, change the interval from 20 to 0.5, and change the Scale to 6, then display.
Select the parameter Scalar_normal_wind and in the box below change "obs" to "geo" and in the more window, change the interval to 10, then display this variable (the normal component of the geostrophic wind).
Select the parameter "GeoWind" under "Vector" and then in the more window, change the symbol type from Arrow (other) to Barb (knots), then display.
Select the parameter Òomega_mbs-1Ó, then display
Now your cross section looks like a complete mess, BUT, you can temporarily take off any of the fields by clicking on the title bars on the bottom of the parameters you want to toggle off.

Questions

2. Using the theta and the normal component of the wind parameters, compare your cross section to the cross section of an upper level front given at the end of this write-up of Shapiro's April 1976 case. Is our case structurally similar in terms of the sloping of the isentropes, the location of the jet core and the cyclonic shear? Now overlay the potential vorticity. To what pressure level does high values of potential vorticity (i.e. greater than 1 PVU) extend?

3. Now look at theta and the geostrophic wind vectors together. Are there any regions of cold/warm advection in the cross section. Where are they in relation to the upper level front? Using the surface map provided and your cross section, does the cross section intersect the surface cold front? If so, where is it relative to the upper level front (i.e., is the cold front connected with the upper level front? ahead? behind? underneath, nowhere?)

4. Now look at omega and theta. Remember that negative numbers correspond to upward motion. Is the upper level front in an environment of rising or sinking motion? Does the omega pattern appear frontogenetical or frontolytical with respect to the upper level front?

Garp Instructions -- Model and Real Soundings

Now select the upper air icon, and in the pop-up window select ÒverticalÓ for sounding. Select the time 981110/0000 and the station Santa Teresa, NM (EPZ). Select Display and the sounding will be plotted. This station is at 32ûN 106.7ûW. On our cross-section, each tick mark at the bottom is 0.75 degrees lat, so that this station is about 1/3 the way from the right on the cross-section.

Questions

5. Eventhough the winds are missing above 600 mb, determine the layer (in pressure) that defines the upper level front.

6. Now select the model vertical profile icon, and create a sounding. Select the position 32;-106, select the correct time, 00 UTC 10 Nov, and display the sounding. Either overlay this sounding on the previous, or clear the screen and plot it. In this model sounding, where is the layer that defines the upper level front.

7. Compare your estimate of the width of the upper level front at EPZ to the model sounding and/or model cross-section at this same location. Does the model do an adequate job of defining the upper level front, or is it too coarse or too broad compared to the observation?

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