Case 14: 17 - 18 October 1996
Jetstream and Cross Sections
Objectives
- Gain familiarity with cross sections through fronts
- Gain deeper understanding of how thermal wind relationship affects cyclone structure.
Plots
- Create a loop of the 300 hPa heights and isotachs using the Eta model output every six hours for the period 12 UTC 16 October through 00 UTC 18 October. Use the times F000 and F006.
Questions
1. For the first time period (12 UTC 16 October), describe the locations and strengths of the jet(s) in relation to the upper level trough situated over Idaho. For the last time period (00 UTC 18 October), describe the locations and strengths of the jet(s) in relation to the upper level trough situated over Nebraska and Kansas. Has the jet(s) strengthened or weakened over time?
Plots
- Clear screen.
- Create a single map of 300 mb geopotential heights and isotachs, and sea level pressure for 18 UTC 17 October (using the 6 hour forecast field from the 12 UTC model run).
- Overlay the locations of the upper level stations by selecting the upper air data button. Select the station layout button and turn off all the options EXCEPT station identifier and wind. Select the 00 UTC 18 October time period (it really doesn't matter which time period we use since we are not using the upper air data, just the station identifiers and the locations as shown by the wind barbs) and plot the data.
Questions
2. You will be creating two cross sections, one from North Platte, Nebraska (LBF) to Wilmington, Ohio (ILM\N) and the other from Dodge City, Kansas (DDC) to Nashville, Tennessee (BNA). Describe the locations of these two cross sections with respect to the horizontal features (i.e. does the cross section intersect any fronts, surface lows, jet streaks, etc.).
Plots
- Select the model cross section button and select the time 18 UTC 17 October.
- Have the end points of the cross section be: lbf>iln (i.e. from North Platte to Wilmington).
- Plot potential temperature (theta_k) using a 2.5K contour interval. Plot the vector, geostrophic wind (GeoWind) using windbarbs.
- Plot the scalar product, Scalar_normal_wind. Edit the phrase NORM(obs) to read NORM(geo), and change the contour interval to be 10.
- Now the cross section should include: potential temperature every 2.5K, profiles of geostrophic wind and isotachs every 10 m s-1 of the normal component of the geostrophic wind.
Questions
3. Locate regions of cold and/or warm advection using the profiles of the geostrophic wind and locate any fronts.
4. Locate any regions of moderate to strong horizontal temperature gradients in the lower troposphere (below 500 mb or so).
5. Relate the structure of the jet (i.e. strength and location of the jet and the location of the strongest shear) to the horizontal temperature gradients. Using the thermal wind relationship as your guide, is the strength and location of the jet similar to, or different from your expectations.
Plots
- Clear screen and create a different cross section from Dodge City to Nashville: ddc>bna.
- Using the instructions above, plot the same three parameters, potential temperature, geostrophic wind barbs and isotachs of the normal component of the geostrophic wind.
Questions
6. Locate the regions of cold and/or warm advection using the profiles of the geostrophic wind and locate any fronts.
7. Relate the structure of the jet to the horizontal temperature gradients. How does the jet in this cross section compare to the jet in the previous cross section (strength, shear, etc.)? Explain any similarities/differences.
Main Points
- Regions of warm/cold advections are evident in cross-sections as veering or backing of the wind with height.
- Jet strength is directly related to the strength of the horizontal
temperature gradient.
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