17 - 18 October 1996

Midwest Cold Season Synoptic Storm

Laboratory Exercises

The following laboratory exercises are meant as guidelines for instructors for using this case in meteorology courses teaching basic structure and development of midlatitude synoptic scale storms. Please use or modify them to suit your needs.

They are written assuming the student has access to the gridded and observed fields for this case and is able to display and loop the fields as described below. The following instructions are written as if the student is running the interactive program, GARP, written and maintained by COMET at UCAR. We find that having students create the fields themselves is a major part of the student's learning experience, and we would encourage instructors to use the capabilities of programs like GARP (or others such as GEMPAK, or GRADS) when using these exercises. However, if this is not possible, we have provided links to the pre-created fields so that these exercises could be completed here on the web.

The exercises include the following topics:

• Basic Cyclone Structure

• Geostrophic vs. Observed Winds

• Thermal Wind and the Jetstream

• Quasi-Geostrophic Diagnostics

Basic Cyclone Structure

Goals

• Compare the life cycle of a real case with the idealized case
• Be able to describe the intensity of a real case compared to the idealized case

Instructions

1. Create a loop of 500 mb heights, sea level pressure, and 1000-500 mb thicknesses overlayed every 6 hours using the ETA model grids using the following times: 00 UTC October 17 (in GARP select the time period: 96101700F00), 06 UTC October 17 (96101700F06), 12 UTC 17 October (96101712F00), 18 UTC 17 October (96101712F06), and 00 UTC 18 October (96101800F00).
2. Compare these images to the idealized picture of the development of a typical baroclinic cyclone given in the figure reproduced below from Holton, 1992.

Questions

1. Identify which time periods of the October case most closely resemble each of the three stages shown in the figure from Holton.
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 Holton?
3. Describe qualitatively the relative strengths of the low level fronts (as illustrated with the 1000 - 500 mb thicknesses) as the cyclone develops. Are the intensities similar to or different from the ideal case?
4. Would you describe the October case as (a) near ideal, (b) somewhat ideal, or (c) completely different from ideal?

Goals

• Compare geostrophic and observed winds and explain similarities/differences

Instructions -- Surface Winds

1. Create a map that includes the mean sea level pressure, the geostrophic winds from the ETA model at 1000mb and the observed winds (from station observations) for 00 UTC 17 October 1996. Have the map include at least the midwestern states of Kansas and Missouri.
2. Create a different map of the same three parameters above (slp, geostrophic and observed winds) for 06 UTC 17 October (use the 6 hour forecast of the ETA model from the 00 UTC 17 October model run), but of the area over the northern Pacific ocean.
3. Create a third map of the same three parameters above (slp, 1000mb geostrophic winds and observed surface winds) for 06 UTC 17 October, but of the area of the intermountain region including the states of Nevada, Utah and Colorado.

Questions -- Surface Winds

1. Estimate the average percentage difference in windspeed between the observed winds and the 1000 mb geostrophic winds in the region including Missouri, Kansas and Arkansas (use the first map). Compare this with the average percentage difference in windspeed between the observed winds and the 1000 mb geostrophic winds over the Gulf of Alaska in the vicinity of the surface low pressure center (second map -- use all the ship reports inside the 1012 mb contour).
2. Compare the average difference in wind direction for the reports over the ocean (map 2) and the reports over the land (map 1). Are these differences between the geostrophic wind and the observed wind similar to what you expected? Explain.
3. There is a strong pressure gradient across Colorado, Utah and Nevada with very strong geostrophic winds (most winds around 40 knots). However, the observed winds in this region are much lighter and often from a completely different direction. What are the factors contributing to these large differences in speed and direction? Is comparing 1000 mb geostrophic winds to observed surface winds a fair comparison in this region at this time?

Instructions -- Winds aloft

Create a map of the 300 mb geopotential height, geostrophic wind and observed winds for 12 UTC 18 October. Include the region from Montana to western Pennsylvania.

Questions -- Winds aloft

1. What are the observed and geostrophic windspeeds at the following locations: Springfield MO (SGF), Little Rock AR (LZK), Glasgow MT (GGW), Bismard ND (BTS), and Amarillo TX (AMA).
2. Explain why the observed wind is less than the geostrophic wind at some locations and greater at others.

Goals

• Become familar with the vertical structure in a developing cyclone
• Be able to identify regions of warm and cold advection using the wind barbs in cross sections.
• Relate the strength of the jetstream to the strength of the horizontal temperature gradient using real data.

Instructions -- jet evolution

1. Create a loop of the 300 mb heights and isotachs using the ETA model output for the period of 12 UTC 16 October through 00 UTC 18 October (map1, map2, map3, map4, map5, map6,map7.
2. Overlay the temperature at 850 mb on the maps of the loop (map1, map2, map3, map4, map5, map6, map7).

Questions -- jet evolution

1. Describe the locations and strengths of the jet(s) in relation to the upper level trough situated over Idaho for the first timjet_300mb_temp_850mb01.gife period (12 UTC 16 October) and over Nebraska and Kansas for the last time period (00 UTC 18 October). Have the jet(s) strengthened or weakened over time?
2. Describe the evolution of the jet east of the trough over time. Discuss the role thermal wind may have had in contributing to the increase of the strength of this jet.

Instructions -- cross sections

1. Create a cross section from 1000 mb to 200 mb from North Platte, Nebraska (LBF) to Wilmington, Ohio (ILN) that includes potential temperature every 2.5K, geostrophic wind barbs and isotachs every 10 m s-1 of the normal component of the geostrophic wind for the time 18 UTC 17 October 1996.
2. Create a different cross section of the same three parameters above from Dodge City, Kansas (DDC) to Nashville, Tenessee (BNA) for the same time.
3. Create a map of 300 mb heights and isotachs, and a second map of 850 mb heights and temperatures for the 18 UTC 17 Oct to give some perspective to the vertical cross sections.
4. Create a third cross section along 92W from 42N to 55N for 12 UTC 17 October 96. Include the same three paramenters (potential temperature, geostrophic wind barbs and isotachs of the normal component of the geostrophic wind) as before.

Questions -- cross sections

1. For each cross section,
   • Indicate regions of significant cold and/or warm air advections (use the turning of the wind)
   • If present, state type of front and sketch it's location on the cross section up to about 700 mb.
2. For the first two cross sections, 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. Is the observed structure in agreement with what is predicted by the thermal wind relationship?

Instructions

1. Create a series of maps valid at 12 UTC 17 October 1996 of the forcing terms for geopotential tendency (eqn. 6.23 of Holton, 1992) as follows:
   • Geostrophic vorticity advection at 500 mb (term B in eqn. 6.23) with 500 mb heights. Negative values mean lowering heights and positive values mean rising heights.
   • Differential thickness advection (term C in eqn. 6.23) where the upper level is the thickness between 200 mb and 400 mb and the lower level is the thickness between 600 mb and 800 mb.
   • Total forcing (the two terms above together) with 500 mb heights at both 12 UTC and 18 UTC.
2. Create a series of maps valid at 18 UTC 17 October 1996 of upward motion as follows:
   • Divergence of Q (see equation 6.35 and 6.36 of Holton) at 500 mb, the 500 mb heights and model derived vertical motion. The divergence of q will have units of order e-17 and are shaded where red colors mean upward motion and blue colors are downward motion. The model field is omega so that negative values mean upward motion and positive values downward motion.
   • Satellite image + DivQ
   • Satellite image + model produced upward motion

Questions

1. Within the main trough in the midwest, there are two shortwaves, one over eastern Colorado/western Kansas and a weak one over Iowa at 12 UTC 17 October. Describe where the first forcing term shows rising or lowering heights with respect to these short waves only (use the first map).
2. Describe where the second forcing term of the geopotential height tendency (second map) also shows rising or lowering heights in association with the short waves.
3. Compare the total forcing of geopotential tendency (third map ) to the heights at 18UTC (blue contours on the map). How well does the equation predict the heights at the later time?
4. How well does the divergence of Q at 500 mb relate to the model derived vertical motion (fourth map )? Does the quasi-geostrophic forcing of vertical motion capture all the major areas of upward and downward motion?
5. How well do the divergence of Q and the model produced upward motion relate to the cloudy regions on the satellite image (fifth and sixth maps)? Do the two methods capture the major areas of cloudiness and dry areas (such as the dry slot behind the cold front and the comma head)? If there are any regions where the divergence of Q does NOT explain the cloud features, give two or three reasons why this might be the case.

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