This lab is an introduction to locating and interpreting Jet Streams and Jet Streaks.
You should carefully read Chapter 4 in Vasquez, and pay particular attention to the section on jets, pages 62 to 68, and the section on jet streaks, pages 75 and 76.
You have now had some experience with analyzing weather maps and drawing isolines. For this lab, you need to take your time, increase your accuracy, and draw your map features neatly.
The jet stream is located at high levels in the atmosphere -- usually at or between 300mb and 200mb. A cross section is often the easiest way to locate the level of the jet stream in a particular region.
For this lab, you will analyze a 300mb chart. On the 300mb chart, when plotting geopotential heights, the last zero is customarily dropped, as with the 500mb chart. It is also common to analyze the wind speed using contours of equal wind speed, called Isotachs. These contours traditionally plotted every 20 knots, starting with 70 knots (contour lines less than 70 knots are often not plotted).
The jet stream can be seen as an elongated ribbon of higher winds. A jet streak is defined as a closed isotach within the jet stream -- in other words, a localized maximum in the wind speed. The center of the jet streak is at the highest wind speed.
Jet streaks are analyzed using the quadrant method. To obtain the quadrants, identify the point of the highest wind speed within one or more closed isotachs. Then draw a line perpendicular to the wind flow that crosses the point of highest wind, and extends in both directions to the lowest valued closed isotach. This line is typically not curved. Then draw a line that extends from the point of highest wind speed that runs paralled to the wind flow, and extends both directions to the lowest valued closed isotach. This line is curved along the wind flow and passes along the axis of higher wind speed.
For identifying which parts of the quadrants indicate divergence, and which indicate convergence, see the diagram on page 67 in Vasquez, which shows the idealized "model" of a jet streak. Also see the sample plot later on this web page.
It is also customary to draw streamlines on the 300mb chart, instead of height contours. Streamlines are lines that are parallel to the wind flow at every point. The spacing between streamlines is arbitrary, and does not signify anything in particular (unlike the spacing between contour lines, which indicates the gradient).
1) Draw isotachs, starting at 70 knots, at every 20 knots. Eg: 70 knots, 90 knots, 110 knots, etc.
2) Draw streamlines on the 300mb chart. Look closely at the sample plot below to see how this is done. It is not necessary to attempt to draw streamlines in areas of low wind, where the wind speed is less than 20 knots.
3) Identify jet streaks on the map. Draw the lines that show the jet streak quadrants, and label each quadrant with DIV for divergence, and CON for convergence.
The radiosonde reports are represented by station model plots. Remember that the various values associated with a station model are valid at the location of the observation , which is at the circle in the middle of the station model! Do not get confused and think the value of the wind velocity is located where the flag is! The value is located at the small circle that is at the base of the flag.
The map you will analyze is linked HERE.
It can be seen, from a careful analysis of the data, that there are two separate jet streaks in the larger jet stream over the south central US. (The computer analysis of the data suggests this as well.)
Below is my own analysis of the 300mb chart that is 12 hours prior to the chart you are analyzing. Use this as a guide for how to analyze your own chart.
In addition, provided below are computer produced plots of the isotachs and streamlines for this case.
HOWEVER: you are to do your own analysis from the raw radiosonde reports, as plotted on the provided map. While the computer plots will be fairly close to what you want, they are not entirely correct for this situation.
The first plot is the computer (objective) analysis for 00Z. The 2nd plot is my own analysis for 00Z. The 3rd plot is the computer (objective) analysis for 12Z. The 12Z case is the one you are analyzing.
