SO441
Homework 1 (52 pts)
Name ___________________________
The objective of this homework assignment is to get you thinking about the ingredients for
severe convective storms. The answers to these questions can be found in a variety of places:
your class notes, the Rasmussen paper itself, and of course online. You are encouraged to work
together, but every student must turn in his/her own work. (Each question is worth 4 points.
1. Define “storm-relative helicity”.
2. Give an equation to compute the storm-relative helicity from 0-1 km, and again from 0-3 km.
In your equation, identify every variable and give its units.
3. Why is it important to include storm motion in the calculation of storm-relative helicity?
4. Using your equation from problem 2, show that the units of storm-relative helicity are m2 s-2.
5. What is Convective Available Potential Energy (CAPE)?
6. Give an equation to calculate CAPE. In your equation, identify every variable and give its
units.
7. Verify that the units in the equation you gave in #6 are J kg-1. Also verify that these units are
m2 s-2 (the same as helicity. Hmm. Interesting!)
For the remainder of these questions, read the paper: Refined Supercell and Tornado Forecast
Parameters, by Erik Rasmussen (Weather and Forecasting, 2003), available at
http://www.usna.edu/Users/oceano/barrett/rasmussen_paper.pdf
8. Explain the primary results in Figure 1 of Rasmussen (2003).
9. Assess the Fig. 3 in Rasmussen (2003). What are the main points conveyed in this figure?
How does the data in the figure relate to tornadogenesis?
10. Based on Table 2 in Rasmussen (2003), what are the most important parameters to
discriminate between supercell- and tornado-environments, and “ordinary” environments? What
parameters are least important? Of the top 3 parameters (those with highest Heidke skill score),
what is common among all three of them?
11. What is Heidke skill score? What does it measure? What is the equation for it?
12. What is the Lifting Condensation Level (LCL)? Also, how is it found from a sounding?
13. What do you notice about the geographic distribution and average LCL height for all
supercell- and tornado-producing thunderstorms (Fig. 7 in Rasmussen 2003).