1. SO441 Lesson 10: Tornadoes
Week 15

2. What exactly causes tornadoes to form?
We know the ingredients that cause supercell thunderstorms to form:
Conditionally unstable air
Strong vertical wind shear (both speed & directional shear)
Lifting mechanism: a dryline or warm front
And this “trigger mechanism” must be enough to overcome the capping inversion (region where temperature increases w/height) that typically exists ~1000 m above ground
What seems to be missing in the ~90% of supercells that do not produce tornadoes?
Low-level (near-surface) vorticity seems to hold the key!
Let’s examine how tilting leads to the formation of the rotating mesocyclone, and how “baroclinic” generation of vorticity at the surface leads to a possible tornado

3. Markowski and Richardson (2013)
Step 1: form a rotating supercell: the updraft tilts environmental vorticity in the vertical
Need to mention how tilting by updraft alone can’t produce vertical vorticity at surface. A downdraft is key, and there’s more vorticity generated by the downdraft itself. Be sure to explain that storms have both updrafts and downdrafts…point out that they’re all familiar with downdrafts whether they know it or not (they all have noticed the wind shift and T drop as they watch a storm roll in on a summer day).
streamwise vorticity 3

4. What is the process of supercell thunderstorm formation?
TILTING &
STRETCHING
“Horizontal roll”
Image from Meteorology Today
Low-level wind shear causes large areas of air to slowly turn. The thunderstorm updraft “tilts” this area of rotation, and it becomes the mesocyclone (rotating updraft in a supercell thunderstorm).

5. Now we have rotation at the surface but not necessarily a tornado
Markowski and Richardson (2013)
Step 2: Generate low-level vorticity via difference in temperatures between the warm inflow and the cool downdraft
Need to mention how tilting by updraft alone can’t produce vertical vorticity at surface. A downdraft is key, and there’s more vorticity generated by the downdraft itself. Be sure to explain that storms have both updrafts and downdrafts…point out that they’re all familiar with downdrafts whether they know it or not (they all have noticed the wind shift and T drop as they watch a storm roll in on a summer day).
warm
streamwise vorticity
Now we have rotation at the surface but not necessarily a tornado
cool
Photo by Eric Nguyen 5

6. Step 3: contraction to tornado strength
This requires getting the circulation coincident with the updraft and lifting the outflow air. This is easier if the outflow is not too cold (linked to low LCL heights).
So we know that downdrafts are important. We can see downdrafts in the videos and in the tornado pictures we saw earlier. But even nontornadic stormshave downdrafts. Something else has to happen. The rotation still must be intensified via the figure-skater effect. We think the temperature of the downdraft air is important, as well as the sucking from above.
not too cold
Markowski and Richardson (2013)
too cold
Photo by Eric Nguyen

7. The importance of thunderstorm downdrafts to tornado formation has been hypothesized for 40+ years
FIG. 9. Fujita’s revised conceptual model of left-turn and right-turn tornadoes and their
relation to the mesocyclone [(a)–(c): reprinted from Weatherwise with permission of the
Helen Dwight Reid Educational Foundation (see acknowledgments)] (a) formation of leftand right-turn tornadoes relative to the mesocyclone (Fujita 1974)
From: Forbes, G. S., and Bluestein, H. B., 2001: Tornadoes, Tornadic Thunderstorms, and Photogrammetry: A Review of the Contributions by T. T. Fujita. Bull. Amer. Meteor. Soc., 82,

8. Some US tornado statistics and climatology
What spatial patterns do you notice in the plot of all (F0-F5) tornadoes? Any differences in the F1/F2 plot? What about F3/F4? Finally, what patterns do you see in the F5 plot?

9. Number of days per year with a tornado nearby (25 miles)
Source: Brooks et al. (2003) and Oklahoma Climatological Survey

10. Where do U.S. tornado fatalities occur?
(Ashley, WAF, 2007)

11. Where does this year (2017) rank with climatology?

12. US tornado statistics
Number of EF-1+ tornadoes has stayed fairly steady (with noted interannual variability) from
Number of EF-0+ tornadoes has nearly doubled since the 1950s. Why?

13. More US tornado statistics
What hour of the day is most likely to have a tornado? What hour is least? Why?
What month of the year is most likely to have a tornado? What month is least? Why?

15. More US severe statistics: # of tornado and severe thunderstorm watches per year
Number of severe thunderstorm watches issued per year by the NOAA Storm Prediction Center
Number of tornado watches issued per year by the NOAA Storm Prediction Center

16. More U.S. tornado statistics
Note: while the number of days with at least one tornado has increased in the last 80 years (to where a tornado occurs on ~ 200 days, or ~ 55%, of the year), the annual number of people killed by tornadoes has declined, especially since 1940.

17. Global tornado statistics: Tornadoes around the world
From: Forbes, G. S., and Bluestein, H. B., 2001: Tornadoes, Tornadic Thunderstorms, and Photogrammetry: A Review of the Contributions by T. T. Fujita. Bull. Amer. Meteor. Soc., 82,

18. Source:
The Onion