Severe and Unusual Weather ESAS 1115

1. Severe and Unusual WeatherESAS 1115 Spotter Training and Radar Meteorology Part 6 – Tornadoes ESAS 1115 Severe and Unusual Weather

2. Supercell Tornadoes • Supercells most likely to produce tornadoes form where low-level wind shear is strong and cloud bases are relatively low • Most supercells do not produce tornadoes • Tornado production does not need mid-level circulations ESAS 1115 Severe and Unusual Weather

3. Gust Front Effects of a Mesocyclone ESAS 1115 Severe and Unusual Weather

4. Supercell Features and Schematic • A supercell, which has a mesocyclone, may undergo an occlusion process much like a synoptic scale cyclone • Cold air from the RFD wraps around the storm • This air must still have buoyancy and thus the air cannot be “too” cold • The occlusion process is marked visually by the appearance of the clear slot, perhaps the most important visual feature for storm spotters to understand • If the air is too cold, tornadogenesis may fail ESAS 1115 Severe and Unusual Weather

5. Tornado Occlusion Cool air from the RFD wraps around the low. Most likely it is now when the tornado will first form. The cooler air clears the cloud base resulting in the horseshoe updraft. The tornado will begin to “rope out” as it gets entrenched in the colder air. A new mesocyclone may form at the triple point. Triple point of the mesoscale occlusion A new mesocyclone (along with the wall cloud and tornado) may form along the triple point of the mesoscale occlusion leading to a cyclic supercell ESAS 1115 Severe and Unusual Weather

6. Clear Slot and Tornado ESAS 1115 Severe and Unusual Weather

7. The Clear Slot – Look for This! Horseshoe updraft Clear slot Preferred site for tornadogenesis ESAS 1115 Severe and Unusual Weather

8. First Clue • No wall cloud • No apparent rotation • Only visible clue was a rain free base • Small scud tags started showing upward motion • Clear slot and horseshoe updraft clearly visible Horseshoe Updraft Clear Slot ESAS 1115 Severe and Unusual Weather

9. Springfield, IL 1995 ESAS 1115 Severe and Unusual Weather

10. Less then 10 Minutes of Time ESAS 1115 Severe and Unusual Weather

11. Horseshoe Updraft ESAS 1115 Severe and Unusual Weather

12. In the Clear Slot • Different perspectives of the mesocyclone and supercell features can be very dififcult to interpret • This above tornado appears almost entirely in the clear air and away from the storm • The picture below is looking south from the forward flank and a clear slot is not apparent ESAS 1115 Severe and Unusual Weather

13. Clear Slot is Visible ESAS 1115 Severe and Unusual Weather

14. Incorrect Representation of Inflow ESAS 1115 Severe and Unusual Weather

15. What do Tornadoes Need*? *Controversy exists about the true necessary conditions! • Research hints at a necessary condition being an RFD (rear flank downdraft), at least for a supercell tornado • An RFD may occur and create a tornado without being associated with a supercell • Supercells are the most likely storms to produce RFD’s • Boundaries can play an important role ESAS 1115 Severe and Unusual Weather

16. RFD – Rear Flank Downdraft RFD air is brought back into the tornado updraft! Courtesy Paul Markowski ESAS 1115 Severe and Unusual Weather

17. Which Resulted in Tornadoes? N N N T T T ESAS 1115 Severe and Unusual Weather

18. Fujita’s Work from 1973 ESAS 1115 Severe and Unusual Weather

19. The DRC (aka The BLOB!) • “Blob Echo” -(defined in Rasmussen et. al. 2006)The descending reflectivity core, or DRC, is a protuberance of reflectivity that descends from the echo overhang in the right-rear flank of a supercell • To insure separation from the main precipitation core, the DRC must have a reflectivity 4dB greater than the path of maximum reflectivity along the appendage to the core. This 4dB requirement is an arbitrarily chosen value that appears to encapture most DRCs. • Pendant from supercell echo overhang aloft and descends with time ESAS 1115 Severe and Unusual Weather

20. Storm Drafts ESAS 1115 Severe and Unusual Weather

21. Enhanced Part of RFD • The descent of a DRC is associated with… • Locally stronger outflow; • A gust front that surges; • Counter-rotating vortices to the ground. • A locally intense downdraft embedded in the RFD is the key feature, and the DRC is associated with this downdraft. ESAS 1115 Severe and Unusual Weather

22. Examples of DRC Using Gibson Ridge software GR Analyst with reflectivity isosurfaces ESAS 1115 Severe and Unusual Weather

23. The DRC is Not the Tornado Strongest rotation north of DRC ESAS 1115 Severe and Unusual Weather

24. Let’s Take a Look at a Storm Courtesy Erik Rasmussen ESAS 1115 Severe and Unusual Weather

25. Let’s Take a Look at a Storm Horseshoe Updraft Precipitation Curtain – Hook Echo ESAS 1115 Severe and Unusual Weather

26. Two Areas of Rotation Mesocyclone Mesoanticyclone ESAS 1115 Severe and Unusual Weather

27. Clear Slot “Cuts into” Updraft Preferred site for tornadogenesis Clear Slot ESAS 1115 Severe and Unusual Weather

28. RFD and DRC • Many questions remain about the nature of the RFD and DRC • DRC is most likely a specialized feature of the RFD • Questions remain as to what kind of precipitation is generated and how it is generated • Big rain drops vs. many raindrops • Characteristic of Supercell (i.e. HP or LP) ESAS 1115 Severe and Unusual Weather

29. Vortex Arches • Describing the flow as having a vortex line arch is shorthand for saying… • Cyclonic vortex to the left (north), looking downshear. • Gust front trailing to the right (~south), with rising ahead and sinking behind. • Anticyclonic vortex to the right (south). • The combination of these three features is the kinematic signature of vortex line arching. ESAS 1115 Severe and Unusual Weather

30. Tornado Cyclone • The arching process does not consider the additional effects of ingesting streamwise vorticity • Without the arching, it is difficult to know how the tornado cyclone forms • The tornado is different than the tornado cyclone ESAS 1115 Severe and Unusual Weather

31. The Tornado Cyclone Forms Without Shear in the Environment at least in the simulation! ESAS 1115 Severe and Unusual Weather

32. Tornadoes with or without DRC • The DRC is an interesting feature that is not always visible on radar • Not all tornadoes from with a DRC • Do tornadoes need an RFD or a DRC within? ESAS 1115 Severe and Unusual Weather

33. Rotating Wall Cloud - No Tornado We used to wonder why storms like this did not produce a tornado ESAS 1115 Severe and Unusual Weather

34. According to Rasmussen, Markowski, Kennedy… • It is possible that the mesocyclone does not play a direct role in tornado formation. • It is possible that the mesocyclone mainly indicates that the low-level environment has a lot of shear, and if this shear is especially large near the ground, the environment can support a tornado as well (SRH augmenting the tornado cyclone). ESAS 1115 Severe and Unusual Weather

35. According to Rasmussen, Markowski, Kennedy… • It is probable that the mesocyclone plays some role in allowing blob and RFD formation to occur. • Perhaps stagnation at the rear of the updraft, caused by the meso, allows precip to descend there and not be swept around the sides. • Perhaps the meso advects precip into a position where it can descend in a DRC. • Is their some magic combination of advection/descent? • The data are fairly convincing that tornado formation is not the result of mesocyclone rotation “somehow” developing down to the ground. ESAS 1115 Severe and Unusual Weather

36. Non-Tornadic Supercell According to research, it appears that one of the key characteristics of an RFD in a tornadic storm is that the air in the RFD is not too cold and still fairly buoyant. ESAS 1115 Severe and Unusual Weather

37. Theta-v Plots in the RFD ESAS 1115 Severe and Unusual Weather

38. High-Based Storms High based storm implies a high dew point depression (T-Td) and a low relative humidity ESAS 1115 Severe and Unusual Weather

39. Tornadogenesis Whatever the cause, identifying the clear slot is one of the most useful clues to anticipating tornado formation… ESAS 1115 Severe and Unusual Weather

40. Tornado Birth and Death (Roping Out) ESAS 1115 Severe and Unusual Weather

41. Great Storm, No Tornado …and an important clue indicating tornadogenesis failure ESAS 1115 Severe and Unusual Weather

42. Landspout at Best No clear slot Too much cold outflow! http://www.tapestryweb.org/tornado/wizardoz.html ESAS 1115 Severe and Unusual Weather

43. HP Occlusion ESAS 1115 Severe and Unusual Weather

44. In The Bear’s Cage Looking South Looking South Probably a similar-looking echo to the Plainfield tornadic supercell Not a wall cloud but instead the shelf cloud along the rear flank gust front ESAS 1115 Severe and Unusual Weather

45. Location, Location, Location Looking North Looking Southwest ESAS 1115 Severe and Unusual Weather

46. Moderate to Strong Thunderstorm • Vertically-aligned reflectivity pattern with storm top directly on top of low-level reflectivity maximum • Updraft is tilted due to the presence of wind shear • Storm is in non-severe “multicell mode” ESAS 1115 Severe and Unusual Weather

47. Supercell vs. Regular Storms ESAS 1115 Severe and Unusual Weather

48. Strong Updraft – Severe Storm • As updraft becomes vertically erect, the precipitation pattern exhibits an elongation down the mean wind and a WER • There is a strong reflectivity gradient on the inflow side which is underneath the top • The storm is higher and more intense and exhibits deviant motion • Storm is most likely severe ESAS 1115 Severe and Unusual Weather

49. WER ESAS 1115 Severe and Unusual Weather

50. Deviant Motion ESAS 1115 Severe and Unusual Weather