Tornadoes

Ø  Tornado Formation in Supercells

̶        Tornadoes are violently rotating columns of air that extend from a thunderstorm cloud to the ground

̶        First indications may be a funnel cloud or a gust swirl on the ground

̶        Supercell thunderstorms rotate around a vertical axis because of a process called  Vortex Tilting

̶        Mesocyclone: rotating updraft part of a storm circulation

̶        Vortex stretching is required to concentrate the mesocylone rotation

̶        Conservation of angular momentum (ice skater spin)

̶        Tornadogenesis: the formation of a tornado. Believed to occur in 3 steps:

1)    Mid-level Mesocyclone: tilting of the vertical wind shear that causes the storm’s updraft to rotate

2)    Mid-level Mesocyclone: Tilting of the horizontal circulation generated along the forward-flank gust front

3)    Low-level rotation: associated with the development of the wall cloud, develop rotation at the ground. Three (Tornadogenesis) mechanism have been proposed for how this process occurs:

a) Bottom-up process: typically occurs near the time that the supercell’s rear-flank downdraft moves under the mesocyclone - believed to be the most common

b)    Top-down process: Tornado descends from mid-levels within the low-level mesocyclone and then emerges from the base of the wall cloud (dynamic pipe effect)

c)    Based on a study of the Garden City, Kansas, tornado during VORTEX. Analysis showed that similar process occurred in the mesocyclone during tornadogenesis. Tornado developed as the central downdraft occurred within the mesocyclone merged with rotating air in the outer part of the surface mesocyclone

̶        Occlusion Downdraft: downdraft occurring in the vicinity of the mesocyclone

̶        Tornadoes may be on the ground from a few minutes to as long as hours

̶        Tornado vortex can stretch over a kilometer horizontally across the sky

̶        Tornado Family: tornadoes emerging from the supercell over its lifetime

Ø  Tornado Formation within Non-Supercell Thunderstorms

̶        Tornadoes sometimes develop within squall-line thunderstorms aligned along fronts, along outflows from mesoscale convective systems (MMSs), or even in thunderstorms aligned along the sea-breeze front (particularly in Florida)

̶        Sometimes called: non-supercell tornadoes, landspout tornadoes, waterspouts, mesovortices, or gustnadoes

̶        Landspout tornadoes: generally short-lived and not as intense as their supercell tornado counterparts (still dangerous though)

̶        Waterspouts: A class of tornadoes that are commonly observed off coastlines, believed to  associated with a spin up of circulations created by breakdown of the flow in regions of low-level horizontal wind shear (like landspout tornadoes)

Ø  Tornado Statistics

̶        Tornadoes occur most frequently over the Great Plains and Midwestern states, oriented along a southwest-northeast region (Tornado Alley)

̶        Only about 25% of all tornadoes occur outside the U.S.

̶        Fujita Scale/F-Scale (developed by Dr. Theodore Fujita in 1971) was based on the damage cause by a tornado and served as a measure of tornado intensity

̶        F-Scale Weaknesses: overestimated the winds in the more violent tornadoes, educated guesses, and did not account for differences in construction techniques that are common in many structures

̶        Enhanced Fujita Scale(EF Scale): Created to more accurately rate the damage and winds associated with a tornado

Fujita Scale	3-Second Gust Speed (mph)	Operational Enhanced Fujita Scale	3 Second Gust Speed (mph)
F0	48-78	EF0	65-85
F1	79-117	EF1	86-110
F2	118-161	EF2	111-135
F3	162-209	EF3	136-165
F4	210-261	EF4	166-200
F5	262-317	EF5	>200

̶          Tornadoes occur every month of the year but occur most often in April, May, and June (often the best combinations for vertical wind shear and instability)

Ø  Tornado Detection

̶        Storm Spotters: volunteers who are developed at key locations around threatened cities during severe storm outbreaks, report dangerous weather conditions and tornado locations

̶        Prior to the Doppler radar, the Hook Echo was the only way to identify a possible tornado with radar

̶        Hook Echoes do not exist with non-supercell tornadoes, only about 25% of supercells exhibiting a hook echo will produce a tornado

̶        Doppler radar can measure the component of the wind that is moving toward or away from the radar

̶        Mesocyclone Signature: often a precursor to tornado formation

̶        Tornado Vortex Signature: a tiny area will show up on the screen with unusually large velocity next to the pulse volume with a large velocity in the opposite direction and marks the location of a tornado

Ø  Tornado Forecasting

̶        Impossible to forecast the precise location of a tornado will occur, identifying potentially tornadic storms are done routinely

̶        CAPE (convective available potential energy) measures how unstable the atmosphere is and now strong a thunderstorm’s updraft will be (kinetic energy  that buoyant air parcels will obtain as they rise through the atmosphere)

̶        Storm-Relative Helicity (SRH): measures the horizontal rotation in the lower atmosphere relative to the motion of a thunderstorm

̶        No clear SRH thresholds or “boundaries” between non-tornadic and significant tornadic supercells

Ø  Low level wind shear is strong

Ø  Supercells

̶        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

Ø  Characteristics of a Tornadic Wall Cloud

̶        Surface-based inflow

̶        Rapid vertical motion (scud-sucking)

̶        Persistent

̶        Persistent rotation

̶        The key, however, is the development of a clear slot

Ø  What’s the difference between the 2 green lines?

̶        One is further away, the other is closer

̶        When you see the clear slot there is occlusion

Ø  Clear Slot

̶        Depends how cold the air is for a tornado to form (A tornado will most likely form, not always)

̶        There is precipitation but we can’t see it because they are very small drops (evaporate a lot)

̶        The radar can see the few big drops (don’t evaporate that much – not as cold = air ingested into the tornado is not that cold)

̶        Too cold = no tornado

Ø  Where does the new wall cloud form?

̶        Triple point of the Mesoscale occlusion

     

Ø  What does a tornado NEED?

̶        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

Ø  DRC – Descending Reflectivity Core

̶        “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

̶        Associated with…

o   Locally stronger outflow

o   A gust front that surges

o   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.

̶        Not all tornadoes from with a DRC

Ø  Vortex Arches

̶        Describing the flow as having a vortex line arch is shorthand for saying…

̶        Cyclonic vortex to the left (north), looking down shear.

̶        Gust front trailing to the right (~south), with rising ahead and sinking behind.

̶        Anti-cyclonic vortex to the right (south).

̶        The combination of these three features is the kinematic signature of vortex line arching.

Ø  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).

̶        It is probable that the mesocyclone plays some role in allowing blob and RFD formation to occur.

o   Perhaps stagnation at the rear of the updraft, caused by the meso, allows precip to descend there and not be swept around the sides.

o   Perhaps the meso advects precip into a position where it can descend in a DRC.

o   Is there some magic combination of advection/descent?

o   The data are fairly convincing that tornado formation is not the result of mesocyclone rotation “somehow” developing down to the ground.

Ø  Low Base

̶        The Lifting Condensation Level (LCL) helps indicate the relative humidity of the sub-cloud layer

̶        High LCL’s indicate lower RH and more chance for microbursts

̶        Low LCL’s indicate more moisture in the sub-cloud layer

̶        Lower LCL storms have a higher tornado potential

Ø  HP Occlusion