Radar

Ø  Radar (radio detection and ranging):

̶        Sends out a signal .95/.96 degree beam width (older radar had a 2.0 degree beam width) that are trying to detect precipitation when it turns itself off to listen for an echo.

̶        Radar hits a raindrop, when it hits the raindrop the radar beam scatters, some of it comes back

̶        Not able to resolve/resolution the details when the storms are far away with one beam but when you are close and you can distinguish with more than one beam

̶        Decreased beam width=decreased resolution.

̶        Parabola dish.

Speed of light * time = distance ÷ 2

̶        Beam: Arches/normal refraction (not linear) – the way density changes with height is what makes the radar arch more or less

̶        Smaller pulse length = better resolution = better sensitivity

Low-level Rotation and Storm Top Divergence

̶        Each box is at different heights in the atmosphere happening at the same time

1.    Shows rotation

2.    ____

3.    Rotation, a little divergence, Strong asimuthial shear, faster in, faster out – TVS

4.    Radial shear, divergence

̶        Radial Shear: Traveling along a radius (convergence or divergence)

̶        Radial Velocity: component of the actual velocity moving towards or away from the radar

̶        Azimuth angle: the angle the radar is at

̶        Volume Coverage Pattern (VCP): Telling the radar how to operate (scanning at a certain height, then raise the beam and scan again, then raise the beam and scan again…)

̶        Algorithms: algorithms that tells the radar to do something

Ø  Azimuth Resolution Considerations

̶        The further something is from the radar the less intense the rotation may seem

̶        When the tornado is too small or too far from the radar = resolution is not fine enough = get a closer radar or a smaller beam

̶        You have to use other things that the tornado is there, like the mesocyclone

̶        FAR – False Alarm Ratio – Better to have a false alarm then no alarm when a tornado is there

̶        An event is warned for but does not occur results in a false alarm

̶        POD – Probability of Detection -  Ratio of how many times I got a warning to the amount of events (Something is there and I have warning out for it)

̶        An event that occurs and has been warned for results in a 100% POD

̶        We want FAR down (40%) and POD up (70%)

̶        FAR and POD can be used for either Thunderstorms or Tornados

̶        CSI – Critical Success Index

Ø  SRV vs. Base Velocity with Subtle Rotation

̶        When I want to know if there is damaging winds on the ground or rotation, what’s going on, on the ground?

Base Velocity	Storm Relative
When diagnosing straight line winds use base velocity	When diagnosing rotation, use storm relative velocity
The strength of an advancing line of storms producing straight line winds is the sum of the winds produced by the storms, plus the movement of the storms.	SRV subtracts out the motion of a storm to display pure rotational characteristics of that storm.
How fast winds actually are	How fast something is rotating
Use: Actual Wind	Use: Rotation

̶        Rmax (unambiguous [clear] range): The furthest distance the beam can travel away from the dish and back before the next beam is sent out – c / (2 * PRF)

̶        Range Folding: Radar displaying and echo 1 Rmax closer (ping pong balls)

̶        PRF (pulse repetition frequency): how often a pulse is sent out

̶        Vmax (velocity Interval):

o   The faster something is moving, then I need more samples to measure it accurately (PRF)

o   As Vmax goes up, Rmax goes down (Doppler Dilemma)

̶        Doppler Dilemma: There is no single PRF that maximizes both Rmax and Vmax

o   High PRF’s = short unambiguous ranges and vice versa

o   Low PRF’s = velocity aliasing and vice versa

̶        How does a Doppler radar determine if an object is going towards or away from the radar?

o   The shift in frequency determines whether an object is moving toward or away from the radar

o   Frequency of what? Wave length/Radio waves

̶        Red Shift: Everything is moving away from each other (Big Bang)

̶        Bigger Rmax = Less Range Folding

o   How do I get a large Rmax?

̶        Aliasing:  Bad velocity data (wall paper example) “fold over”

̶        Isodop: “S” shaped, winds are veering with height (hurricane Katrina)

̶        Veering (VW – Veering warm): Turning clockwise with height

̶        Backing (BC – Backing Cold): Counter-clockwise

̶        TBSS (Three-Body Scatter Spike):

o   Beam hits the stone = some comes back, some scatters = hits ground, comes back to hail = returns to radar

o   Radar thinks it’s further away because it takes longer to return (flare)

o   Best indication of large hail (~1.5in. diameter hail)

̶        Stone: Hail stone

̶        Flare: nothing is there, not real

̶        AP (Anomalous Propogation): a low-level inversion created by the cold pool results in superfraction and thus AP

Ø  Reflectivity:

̶        dB: decibel – 10 log (power returned / reflected power)

̶        Hail: Big raindrop on the radar

̶        m: Milliwatt (thousandth)

̶        D: Change in diameter

̶        Z:

o   is reflectivity (of a single raindrop)

o   = D^6 (64 times more power back)

o   Proportional to D⁶

̶        Log: 10^0=1   -   Log1=0

̶        Reference Power: The amount of power you get back with a 1millimeter raindrop per 1cubic meter space – changes based on the distance to the radar

̶        dBZ: decibel of recent activity/reflectivity – 10 log (power / reflected power)

o   0 dBZ = 0

̶        dBm: measuring the power of the return – 10 log (power returned / 1 milliwatt)

o   0 dBm = 1 Milliwatt

̶        Size of raindrops determine if there will be a tornado

Every time I double the power, I add 3: Doubling of power results in a linear increase of 3dBZ

̶        Sends out a signal (750,000 watts)

̶        Double the size of a raindrop = gives back D⁶

   30dBZ

+ 3(six times)

   48dBZ

Ø  VCP (Volume Coverage Patterns)

̶        Clear Air Mode:

o   Want to be sensitive

o   Longer pulses = Higher sensitivity

Ø  Doppler

̶        Radial Velocity: Velocity toward or away from the radar, shifts in the frequency

o   Toward: higher frequency

o   Away: lower frequency

̶        Frequency of Sound: pitch

̶        Doppler Shift/Effect: frequency of radar energy caused by the movement of precipitation or other objects in the radar beam toward or away from the radar

̶        Vector in the Components: (ping pong balls with someone walking)

Ø  NIDS - Nexrad Information Dissemination Service

̶        Dissemination: Distribute information

̶        Base Velocity (storm relative):

o   Various elevation angles

o   When diagnosing Straight Line Winds (bow echo, derecho, microburst’s)

̶        VIL (Vertically Integrated Liquid): Sum up how much liquid is in a storm

̶        VWP (VAD Wind Profile)

̶        VAD (Velocity Azimuth Display)

̶        Composite Reflectivity: “here’s the big storms” – all on the same display, doesn’t care where in the storm is strong or weak

̶        Radial Velocity: Velocity toward or away from the radar, shifts in the frequency

o   Toward: higher frequency

o   Away: lower frequency

Ø  Interpreting Doppler Radar

̶        Zero Isodop: winds are perpendicular from green to red, looks like an “S” on the wind display, veering winds with height = warm air advection = rising air

̶        Veering: Clockwise shifting

̶        Advection: Horizontal movement of air

Ø  Backwards “S”: backing winds with height = cold air advection = sinking air (subsidence)

Ø  Blow from green to red

Ø  Duel-Polarization Radars

̶        http://wdtb.noaa.gov/courses/dualpol/outreach/non-mets-intro/player.html

̶        Hole = debris from tornado

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