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