Radar Exam 1

What does RADAR stand for? When/how/why was it developed?

Stands for Radio Detection and ranging. It was developed during WWII for military purposes after planes interrupted radio communication in 1934. The first storm was tracked in England on Feb. 20th 1941 and the first hook echo was identified in 1953. Most radars are pulsed radar systems.
Functions over many conditions; day or night, clouds/fog have little impact and works at both long and short ranges.

What are the functions of the radar system? How do meteorologist use it?

They detect/locate weather targets, measure reflectivity, identify rotation, and display distributions of weather targets. Mets use it for looking at storm structure, precipitation patterns and distributions, precipitation intensity, and storm development.

Describe the overall radar coverage area. What are some limitations/issues with the coverage area of lack of coverage area?

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What are Electromagnetic waves? How are they produced?

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What are the different types of electromagnetic waves?

-All travel at c=3x10^8 m/s
-E and B fields are perpendicular to each other
-E and B fields are in phase (both reach a max and min at the same time)
-The E and B fields are perpendicular to the direction of travel (transverse waves)

What are the common frequencies/wavelengths of weather radars?

The s-band is a typical wavelength/frequency with a frequency of 2,000-4,000Mhz and a wavelength of 7.5-1.5 cm.

What are the advantages/disadvantages of using a shorter or longer wavelength?

-Most radars have wavelength between .8 cm and 10 cm
-Short wavelengths mean smaller and less expensive equipment
-Shorter wavelengths are effective in detecting smaller particles (cloud & drizzle) droplets, but tend to absorb more
-Longer wavelength has reduction of smaller particles being absorbed and can see thunderstorms when behind another one

Polarize vs. Unpolarize waves

When light is polarized, the electric field always points in the same direction.
-Polarized in the vertical direction: The electric field points in the vertical direction.
-Unpolarized: Superposition of many beams, approximately parallel, but each with random polarization.

What are the basic characteristics of the radar system?

-Remote sensing
-Active sensor (Transmits EM wave)
-Observing tool with exceptional nowcasting abilities
-Continuous sensing
-Reasonably Resolution (~5 min, 500 m)
-Ability to sense total 3D structure and variability of target
-Coherent observation (velocity) of in-storm motion field (Doppler Only)

How does radar work? Describe the beam process.

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What are the types of radar?
-Monostatic vs. Bistatic
-Continuous wave vs. Pulsed
-TDWR
-Doppler

Monostatic
-Single antenna (Modern WSR-88D)
Bistatic
-2 antennas
-Antennas must be aimed at the same region, but can be quite some distance apart
-Used in military for detecting aircraft of missiles at ranges of several thousand km
Continuous Wave (CW)
-Received signal is also continuous
-Only way to detect anything is if signal is different
-Occurs when target is moving
-Shift in the frequency proportional to the speed of the target relative to the radar (police radar)
Pulsed Radar
-Transmits a short pulse and waits for a return echo
-Modern radars
Doppler Radar
-Measures velocity of a target
-Compares the received frequencies with the transmitted frequencies and deduces the velocity

What is the Doppler shift? How does it relate to weather radars?

The frequency increases as the object emitting sound moves closer and decreases as it moves away.

What are the four main components of radar?
-Their function, purpose in radar systems and different types.

-Transmitter to generate high frequency signal
-Antenna to send and receive signal
i. Designed for specific wavelength, determined by transmitter, antenna must match transmitter
ii. Size of the reflector ~1-30 ft
iii. Rotate between 10 and 70 degrees per second about a vertical axis
iv. Does a complete circle at every elevation angle (tilt)
v. Volume scans
-Receiver to detect and amplify the signal
-Display system to view results

What are beamwidth and gain? How do they change?

-Beam width - width of beam at half the max beam intensity, also called 3 dB beam width.
-Gain - Ratio of the power that is received at a specific point in space (usually the center of the beam axis) with the reflector in place (p1) to the power that would be received at the same point from an isotropic antenna (p2). Gain represents the capacity of the antenna to concentrate energy (including losses) (typical value = ~1000), how much power you gain by focusing the energy.
-Changes - Gain is measured logarithmically in decibels and represents the ability to concentrate energy between points. The .5 power points and the max beam at the center determine beam width.

What are side lopes? How do they see them on the radar?

Side lobes - Are the spreading energy around the beam width responsible for most of the ground-clutter.

Explain 2 types of display systems/scanning strategies? How do they differ?

1. PPI�Plan Position Indicator
-What 88D commonly uses
-Range, Azimuth display at constant elevation angle
-Good surveillance scans
-Good in operational setting
-Antenna rotates through 360� sweep at constant elevation angle
-Allows detection/intensity determination of precipitation within given radius from radar
2. RHI-Range Height Indicator
-Range, elevation display at constant azimuth angle
-Good vertical scan

How is a target's location determined?

Three pieces of information
-Azimuth angle
-Elevation angle
-Distance to target
From these data radar can determine exact target location and display
r = ct/2 (Distance to target)
t ? pulse's round trip time

What is the cone of silence? Where does it happen?

Where the radar cannot see directly overhead. Appears as ring of minimal/non-returns around radar, esp. with widespread precipitation

PRF
-How to find them and what their function is.

Pulse Repetition Frequency is the number of pulses per second transmitted by the radar.

Pulse Duration
-How to find them and what their function is.

The actual time of the complete pulse, minus the time between pulses.

Range folding (second trip echos)
-How to find them and what their function is.

If the range to the target is smaller than the unambiguous range, it will be displayed at the correct location, if it is greater, than it will not be displayed correctly.

Range resolution
-How to find them and what their function is.

The target resolution of radar is its ability to distinguish between targets that are very close in either range or bearing. (C*T)/2

Unambiguous velocity
-How to find them and what their function is.

(wavelength*PRF)/4

What are the consequences/results of a widening pulse/beam with range?

a. If any portion of beam hits something, plots it in the center of the beam
b. Elongate along the beam
c. Becomes worse with range
d. Washes out the fine details since radar plots in the middle of the highest reflectivity it see.

How is a radar beam propagated?

The beam propagates from its original path by refraction from the atmosphere and curvature of the earth.

What 2 factors contribute to beam bending?

Sub refraction - Beam not bending as much towards surface or bends up

What is refractivity? How is it calculated?

It is calculated by N = (n-1) 10^6

What are the state variables that affect refractivity? How do they?

The variable affecting propagation include T (Temp in K), p (Pressure in hPa or mb), e (vapor pressure), Ne (Density free electrons), f (operating frequency in Hz). T, p, and e come from the sounding and contribute to the lapse rate.

What is the standard N lapse rate? How is it calculated?

The standard lapse is -39 N-units/km. The lapse rate is a result of the change in N over Z.

Subrefraction
-What, where, when and how does it occur?
How is it seen on radar?

-Beam not bending as much towards surface or bends up
-The beam will be higher than the radar calculation and, hence, target heigh will be underestimated
-Most common late afternoon
-"Inverted V" environment

Super-refraction
-What, where, when and how does it occur? How is it seen on radar?

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Ducting
-What, where, when and how does it occur? How is it seen on radar?

-Extreme cases: extra-super-refraction
-Radar beam becomes trapped within the inversion
-Beam propagates nearly parallel to the earth's surface
-Most common under fair weather conditions
-Can result in anomalous propagation and will be worst in morning

What is attenuation? What environments/features/wavelengths attenuate more/less and why?

-The diminishing intensity of the EM wave along its path.
-This occurs as the backscattered radiation travels through the atmosphere, where some is scattered, absorbed, or reflected into another direction, thus reducing the intensity.
-This loss is easiest related in the loss of decibels, which is the measure of relative power or relative values of two flux densities.
-This happens most commonly with atmospheric gases that cause scattering and absorb the beam, which is more defined over very long ranges (several dB). Clouds also cause attenuation varying directly with the h20 content of the cloud. For a cloud of average wetness (.5g/m^3 of air) attenuation over 100 km would be 5dB for 3cm and .5 for 10cm radar. For most cases, this attenuation is less than the above estimates, because the beam travels in the upper part of the atmosphere where water content is lower.
-Precipitation can also attenuate 10 cm radar if the rain is heavy and over a long path. Higher precipitation rates have bigger drops, lower precipitation rates have smaller drops.

How are rainfall rates determine?

Rainfall rates are determined by R = (?/6) ? Ni Di3 wt where:
-R- rainfall rate (mm hr-1)
-(?/6 = .5236)
-wt is the terminal velocity of the hydrometeor in m/s,
-D - diameter in mm,
-N - number of hydrometeors of this diameter per cubic meter

What is the formula for generic Z-R relationship? How is it used to estimate rainfall rates from radar reflectivity?

The formula for the generic z-r relationship is Z ? ARb
-Z- radar reflectivity factor (mm6/m3)
-R- rainfall rate (mm/hr)

What are the main Z-R relationships? Why is there no universal Z-R relationship?

A and B- empirical constants
It is used to estimate rainfall rates from reflectivity by multiplying the empirical constant a*rainfall^empirical constant b.
The most common solution to this problem is using Marshall Palmers method for finding precipitation rates in which z= 200R^1.6 for strati-form regions and z = 55R^1.6 for convective regions.

Which Z-R relationship is used for WSR-88D?

The WSR-88D uses z = 300R^1.4 and there is no one universal relation because the a and b values depend on precipitation character, location, and warm/cold seasons. T

What are the errors associated with Z-R relationships?

The errors in Z-R Relationships include: Anomalous propagation (Causes overestimates) and partial beam filling which further results in underestimation, attenuation, and incorrect hardware calibration.
More errors in Z-R relationships include: Variations in drop size distribution, mixed precipitation types, bright banding due to precipitation near and below the melting level, and hail. Sleet, wet snow, and bright banding can cause overestimation of precipitation amounts where strong winds below the beam cause an underestimation. Evaporation is also an error at long ranges, which is seen as a donut around the radar, if the donut grows, it won't precipitate, and vice versa. Coalescence (merging of droplets) and snow also throw off values. Relate snow reflectivity to those measurements of ground observers.

What is bright banding? Where, when and how does it occur? What does it look like? How can it be minimized?

Bright band - The region of relatively high equivalent reflectivity that usually appears as an elevated layer at the height where falling ice particles begin to melt and thus become water-coated. This bright-band depicts the melting layer and is often 3,000 ft in depth. As frozen precipitate fall into an area with temperatures in the -5C to 0C range there is a rapid increase in the coalescence of individual snow crystals.
This causes a fivefold increase in the efficiency of the particles to return energy to the radar (Green 1993). This is the primary cause of the bright band.

Base reflectivity
-What are they? How are the derived? What do they show? How/when are they used? Advantages and limitation/issues?

-Single elevation angle scan (5-14 available)
-Useful for precipitation detection/intensity
-Usually select lowest elevation angle for this purpose
-High reflectivity's ? heavy rainfall
-Usually associated with thunderstorms
-Strong updrafts ? larger raindrops
-Large raindrops have higher terminal velocities
-Rain falls faster out of cloud ? higher rainfall rates
-Hail contamination possible > 50 dBZ

Composite reflectivity
-What are they? How are the derived? What do they show? How/when are they used? Advantages and limitation/issues?

Composite reflectivity - Shows highest reflectivity over all elevation scans
-Good for severe thunderstorms
-Strong updrafts keep precipitation suspended
-Drops must grow large enough to overcome updraft

Base velocity
-What are they? How are the derived? What do they show? How/when are they used? Advantages and limitation/issues?

_Can be used to ascertain large-scale and small-scale flows/phenomena
-Fronts and other boundaries
-Mesoscale circulations
-Micro-bursts
-"Purple haze" can occur with base velocity when a proper velocity cannot be assigned due largely to multi-trip echoes.

Storm relative velocity
-What are they? How are the derived? What do they show? How/when are they used? Advantages and limitation/issues?

Storm Relative Velocity - SRV subtracts out the motion of a storm to display pure rotational characteristics of that storm
-Often, the motion of the storm will mask or "dilute" the rotational information.
-This is especially true when rotations are subtle.
-Good for viewing structure of fast moving storms
-Takes the average storm motion of all storms seen
-Even if storms moving in different directions

VAD wind profile
-What are they? How are the derived? What do they show? How/when are they used? Advantages and limitation/issues?

VAD Wind Profile - This is the velocity Azimuth Display which with respect to height, increases with time from left to right. (Height vs. time (with current winds on the right))
Needs at least 25 scatters to plot winds
To determine mean wind at height level:
-Applies winds to sine curve then
-Looks at the root mean square of the azimuth and velocity of all data points
-If large variability in winds- no winds plotted
Limited data closer to radar
Good for analyzing winds:
-Finding wind shear
-Upper- level flow and helicity trends
-Frontal inversions, coupling

One/three hour precipitation estimate
-What are they? How are the derived? What do they show? How/when are they used? Advantages and limitation/issues?

One hour precipitation estimate - Covers 1h period ending at time of image
-Can help to track storms when viewed as a loop
-Highlights areas for potential (flash) flooding
-Interval too short for some applications
Three hour precipitation estimate -
-Covers 3h period ending at time of image
-Can help to track storms when viewed as a loop
-Highlights areas for potential (flash) flooding
-Interval too short for some applications

Storm total precipitation estimate
-What are they? How are the derived? What do they show? How/when are they used? Advantages and limitation/issues?

Storm Total precipiation estimate -
-Cumulative precipitation estimate at time of image
-Begins when radar switches from clear-air to precipitation mode
-Ends when radar switches back to clear-air mode
-Can help to track storms when viewed as a loop
-Helpful in estimating soil saturation/runoff
-Post-storm analysis highlights areas of R+/hail
-No control over estimation period
-Hail contamination
-Mixed precipitation in the same area

VIL
-What are they? How are the derived? What do they show? How/when are they used? Advantages and limitation/issues?

VIL - Vertically integrated liquids are the total mass of precipitation in a cloud/clouds within a vertical column of the atmosphere. High VIL values are a good indication of hail.

VIL density
-What are they? How are the derived? What do they show? How/when are they used? Advantages and limitation/issues?

VIL Density - Simply the VIL/ Echo tops.
a. Normalize the VIL using the height and depths of echo top
-Eliminates the air mass dependency of VIL alone
-Eliminates problematic assumptions of VIL of the day
b. Can be used to identify thunderstorms with high reflectivities relative to their height
c. VIL density increases primarily due to increase in target size
d. As VIL density increase, hail cores tend to be deeper, more intense and reported hail sizes tend to be larger.

Echo tops
-What are they? How are the derived? What do they show? How/when are they used? Advantages and limitation/issues?

Echo Tops - Height in mean sea level estimate of the highest grid box that exceeds 18.5 dBz reflectivity. This means that the center of the beam for the highest scan meeting the 18.5 dBz threshold is the echo top.
a. Often has a stair-step appearance due to uneven data between elevation scans.
b. Error - Inaccurate data close to radar because there is no beam angle high enough to see tops.
i. Tops are usually underestimated
ii. Only detects heights over 18.5 dBz
iii. Cone of silence
iv. Overestimate if severe Hail
v. Increase in height may indicate strengthening
vi. Opposite true as well

MDA
-What are they? How are the derived? What do they show? How/when are they used? Advantages and limitation/issues?

MDA - meso-cyclone detection algorithm
-Detects rotation� meso-cyclones
-Need a minimum of 2 volume scans of at least 10,000 ft. depth
-Only detects cyclonic rotation since adopted for super-cells

TVS
-What are they? How are the derived? What do they show? How/when are they used? Advantages and limitation/issues?

TVS - tornado vortex signature
-A specific type of MDA that looks lower in atms to identify tornadic scale rotation
-Looks at the strength and depth of gate to gate shear
-High false alarm rates in squall-lines and tropical cyclones

Spectrum width
-What are they? How are the derived? What do they show? How/when are they used? Advantages and limitation/issues?

Spectrum Width - Measures the range of velocity at each bin, basically a measure of the turbulence Wider range of velocity � greater spectrum width values, which may indicate wind shear
Common SW values:
-Stratiform precipitation:
-Turbulent flow:
Good for evaluating velocity data
-For example, if you see high SW, definitely need to look closer at velocity data locates areas of turbulence and shear. Same limitations as velocity data
Precipitation Accumulation Algorithm - makes up for data missing by 30 minutes or less by averaging or extrapolation.

How is velocity determined?

A Doppler radar with continuous pulse is used to determine velocity of bodies nearing it by the change in frequency of its waves.

What does negative/positive velocity mean?

A negative value means that the storm is approaching the radar (in green) and a positive value means that the storm is moving away from the radar (in red/warm colors).

What percentage of actual wind will the radar detect?

The radar will only detect roughly 100% of the wind at 0 degrees, 97% at 15 degrees, 87% at 30 degrees, 71% at 45 degrees, 50% at 60 degrees, 26% at 75 degrees, and 0% at 90 degrees.

What is the Doppler dilemma?

The Doppler dilemma is when there is no single PRF to satisfy both the max range and max mean radial velocity. (As PRF increases, Rmax decreases and Vmax decreases)

What is a zero isodop? How does it change in relation large scale wind patterns?

The Zero Isodop is when the radial is perpendicular to the wind in which the radar displays zero velocity. This can change with veering, backing, shear line, convergence, and divergence. (Study those funny S shape velocities from Presentation 5). The S-shape indicates veering winds with height (warm air advection). The backward S shape indicates backing winds with height (Cold air advection). The straight isodop indicates uniform direction at all levels.

How is rotation seen on velocity display? What does it look like?

Rotation is seen in velocity using the Storm Relative Velocity and seen in the form of a couplet (red vs. green).

How is storm relative velocity derived?

Storm relative velocity is derived from taking out the actual full overall movement of the storm and focuses just on the rotational aspects.

When would you use SRV vs. BV?

You would use SRV when looking solely at the rotation of the storm, but you should use the base velocity when looking at straight line winds (The sum of the movement of the storm plus winds produced by the storm).

What does the VAD wind profile show?

The Velocity Azimuth Display (VAD) displays the root mean square of the azimuth and velocity of all data points, which determines mean wind at height levels over time. This is helpful in determining wind shear, upper level flow, frontal inversions, and coupling.

How can hail be detected? What products are most useful when detecting hail?

Returns greater than 55dBz usually indicate hail. Whether the hail will reach the ground or not depends on the freezing altitudes, cause hail usually melts before reaching the surface.
The freezing level can be determined from upper-air soundings.
Probability of hail (POH): Looks at the freezing level for reflectivity greater than 45dBz, the larger the reflectivity, and the more likelihood of hail.
Probability of Severe Hail (POSH): Finds hail greater than � of an inch and looks at layer between freezing and -20 C for reflectivity greater than 50 dBz.
Maximum Expected Hail Size (MEHS): Given in � of an inch increments
Errors included with POH, POSH, and MEHS include: accurate and timely measurements between -20 C and freezing, they fluctuate at short and long ranges with storm tilts, cells greater than 124 nm are unknown, and weak winds and tropical environments lead to overestimation.

In-situ

the instrument is measuring the state of the variable at the location of the instrument

Remote sensing

the instrument is measuring the state of the variable at a location other than that of the instrument (active or passive)
Remote sensing is more efficient, but can also be very expensive

Active remote sensing

Transmits a signal, measures return signal (e.g., radar)

Passive remote sensing

Does not transmit, only measures signal emitted or reflected by objects in atmosphere (or atmosphere itself) (e.g., satellite)
Emitted: infrared, water vapor
Reflected: visible

Azimuth Angle

Angle of beam with respect to north

Elevation Angle

Angle of beam with respect to the ground
-Radar usually aimed above horizon, minimizes ground clutter, but not perfect
-Beam gains altitude as it travels away from radar
-Radar can't see directly overhead (cone of silence)
-Sample volume increases as beam travels away from radar

Second Trip Echo Detection

-Conserve azimuth angle
-Weaker
-Sometimes associated with "strange" Doppler velocities
-Eliminated by changing listening time (PRF) or use a different PRF every 2-3 pulses

Radar Beam Propagation

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Clean-Air Mode

-Slower antenna rotation
-5 elevation scans in 10 minutes
-Sensitive to smaller scatterers (dust, particulates, bugs, etc.)
-Good for snow detection

Precipitation Mode

-Faster antenna rotation
-9-14 elevation scans in 5-6 minutes
-Less sensitive than clear-air mode
-Good for precipitation detection/intensity determination