Radar Meteorology Glossary

Most of this glossary is reprinted with the kind permission of Ronald E. Rinehart of the University of North Dakota, from:

Entries beginning with a "*" are additions or modifications to the above reference.

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Ascope:: A deflection-modulated display in which the vertical deflection is proportional to target echo strength and the horizontal coordinate is proportional to range.
accuracy:: Degree of conformity of a measure to a standard or true value.
A/D converter:: Ananlog-to-digital converter. The electronic device which converts the radar receiver analog (voltage) signal into a number (or count or quanta)
aliasing:: The process by which frequencies too high to be analyzed with the given sampling interval appear at a frequency less than the Nyquist frequency.
amplitude:: The maximum magnitude of a quantity.
analog:: Class of devices in which the output varies continuously as a function of the input.
angular area of sphere:: Equals 4*pi steradians.
anomalous propagation (AP):: When nonstandard refractive index distributions prevail, "abnormal" or "anomalous" propagation occurs.
antenna:: A transducer between electromagnetic waves radiated through space and electromagnetic waves contained by a transmission line.
antenna gain:: The measure of effectiveness of a directional antenna as compared to an isotropic radiator; maximum values called 'antenna gain' by convention. Gain can be defined as: (power at point with antenna) / (power at same point with isotropic antenna).
antenna reflector:: The portion of an antenna system which reflects the energy from the radiating element into a focused beam; generally circular parabolas for weather radars.
attenuation:: Any process in which the flux density (power) of a beam of energy is dissipated.
autocorrelation:: A measure of similarity between diplaced and undisplaced (in time, space, etc.) versions of the same function.
automatic gain control (AGC):: Any method of automatically controlling the gain of a receiver, particularly one that holds the output level constant regardless of the input level.
average power:: *Pulsed radars transmit over a very low duty cycle; i.e., many intense but short and widely separated pulses. The average power is a radar's peak power * its PRF * its pulse length (duration).
azimuth:: A direction in terms of the 360 degree compass; north at 0, east at 90, south at 180, west at 270, etc.
B:
B scope: An intensity-modulated rectangular display with azimuth angle as the horizontal coordinate and range as the vertical coordinate
backing wind:: A change in wind direction in a counterclockwise sense representing cold air advection
backscatter:: That portion of power scattered back in the incident direction
band:: See frequency band.
bandpass filter:: A filter whose frequencies are between given upper and lower cutoff values, while substantially attenuating all frequencies outside these values (this band).
band width:: The number of cycles per second between the limits of a frequency band.
base data:: See NEXRAD base data.
basin:: See watershed.
beam filling:: The measure of variation of hydrometeor density throughout the radar sampling volume. If there is no variation in density, the beam is considered to be filled. The fraction of the radar sample volume filled.
beam width:: Angular width of the antenna pattern. Usually that width where the power density is one-half that on the axis of the beam ("half-power" or "3 dB" point)
bias:: A systematic difference between an estimate of and the true value of the parameter.
bistatic radar:: A radar which uses separate antennas for transmission and reception; usually the transmitter and receiver are at different locations. Bistatic radars depend upon forward scattering of the signal from transmitter to receiver.
boundary layer:: The layer of a fluid adjacent to a physical boundary in which the fluid motion is affected by the boundary and has a mean velocity less than the stream value.
bounded weak echo region (BWER):: A core of weak equivalent reflectivity in a thunderstorm that identifies the location of a strong updraft. The updraft is so strong that large precipitation particles do not have time to form in the lower and mid-levels of the storm and are prevented from falling back into the updraft core from above. The weak echo region is bounded when, in a horizontal section, the weak echo region is completely surrounded or bounded by higher reflectivity values.
bow echo:: Rapidly moving, crescent shaped echo that is convex in the direction of motion. Typically associated with strong, straight-line winds.
Bragg scattering:: Scatter from small-scale fluctuations (i.e., turbulence) in the refractive index of the atmosphere. Bragg scatter comes from fluctuations which are small compared to the radar's wavelength.
bright band:: *The enhanced layer of radar echo caused by the difference in radar reflectivity of ice and water particles, and the sticking/clumping of ice particles when they melt. This echo is interpreted as lying just below melting level.
C:
CAPPI:: *Constant Altitude PPI; a data product providing radar data at a fixed height or altitude, rather than at a fixed elevation angle.
Cartesian coordinates:: *The familiar "x-y" coordinate system, in which the axes are at right angles to each other. Raw radar data, often in polar coordinates, can always be converted to Cartesian coordinates.
cell:: A compact region of relatively strong vertical air motion (at least 10 m/s).
centroid:: The center of mass of a storm.
clutter:: Echoes that interfere with observation of desired signals on a radar display. Usually applied to ground targets.
coherent radar:: A radar that utilizes both signal phase and amplitude to determine target characteristics (*e.g., velocity, spectrum width)
cokriging:: A technique for estimating values of a spatial process (e.g., a precipitation field) given point observations of the process (e.g., rain gage observations) and possibly auxiliary observations (e.g., radar and satellite observations)
complex index of refraction:: m = n + i*k, where n is the normal index of refraction, i is sqrt(-1), and k is the absorption coefficient.
complex signal:: A signal containing both amplitude and phase information.
conjugate of a complex number:: If c = a + i*b is a complex number, then c* = a - i*b is its complex conjugate.
convection:: *In meteorology, the ascent of air which is less dense than its surroundings, usually because of thermal (temperature) differences. Clouds and storms are known as moist convection because the condensation of water vapor releases latent heat, which further heats the ascending air and makes it more buoyant.
convergence:: A measure of the contraction of a vector field. *In meteorology, usually refers to wind, or mass, convergence, when observed near the ground, it is associated with updrafts.
correlation:: A measure of the similarity between variables or functions.
couplet:: Adjacent maxima of radial velocities of opposite signs.
covariance:: A measure of the degree of association between two variables. In Doppler radars, the argument (or angle) of the covariance of the complex signal is a measure of the Doppler frequency.
cross section:: See radar cross section.
curvature:: The reciprocal of the radius of a circle; the rate of change in the deviation of a given arc from any tangent to it.
D:
dealiasing:: Process of correcting for aliases in the velocity measurement. Also known as unfolding.
decibel (dB):: A logarithmic expression for ratio of two quantities. Mathematically, dB = 10 log (x1/x2)
dBm:: *A logarithmic expression for power, referenced to 1 milliwatt. dBm = 10 log (power / 1 mW). See dB.
dBZ:: *A logarithmic expression for reflectivity factor, referenced to (1 mm^6 / 1 m^3). dBZ = 10 log ( z / 1 mm^6 m^3 ). See dB.
dielectric constant:: For a given substance, the ratio of the capacity of a condenser with that substance as dielectric to the capacity of that condenser with a vacuum as dielectric. *The dielectric constant of precipitation partially determines its reflectivity; this is partially the reason the same rate of snow and water precip will have different reflectivities.
differential reflectivity (Z_dr):: *A measure of the difference in reflectivity of a target when the transmitted waves are polarized horizontally or vertically.
diffraction:: The process by which the direction of radiation is changed so that it spreads into the geometric shadow region of an opaque or refractive object that lies in a radiation field.
disdrometer:: Equipment that measures and records the size distribution of raindrops.
distortion:: Change in a signal resulting in gross nonlinearities in signal processing or handling.
divergence:: A measure of the expansion of a vector field. *In meteorology, usually wind or mass divergence. When observed near the ground it is associated with downdrafts.
Doppler dilemma:: Vmax * Rmax = c * l / 8, where Vmax is maximum unambiguous velocity, Rmax is maximum unambiguous range, c is speed of light, and l is wavelength.
Doppler frequency shift:: f = 2 * V / l, where V is radial velocity of the target, l is the wavelength
Doppler shift:: The change in frequency at a receiver due to the relative motion of the receiver and the energy source.
downburst:: A strong downdraft that induces an outburst of damaging winds on or near the ground.
downdraft:: Currents of air with marked vertical downward motion.
dryline:: A mesoscale feature with its own associated vertical circulation. It is a narrow, almost vertical zone, across which a sharp moisture gradient, but little temperature gradient, occurs at the Earth's surface.
Dual Doppler: *The use of two Doppler radars to measure two different radial velocities; with some math, these two radial wind components can be synthesized to a spatial distribution of fully 2-D (horizontal) winds.
ducting:: The phenomenon by which the radar signal propagates along the boundary of two dissimilar air masses. The radar ranges with ducted propagation are greatly extended; holes can also appear in the coverage. Ducting occurs when the upper air is exceptionally warm and dry in comparison with the air at the surface. Ducting occurs when dN/dh <= - 157 N-units/km
duplexer:: A device in the waveguide which protects the sensitive receiver from the full power of the transmitter; usually contains one or more TR (transmit-receive) tubes.
dwell time:: Time over which a signal estimate is made. Usually, the time required for the antenna to traverse one degree.
dynamic range:: The ratio, usually expressed in decibels, of the maximum to the minimum signal that a system can handle. Used to describe the limits of receivers.
E:
echo:: Energy backscattered from a target as seen on the radar display.
effective radar reflectivity factor:: See equivalent radar reflectivity factor.
elevation angle:: The vertical pointing angle of the antenna.
equivalent radar reflectivity factor (z_e,Z_e):: The concentration of uniformly distributed small (diameter = one sixteenth wavelength or less) water particles that would return the amount of power received. In this text, this is expressed in mm^6 / m^3 for linear values of equivalent radar reflectivity factor z_e and in dBZ for logarithmic values of equivalent radar reflectivity factor Z_e.
estimate:: A statement of the value of a quantity or function based on a finite number of samples.
EVAD:: See VAD.
F:
feeder cloud:: The flanking lines of developing cumulus congestus that sometimes merge with and appear to intensify supercells.
folding:: Aliasing; applied to both velocity and range aliasing.
Fresnel reflection:: The reflection of a radar signal froma single, dominating discontinuity of the refractive index, usually with a large horizontal extent. Also called "partial reflection" because only a small fraction of the incident power is reflected, "specular reflection" if the horizontal surface discontinuity is assumed to be smooth, or "diffuse reflection" if the discontinuity is assumed to be corrugated or somewhat rough.
Fresnel scatter:: Scatter which occurs if several or many refractive index discontinuities exist along the pointing direction. The difference between Fresnel reflecion and scatter may be primarily a matter of resolution of the sampling volume compared to the size of the reflecting target.
frequency:: The number of recurrences of a periodic phenomenon per unit time. Electromagnetic energy is usually specified in Hertz (Hz), which is a unit of frequency equal to one cycle per second. Weather radars typically operate in the GigaHertz range (GHz). See also wavelength.
frequency band:: A range of frequencies, between some upper and lower limit.
frequency carrier:: The fundamental transmitted microwave frequency between 2700 and 3000 MHz. It is modulated so that it exists for a few microseconds each pulse.
G:
gain:: A change in signal power, voltage or current. Usually applied to a change greater than one and frequently expressed in decibels.
gate:: See range gate.
gating (range gating):: The use of electric circuits in radar to eliminate or discard the target signals from all targets falling outside certain desired range limits.
Gaussian:: Refers to the normal distribution; phenomena whose events are "normally" distributed are "Gaussian" distributed. This is the most common distribution encountered in physical processes.
GPS:: *Global Positioning System. A network of satellites which provide extremely accurate position and time information. Useful in remote locations or for moving platforms.
graupel:: A lightly rimed ice aggregate often found in vigorous storms. Formed when an ice aggregate collects supercooled liquid water droplets.
ground clutter:: The pattern of radar echoes from fixed ground targets.
gyro:: A device used for measuring changes in direction. Often used in antenna stabilization.
H:
homodyning:: The transfer of signal intelligence from one carrier to another by mixing of signals at different frequencies.
hook echo:: A pendant or hook on the right rear of an echo that often identifies mesocyclones on the radar display. The hook is caused by precipitation drawn into a cyclonic spiral by the winds, and the associated notch in the echo is caused by precipitation-free, warm, moist air flowing into the storm.
hydrometeor:: *A particle of condensed water (liquid, snow, ice, graupel, hail) in the atmosphere.
I:

i:: i = square root of (-1); a mathematical operator which, when multiplied with a number or parameter, has the effect of turning the vector 90 degrees counter clockwise from its original position
'I':: See inphase.
incident power density:: Energy per unit area incident on the radar target.
index of refraction:: See refractive index.
inphase or 'I' component:: The component of a complex signal along the real axis in the complex plane.
INU or INS:: *Inertial Navigation Unit or System: A highly accurate tool for measuring and keeping track of motions and accelerations. Often composed of laser gyros. Can be used in stabilizing antennas on moving platforms.
isodop:: Contour of constant Doppler velocity values.
isolated storm:: An individual cell or a group of cells that are identifiable and separate from other cells in a geographic area.
isotropic:: *Having the same characteristics in all directions, as with isotropic antennas. Directional or focused antennas are not isotropic.

J:
K:
|K|^2:: *A dielectric constant term in the radar equation. K = (m^2 - 1) / (m^2 + 1), where |K| is the magnitude of the expression for the complex index of refraction m. For water, |K|^2 = 0.93; for ice, |K|^2 = 0.197.
klystron:: An electron tube used as a low-power oscillator or a high-power amplifier at ultrahigh frequencies. Noted for exceptional stability over long periods of transmission.
L:
logarithmic scale:: 10^x, where x is a number
line echo wave pattern (LEWP):: A radar echo pattern formed when a segment of a line of thunderstorms surges forward at an accelerated rate. A meso-high pressure area is usually present behind the accelerating thunderstorms. A meso-low pressure area is usually present at the crest of the wave.
look angle:: A given radar's "perspective" on a storm; i.e., the angle at which its antenna beam hits it. By using more than one radar with different look angles, multiple Doppler analysis can be performed to retrieve the 2D and 3D winds.
M:
macroburst:: Large downburst with 4 km or larger outflow size, with damaging wind lasting 5-20 minutes.
main lobe:: The envelope of electromagnetic energy along the main axis of the beam.
magnetron:: A self-exciting oscillator tube used to produce the radio frequency signal transmitted by some radars. It utilizes a strong magnetic field to help induce the RF signal generated.
maximum unambiguous range:: The maximum range to which a transmitted pulse wave can travel and return to the radar before the next pulse is transmitted. R_max = c / (2*PRF), where c is the speed of light, PRF is pulse repetition frequency.
maximum unambiguous velocity:: The maximum radial velocity that can be detected without velocity aliasing. V_max = PRF * L / 4, where PRF is pulse repetition frequency, L is wavelength.
mean Doppler velocity:: Reflectivity-weighted average velocity of targets in a given pulse resolution volume. Usually determined from a large number of successive pulses. Also called mean radial (towards or away from the antenna) velocity. Doppler velocity referes to spectral density first moment, radial velocity to base data.
melting level:: *That height in the atmosphere at which ice melts to water; i.e., at which the temperature is 0 degrees Celsius. This may range from 0-5 km above the Earth's surface.
mesocyclone:: A 3-dimensional region in a storm that rotates cyclonically (counterclockwise in NH) and is closely correlated with severe weather.
mesoscale convective complex (MCC):: A quasi-circular conglomeration of thunderstorms having a cloud-top area larger than 100,000 square kilometers and persisting for more than 6 hours.
mesoscale convectve system (MCS):: Precipitation systems 20 to 500 km wide that contain deep convection. Examples in mid-latitudes are large isolated thunderstorms, squall lines, Mesoscale Convective Complexes, and rainbands.
microburst:: Small downburst, <= 4 km in outflow size (i.e., <= 2.5 n mi.), with peak winds lasting 2-15 minutes.
Mie scattering or resonant region:: Radar backscattering by targets having dimensions somewhat greater than 1/10 the wavelength of the radar but less than several radar wavelengths. *Often, hail exhibits in this type of scattering.
modulation:: Variation of the amplitude, frequency, or phase of a wave due to the mixing of two signals.
monstatic radar:: A radar that uses a common antenna for both transmitting and receiving.
multiple Doppler analysis:: *The use of more than one radar (and hence more than one look angle) to reconstruct spatial distributions of the 2D or 3D wind field, which cannot be measured from a single radar alone. Includes dual Doppler, triple Doppler, and overdetermined multiple Doppler analysis.
N:
NEXRAD:: NEXt generation RADar, *a generic term for the WSR-88D deployment program.
NEXRAD base data:: Those digital fields of reflectivity, mean radial velocity and spectrum width data in spherical coordinates provided at the finest resolution available.
notch width:: The 3 dB band width of a rejection filter.
Nyquist frequency:: The highest frequency that can be determined in data that have been discretely sampled. For data sampled at frequency f, this frequency is ( f / 2 ). Doppler radar sampling frequency (rate) is equal to the pulse repetition frequency (PRF).
Nyquist velocity, or interval:: The maximum unambiguous velocity that can be measured by a Doppler radar.
O:
Ohm's Law:: I = E / R, where I is current (amperes), E is electromotive force (i.e., voltage) and R is resistance (ohms)
oscillator:: The general term for an electric device that generates alternating currents or voltages. The oscillator is clasified according to frequency of the desired signal.
overdetermined multiple Doppler analysis:: *A multiple Doppler analysis technique in which there are more radars than the desired number of wind components to be retrieved. The wind retrieval can then be performed as an 'optimization' to reduce the retrieval errors.
P:
peak power:: *The amount of power transmitted by a radar during a given pulse. Note that because these pulses are widely spaced, the average power will be much smaller.
pedestal:: *A generic term for the structure supporting the antenna dish. Usually includes the drive motors and one end of the servo loop.
persistence:: On a radar display, the length of time during which a signal is visible; modern, color displays are no longer concerned with persistence.
phase:: A particular angular stage or point of advancement in a cycle; the fractional part of the angular period through which the wave has advanced, measured from the phase reference.
phase shift:: The angular difference of two periodic functions.
phasor diagram:: A diagram used to represent complex numbers. The x-axis is the real component and the y-axis is the imaginary component. The x-axis can be the in-phase and the y-axis the quadrature components (I and Q components).
platform:: *A generic term, often used to encompass the pedestal and antenna assembly; sometimes including the radar control, display and analysis hardware and software as well.
polar coordinates:: *A coordinate system in which the two directions are distance from the origin and angle (sweeping around the origin like a clock). The "natural" coordinate system in which data is collected by many radars. Data in polar coordinates can always be transformed to Cartesian coordinates.
polarization radar:: *A radar which takes advantage of ways in which the transmitted waves' polarization affect the backscattering. Such radars may alternately transmit horizontal and vertically polarized beams, and measure differential reflectivity.
power:: P = I * V = V * V / R = I * I / R, where I is current (amps), V is voltage (volts), R is resistance (ohms), P is power (watts). *I is not to be confused with the 'I' of 'I and Q', the in-phase and quadrature components.
PPI:: Plan-Position Indicator. An intensity-modulated display on which echo signals are shown in plan view with range and azimuth angle displayed in polar coordinates, forming a map-like display. Each PPI is taken at a single, fixed elevation angle, and thus forms a cone of coverage in space. PPIs may be run in sequence, creating a "volume scan" (here is a movie, in QuickTime or mpeg format).
precision:: The accuracy with which a number can be represented, i.e., the number of digits used to represent a number.
propagation:: Transmission of electromagnetic energy as waves through or along a medium.
pulse:: A single, short-duration transmission of electromagnetic energy.
pulse duration:: Time occupied by a burst of transmitted radio energy. This may also be expressed in units of range (pulse length) or time (pulse duration). Also called pulse width.
pulse length:: h = c * t , where t is the duration of the transmitted pulse, c is the speed of light, h is the length of the pulse in space. Note, in the radar equation, the length h/2 is actually used for calculating pulse volume because we are only interested in signals that arrive back at the radar simultaneously.
pulse-pair processing:: Nickname for the technique of mean velocity estimation by calculation of the signal complex covariance argument. The calculation requires two consecutive pulses, hence "pulse-pair".
pulsed radar:: A type of radar, designed to facilitate range measurement, in which teh transmitted energy is emitted in periodic, brief transmissions.
Pulse Repetition Frequency (PRF):: The number of pulses transmitted per second. Also called pulse repetition rate. Typical PRF's may range from 300-1200 Hz. See also Nyquist frequency.
Pulse Repetition Time (PRT):: The time interval from the beginning of one pulse to the beginning of the next succeeding pulse.
pulse resolution volume:: *A discrete radar sampling volume, of dimensions (horizontal beamwidth * vertical beamwidth * 1 range gate).
pulse width:: The time occupied by an individual pulse broadcast from a radar.
Q:
'Q' or quadrature:: The component of the complex signal that is 90 degrees out of phase with the inphase component. This component lies along the imaginary axis the complex plane.
R:
radar cross section:: The area of a fictitious, perfect reflector of electromagnetic waves (e.g., metal sphere) that would reflect the same amount of energy back to the radar as the actual target (e.g., lumpy snowflake).
radar reflectivity:: *The sum of all backscattering cross-sections (e.g., precipitation particles) in a pulse resolution volume divided by that volume. The radar reflectivity can be related to the radar reflectivity factor through the dielectric constant term |K|^2, and the radar wavelength.
radar reflectivity factor, z:: z = the sum (over i) of (N_i * D_i^6), where N_i is the number of drops of diameter D_i in a pulse resolution volume. Note that z may be expressed in linear or logarithmic units. *The radar reflectivity factor is simply a more meteorologically meaningful way of expressing the radar reflectivity.
radial velocity (V_r):: The component of motion of the target toward or away from the radar. Doppler radars only detect radial components of velocity.
random variable (variate):: A variable characterized by random behaviour in assuming its different possible values. Mathematically, it is described by its probability distribution, which specifies the possible values of a random variable together with thte probability asociated (in an appropriate sense) with each value. A random variable is said to be continuous if its possible values extend over a continuum, discrete if its possible values are separated by finite intervals.
range:: *Distance from the radar antenna.
range folding:: Apparent range placement of multiple-trip return. A multiple-trip return appears at the difference of the true range and a multiple of the unambiguous range, i.e., R_displayed = R_true - n * R_max, where n = 0,1,2,...
range gate:: The discrete pont in range along a single "radial" (beam) of radar data at which the received signal is sampled. Range gates are typically spaced at 100-1000 meter intervals. *A "radial" of radar data is composed of successive range gates, out to the maximum unambiguous range.
range unfolding:: Process of removing range ambiguity in apparent range of a multitrip target.
Rankine vortex:: Velocity profile for a symmetric circulation in which the inner core is in solid rotation and tangential winds outside the core vary inversely with radial distance from the center.
Rayleigh scattering:: Scattering by spherical particles whose radii are smaller than one-tenth the radar wavelength.
receiver:: The electronic device which detects the backscattered radiation, amplifies it and converts it to a low-frequency signal which is related to the properties of the target.
reflectivity:: A measure of the fraction of radiation reflected by a given surface; defined as a ratio of the radiant energy reflected to the total that is incident upon that surface. Lazy radar meteorologists and others working with radar data from storms frequently say "reflectivity" when they should say radar reflectivity factor or equivalent radar reflectivity factor; forgive us!
refraction:: The process in which the direction of energy propagation is changed as a result of a change in the speed of propagation caused by changes in density within the medium or as the energy passes through the interface representing a density discontinuity between two media.
refractive index:: A measure of the amount of refraction. Numerically equal to the ratio of wave velocity in a vacuum to a wave speed in the medium, i.e., n = c / v where v is actual speed, and c is speed of light in a vacuum.
refractivity, N:: N = (n-1)*10^6, where n is refractive index and N is a function of temperature, pressure and vapor pressure (in the atmosphere)
resolution:: The smallest increment of a measurement of a parameter.
RHI:: Range-Height Indicator. An intensity-modulated display with height as the vertical axis and range as the horizontal axis. *A "vertical cross section" in a plane passing through the radar.
S:
sample and hold:: The process of sampling (measuring) the signal strength at a particular point in space (i.e., at a range gate).
sea breeze:: *A current of air flowing inland, associated with warmer surface temperatures inland than at sea. Often shows up as a long, thin radar feature as insects get caught up in the frontal region. Temperature and moisture gradients across the front may also contribute to its reflectivity.
sectorized hybrid scan:: A single reflectivity scan composed of data from the lowest four elevation scans. Close to the radar, higher tilts are used to reduce clutter. At further ranges, either the maximum values from the lowest two scans are used or the second scan values are used alone.
servo loop:: *In radar meteorology, a generic description of hardware needed to remotely control the motion of the antenna dish.
severe storm:: A storm with a tornado, surface hail >= 3/4 inch, or wind gusts >= 50 knots, or all three.
shear:: The rate of change of the vector wind in a specified direction. Vertical shear is the variation of the horizontal wind in the vertical direction.
shelf cloud:: A type of arcus (or roll) cloud. It is a low-level, horizontal, accessory cloud that appears to be wedge shaped as seen along the leading edge of approaching thunderstorms. It is accompanied by gusty, straight-line winds and is followed by precipitation.
side lobe: *Secondary radiated energy "away from" than the radar main beam. Typically contains a small percent of energy compared to the main lobe but may produce erroneous echoes.
signal-to-noise ratio (SNR):: A ratio that measures the comprehensibility of data, usually expressed as the signal power divided by the noise power, usually expressed in decibels (dB).
spearhead echo:: A radar echo associated with a downburst with a pointed appendage extending toward the direction of the echo motion. The appendage moves much faster than the parent echo, which is drawn into the appendage. During the mature stage, the appendage turns into a major echo and the parent echo loses its identity.
specific humidity:: In a system of moist air, the ratio of the mass of water vapor to the total mass of the system.
spectral density:: The distribution of power by frequency.
spectrum width:: A measure of dispersion of velocities within the radar pulse resolution volume. Standard deviation of the velocity spectrum. It contains components from wind shear, the dispersion of drop terminal velocities, turbulence, and the antenna scan rate.
sphere calibration:: *Reflectivity calibration of a radar by pointing the dish at a metal sphere of (theoretically) known reflectivity. The sphere is often tethered to a balloon. Here's an image of a sphere calibration at sea. The sphere is visible just above the horizon.
squall line:: A line or narrow band of active thunderstorms.
standard deviation:: The positive square root of the signal variance. The velocity standard deviation is often called spectrum width.
storm:: Any disturbed state of the atmosphere, especially affecting the Earth's surface, and strongly implying destructive and otherwise unpleasant weather. Storms range in scale from tornadoes and thunderstorms through tropical cyclones to widespread extratropical cyclones.
storm motion:: The velocity at which a storm travels.
sub-refraction:: A condition of atmospheric refraction when radar waves are bent less than normal. Sub-refraction occurs when dN/dh > -39.2 N-units/km.
sun pointing:: *Alignment of the radar antenna by locating the position of the sun in the sky, which has an exactly known position given the radar's location and the present time. This may be necessary to verify that when we think we're pointing "north", we actually are! The sun's signal is usually several dB above the background noise, and this technique is also sometimes used to examine the receiver sensitivity.
supercell:: A large, long-lived (up to several hours) cell consisting of one quasi-steady updraft-downdraft couplet that is generally capable of producing the most severe weather (tornadoes, high winds, and giant hail).
supercooled liquid water:: *In the atmosphere, liquid water can survive at temperatures colder than 0 degrees Celsius; many vigorous storms contain large amounts of supercooled liquid water at cold temperatures. Important in the formation of graupel and hail.
super-refraction:: A condition of atmospheric refraction when radar waves are bent more than normal. Super-refraction occurs when dN/dh < -39.2 N-units/km.
synchronous detection:: Processing that retains the received signal amplitude and phase but that removes the intermediate frequency carrier.
T:
target:: Precipitation or other phenomena that produce echoes.
TDWR:: Terminal Doppler Weather Radar
thin line echo:: A narrow, elongated non-precipitating echo usually associated with thunderstorm outflow, fronts or other density discontinuities; also known as a fine line.
tilt:: *Another name for a PPI; i.e., a conical scan at a given elevation angle.
time-height display:: An intensity-modulated display which has height as the vertical coordinate and time as the horizontal coordinate; usually used for vertically-pointing antennas only.
transmitter:: The equipment used for generating and amplifying a radio frequency (RF) carrier signal, modulating the carrier signal with intelligence, and feeding the modulated carrier to an antenna for radiation into space as electromagnetic waves. Weather radar transmitters are usually magnetrons or klystrons.
Triple Doppler:: *Since any wind has three components (say, in the x, y and z directions), and a single radar measures in only one direction (radial), a single radar cannot give the 3D winds everywhere it samples. However, if three different radars view a storm from three different locations, the 3 measured radial velocities can be transformed into the actual 3D wind field.
U: unambiguous range:: See maximum unambiguous range.
updraft:: Current(s) of air with marked vertical upward motion.
unimodal:: A distribution having only one localized maximum, i.e., only one peak.
V:
VAD:: *Velocity-Azimuth Display. VAD and EVAD (Extended VAD) are methods of guessing the large scale two-dimensional winds from one-dimensional radial velocity data. They are essentially multivariate regressions which fit a simple, large scale wind model to the observed winds. EVAD also estimates the large scale horizontal divergence and particle fall speed.
vapor pressure:: The partial pressure of water vapor in an air-water system.
variance:: A measure of variability.
veering wind:: A change in wind direction in a clockwise sense representing warm air advection.
velocity aliasing (folding):: Ambiguous detection of radial velocities outside the Nyquist co-interval. *On a radial velocity display, this shows up as regions where the radial velocity suddenly switches from one extreme to its opposite (e.g., max towards to max away).
vortex:: In its most general use, any flow possessing vorticity. More often the term refers to a flow with closed streamlines.
vorticity:: A vector measure of local rotation in a fluid flow.
VVP:: *Volume Velocity Processing. A way to guess the large-scale 2-dimensional winds, divergence and fall speeds from one-dimensional radial velocity data. Essentially a multivariate regression which fits a simple wind model to the observed radial velocities. Very similar to VAD and EVAD, except it uses different functions for the fit.
W:
wall cloud:: A local, abrupt lowering of a rain-free cumulonimbus base into a low-hanging accessory cloud, from 2 to 6 km (1 to 4 miles) in diameter. The wall cloud is usually located in the southwestern part of a severe thunderstorm in the main updraft to the southwest of the main precipitation region. Rapid upward motion and visible rotation may be seen in wall clouds from several km away. Almost all strong tornadoes develop from wall clouds.
watershed: The total area drained by a river and its tributaries. Sometimes called a basin.
watt:: The unit of power in the MKS system of units; energy per unit time, one Joule/second.
waveguide:: A hollow conductor, usually rectangular or round in cross-section, used to carry radar waves between various components of a radar.
wavelength:: The distance a wave will travel in the time required to generate one cycle. The distance between two consecutive wave peaks (or other reference points) in space. *Weather radar wavelengths typically range from 1 mm to 50 cm.
weak echo region (WER):: Within a convective echo, a localized minimum of equivalent reflectivity associated with the strong updraft region.
WSR-88D system:: The summation of all hardware, software, facilities, communications, logistics, staffing, training, operations, and procedures specifically associated with tthe collection, processing, analysis, dissemination and application of data from the WSR-88D unit.
WSR-88D unit:: *Weather Service Radar, commissioned in 1988. Includes the associated displays, product generators, archiving facilities and communications services.
X:
Y:
Z:
Z_dr: See differential reflectivity.
Z-R relationship:: An empirical relationship between radar reflectivity factor z (in mm^6 / m^3 ) and rain rate ( in mm / hr ), usually expressed as Z = A R^b; A and b are empirical constants.