RADAP
II (RAdar DAta Processor II) -
equipment attached to some WSR-57 and WSR-74
radar units. It automatically controls the tilt sequence
and computes several radar-derived quantities at regular intervals, including
VIL, storm tops, and accumulated rainfall.
RADAR
- acronym for RAdio Detection And Ranging;
a radio device or system for locating an object by means of ultrahigh-frequency
radio waves reflected from the object and received, observed, and analyzed
by the receiving part of the device in such a way that characteristics
(as distance and direction) of the object may be determined
Radar
Meteorology - branch of meteorology that uses radars for weather
observations and forecasts.
Radar
Mosaic - a radar product that combines information from multiple
radars to give a regional or national view of reflectivity
or precipitation. An individual NEXRAD
radar is limited to a range of about 200 miles. Typically, a mosaic product
is produced for regions spanning several hundreds to several thousands
of miles. Mosaic products are produced by vendors external to the NEXRAD
system.
Radial
Velocity - the component of motion toward or away
from a given location. As detected by Doppler
radar, it is the component of motion parallel to the radar beam. An
object moving perpendicular to the radar beam has a radial velocity of
zero.
Radiance
- a measure of the intensity of the radiant
energy flux emitted by a body in a given direction
Radiant
Energy - the energy produced by
any type of electromagnetic radiation
Radiation
- (1) the process by which radiated energy
moves through space or material media; (2) energy
propagated through space or through material media in the form of an advancing
disturbance in electric and magnetic fields (e.g., visible light, x-rays,
microwaves, radio waves, infrared radiation,
ultraviolet waves, cosmic rays, etc.)
Radiation
Laws - the four physical laws which fundamentally describe
the behavior of blackbody radiation:
Kirchhoff's law, Planck's
law, Stefan-Boltzmann law and
Wien's displacement law
Radiational
Cooling - the process by which the surface of the earth and
its adjacent air cool by emission of infrared
radiation
Radiosonde
- a miniature radio transmitter that is carried by an unmanned
balloon aloft with instruments for the simultaneous measurement and transmission
of meteorological data
Rain
- precipitation in the form
of liquid water drops with diameters greater
than 0.5 mm
Rainbow
- an arc of concentric colored bands, arranged from red to
blue, which may be seen on a "sheet" of water drops (rain, fog, spray)
Rain-Free
Base - a dark, horizontal cloud
base with no visible precipitation
beneath it; it typically marks the location of the thunderstorm
updraft
Rain
Gauge - an instrument for measuring the quantity of rain
that has fallen
Rainfall
Estimates - a series of NEXRAD products that employ a Z-R
relationship to produce accumulations of surface rainfall from observed
reflectivity.
Range
- in radar meteorology, the straight-line
distance from the radar
Range
Folding - a limitation of a pulsed Doppler
radar whereby an echo at a far range (beyond
the radar's maximum unambiguous range)
could be misinterpreted as being at a closer range. Range folding hampers
accurate velocity determinations.
RDA
- an acronym for Radar Data Acquisition. The RDA is the hardware
component of the NEXRAD system that consists
of the radar antenna, transmitter, receiver, tower, and controlling computer.
The RDA collects the unprocessed, analog voltages from the radar antenna
and converts the signal to base reflectivity
, base velocity, and spectrum width
(in polar coordinate form). These "wide-band" products are transmitted
to the RPG, which creates and disseminates end-user
products.
Real-Time
- refers to the rapid retrieval, processing and transmission
of data
Rear
Flank Downdraft (or RFD) - a region of dry
air descending on the back side of, and wrapping around, a mesocyclone.
It often is visible as a clear slot wrapping
around the wall cloud. Scattered large
precipitation particles (rain
and hail) at the interface between the clear
slot and wall cloud may show up on radar as a hook
or pendant; thus the presence of a hook or pendant may indicate the presence
of an RFD.
Reflection
- the process whereby radiation (or
other waves) incident upon a surface is directed back into the medium
through which it traveled.
Reflectivity
- a radar term referring to the ability
of a radar target to return energy; used to
derive echo intensity and to estimate precipitation
intensity and rainfall rates
Reflectivity
Factor - the result of a mathematical equation (called the
Weather Radar Equation) that converts the analog power (in Watts)
received by the radar antenna into a more usable quantity. The reflectivity
factor (denoted by Z) takes into account several factors, including the
distance of a target from the radar, the wavelength
of the transmitted radiation, and certain assumptions
about the kind and size of targets detected by the radar. The reflectivity
factor ranges over several orders of magnitudes, so it is usually expressed
on a logarithmic scale called dBZ (decibels of
reflectivity).
Refraction
- the process whereby radiation experiences
a change in direction as a result of a change in density of the medium
or media through which it travels
Relative
Humidity - a measure of the water
vapor content of the air at a given temperature;
the amount of moisture in the air as compared with the amount that the
air could contain at the same temperature, expressed as a percentage
Retrogression
(or Retrograde Motion) - movement of a weather system in a direction
opposite to that of the basic flow in which it is embedded; usually refers
to a closed low or a longwave trough
which moves westward
Return
Flow - south winds on the back (west)
side of an eastward-moving surface high pressure
system
Return
Period - recurrence interval; a statistical parameter used
in frequency analysis as a measure of the average time interval between
the occurrence of a given quantity and that of an equal or greater quantity
RHI
- an acronym for Range-Height Indicator. The RHI is a radar
display in which the radar scans vertically, with
the antenna pointing at a specific azimuth
or radial. NEXRAD does not support RHI, but
the PUP software allows the NEXRAD operator to
construct a vertical cross-section using data from multiple scans of the
radar.
Ridge
- an elongated area of relatively high atmospheric
pressure; the opposite of a trough
Right
Mover - A thunderstorm that
moves appreciably to the right relative to the main steering winds and
to other nearby thunderstorms. Right movers typically are associated with
a high potential for severe weather.
Roll
Cloud - a low, horizontal tube-shaped arcus
cloud associated with a thunderstorm gust front (or sometimes with
a cold front); roll clouds are completely
detached from the thunderstorm base or other cloud features
Rope
(or Rope Funnel) - a narrow, often twisted condensation
funnel usually associated with the decaying stage of a tornado
Rope
Stage - the dissipating stage of a tornado,
characterized by thinning and shrinking of the condensation
funnel into a rope (or rope funnel); damage still
is possible during this stage
RPG
- the acronym for Radar Product Generator. The RPG is the computer
in the NEXRAD system that receives polar-coordinate
base radar data from the RDA and processes these data
into end-user products. Algorithms are
utilized for pattern-recognition, rainfall
estimation, computation of VIL
and other products. The RPG communicates these products to end-users.
A specific subset of available products is always generated for the NIDS
vendors for distribution outside of the NWS,
DoD, and FAA. Other products are generated by the RPG upon request from
a PUP.
RUC
(Rapid Update Cycle) - a numerical
model simulation conducted at NCEP that focuses
on short-term (up to 12 hours) forecasts and mesoscale
weather features; forecasts are prepared every 3 hours for the contiguous
United States
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