Glossary Compiled with use of Collier, C. (Ed.): Applications of Weather Radar Systems, 2nd Ed., John Wiley, Chichester 1996 Rinehard, R.E.: Radar of Meteorologists, 3rd Ed. Rinehart Publishing, Grand Forks, ND 1997 DoC/NOAA: Fed. Met. Handbook No. 11, Doppler Radar Meteorological Observations, Part A-D, DoC, Washington D.C. 1990-1992 ACU Antenna Control Unit. AID converter ADC. Analog-to-digitl;tl converter. The electronic device which converts the radar receiver analog (voltage) signal into a number (or count or quanta). ADAS ARPS Data Analysis System, where ARPS is Advanced Regional Prediction System. 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. Analog Class of devices in which the output varies continuously as a function of the input. Analysis field Best estimate of the state of the atmosphere at a given time, used as the initial conditions for integrating an NWP model forward in time. Anomalous propagation AP. Anaprop, nonstandard atmospheric temperature or moisture gradients will cause all or part of the radar beam to propagate along a nonnormal path. If the beam is refracted downward (superrefraction) sufficiently, it will illuminate the ground and return signals to the radar from distances further than is normally associated with ground targets. 282 Glossary 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 value is called antenna gain by convention. Antenna reflector The portion of an antenna system which reflects the energy from the radi­ ating element into a focused beam generally circular parabolas for weather radars. ARMAR Airborne Rain MApping Radar. A-scope A deflection-modulated display in which the vertical deflection is propor­ tional to target echo strength and the horizontal coordinate is proportional to range. ASL A bove sea level. Assimilation The process of estimating meteorological or hydrological conditions on a re­ gular grid from observations and a numerical model. Assymmetric Digital Subscriber Line ADSL. A technology allowing transmission rates of up to 2 Mbit/s in WANs by applying different rates for uplinks and downlinks. ATe Air Traffic Control. Attenuation Any process in which the flux density (power) of a beam of energy is dissi­ pated. Attenuator A device or network that absorbs part of a signal and transmits the remain­ der with a minimum of distortion. Autocorrelation A measure of similarity between displaced and undisplaced (in time, space, etc.) versions of the same function. Glossary 283 Automatic gain control AGe. Any method of automatically controlling the gain of a receiver, par­ ticularly 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. Azimuth A direction in terms of the 360° compass, north at 0°, east at 90°, south at 180°, west at 270°, etc. B-scope An intensity-modulated rectangular display with azimuth angle as the hori­ zontal coordinate and range as the vertical coordinate or vice versa. Backing wind A change in wind direction with height in a counterclockwise sense repre­ senting cold air advection. Background field Latest short forecast from an NWP model, used as the starting point for assimilation of observations to produce the next analysis. Backscatter That portion of power scattered back in the incident direction. Backscattering cross section Equivalent area required for an isotropic scatterer to return to a receiver the power actually received. Backscattering phase shift Phase shift between H and V polarised waves induced by the backscattering process. BALTEX Baltic Sea Experiment. Bandwidth The number of cycles per second between the limits of a frequency band. Band-pass filter A filter whose frequencies are between given upper and lower cutoff val­ ues, while substantially attenuating all frequencies outside these values (this band). 284 Glossary Bandwidth, 3 dB The frequency span between the points on the selectivity curve at which the insertion loss is 3 dB greater than the minimum insertion loss. Also called 3 dB passband. Beam filling The measure of variation of hydrometeor density throughout the radar sam­ pling volume. If there is no variation in density, the beam is considered to be filled. Beam width Angular width of antenna pattern. Usually that width where the transmit­ ted (i .e. one-way) 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. Bin Radar sample volume. Bipolar video The phase detection of the Doppler radar backscattered signal using the transmitted signal as a reference. Also known as the I (in-phase) and Q (quadrature phase) component of the Doppler radar signal. Bispectral analysis See multispectral analysis. Bistatic radar A radar which uses separate antennas for transmission and reception. Usu­ ally, the transmitter and receiver are at different locations. Bistatic radars depend upon forward and sideward scattering of the signal from the target to the receiver. Bistatic radar network A network comprising one transmitting pencil-beam radar and one or more passive, low-gain, nontransmitting receivers at remote sites. BMRC Bureau of Meteorology Research Centre (Australia). 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 free-stream value. Glossary 285 Bounded weak echo region BWER. A core of weak equivalent reflectivity in a thunderstorm that iden­ tifies 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. See also weak echo region. Bow echo A radar echo pattern in which a line of echoes, typically around 100 km in length, bulges forward under the influence of a corridor of strong winds taking the shape of a bow. Bragg scattering Scatter from small-scale fluctuations (i.e. turbulence) in the refractive index of the atmosphere. Bragg scatter comes from fluctuations which are wave­ length/2. Bright band A region of enhanced reflectivity in the vicinity of the freezing/melting level caused by the complex interaction of small ice crystals (poor reflectors) coa­ lescing into larger clumps and melting from the outside (water coated ice is a strong reflector) then collapsing into raindrops (smaller cross section) and falling more rapidly (more dispersed droplets). Brightness temperature (also called the equivalent blackbody temperature). The temperature of a blackbody emitting the same amount of radiation as the object being viewed. BUFR Binary Universal Form for the Representation of meteorological data. CAPE Convectively available potential energy, a measure of the instability of the atmosphere. CAPE Convection And Precipitation Electrification experiment. CAMPR CRL Airborne Multiparameter Precipitation Radar. CAPPI Constant altitude plan position indicator, a display of radar data at a con­ stant altitude, constructed from data obtained at several elevation angles. See also PPI. 286 Glossary 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. C-Band Radar wavelength of about 5 cm. CETP Centre d 'Etudes des Environnements Terrestres et Planetaires (Velizy, France). CLARA CLouds And RAdiation Experiment. Clear-air echoes Radar returns from cloud- and precipitation-free (optical clear) air. Clear-air echoes are caused either by Bragg scatter or by returns from discrete targets such as insects or birds. Bragg scatter comes from small-scale fluctuations of the refractive index (i .e. turbulent fluctuations of humidity). Cloud base For a given cloud or cloud layer, it is the lowest level in the atmosphere where cloud particles are visible. Clutter Echoes that interfere with observation of desired signals on a radar display. Usually applied to ground targets. Coherency matrix A matrix giving the generally polarized electromagnetic radiation backscat­ ter from precipitation particles. Coherent radar A radar that utilizes both signal phase and amplitude to determine target characteristics such as velocity, spectrum width. See Doppler radar. COHO Coherent oscillator. Cold front A narrow transition zone separating advancing colder air from retreating warmer air. The air behind a cold front is cooler and typically drier than the air it is replacing. Glossary 287 Combined sewer system Dirty water (sewage) and rainwater drained or processed in the same system. 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. Condensation The process by which water vapor becomes a liquid; the opposite of evapo­ ration, which is the conversion of liquid to vapor. Sublimation is the process by which a solid forms directly from vapor. Confluence zone A region where streamlines of the atmospheric flow at any level come closer together, indicating acceleration and corresponding ageostrophic flow. Convection In general, mass motions within a fluid resulting in transport and mixing of properties of that fluid. In meteorology, convection is referred to the atmo­ spheric motions that are predominantly vertical, such as rising air currents produced by surface heating. Forced convection is the ascent of air induced by some external force.