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METR 2413 4 February 2004. Radar Observations. Radar Basics.
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METR 2413 4 February 2004 Radar Observations
Radar Basics 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. Each Radar system consists of an antenna and a receiver.
Radar History The WSR-57 and WSR-74 NWS Weather Surveillance Radar units were replaced by WSR-88D units. The WSR-88D (Weather Surveillance Radar - 1988 Doppler) is a NEXRAD unit. NEXRAD (NEXt-Generation Weather RADar) is a network of high-resolution WSR-88D Doppler radars operated by the NWS. Other Radar systems exist including the Doppler on Wheels (DOWS), the ELDORA, Polarmetric radar, etc.
Radar History The Norman Doppler radar at NSSL, with an older WSR-57 radar (right)
Radar Fundamentals The WSR-88D radar transmits a stream or "beam" of energy in discrete pulses which propagate away from the radar antenna at approximately the speed of light (~3 × 108 ms-1). The volume of each pulse of energy will determine how many targets are illuminated. This directly determines how much energy (power) is returned to the radar. The shape of the radar antenna, the wavelength (l) of the energy transmitted, and the length of time the radar transmits determine the shape and volume of each radar pulse.
Radar Fundamentals The power density determines how much energy targets intercept and reflect or "backscatter" toward the radar antenna. Two pulses of energy have been transmitted by the radar in the following figure. The pulse on the right was transmitted first. Due to its greater range from the radar, it has a larger volume and lower power density than the second transmitted pulse on the left. If two radars transmitted the same amount of power but had different beamwidths, then the one with the narrower beam would have greater sensitivity due to its greater power density. This would result in the detection of smaller targets at greater ranges.
Radar Fundamentals As pulse volumes within the radar beam encounter targets, energy will be scattered in all directions. A very small portion of the intercepted energy will be backscattered toward the radar. The degree or amount of backscatter is determined by target * size (radar cross section) * shape (round, oblate, flat, etc.) * state (liquid, frozen, mixed, dry, wet) * concentration (number of particles per unit volume).
Radar Fundamentals We are concerned with two types of scattering, Rayleigh and non-Rayleigh (there are several types such as Mie scattering). Rayleigh scattering occurs with targets whose diameter (D) is much smaller (D < l/16) than the wavelength of the transmitted radio waves. The WSR-88D wavelength is approximately 10.7 cm, so Rayleigh scattering occurs with targets whose diameters are less than or equal to about 7 mm or ~0.4 inch. Raindrops seldom exceed 7 mm so all liquid drops are Rayleigh scatterers. Nearly all hailstones are non-Rayleigh scatterers due to their larger diameters. However, since the vast majority of targets sampled by the WSR-88D are raindrop size or smaller, the Rayleigh assumption is used in all computations of radar reflectivity.
Radar Fundamentals The Probert-Jones (P-J) radar reflectivity equation will help to quantify the physical aspects of pulsed E-M energy and the associated limitations of target (e.g., precipitation) detection. The P-J equation is described below as where: Pr = power returned to the radar from a target (watts) Pt = peak transmitted power (watts) G = antenna gain, q= angular beamwidth H = pulse length, p = 3.14159 K = physical constant (target character) L = signal loss factors associated with attenuation and receiver detection Z = target reflectivity, l= transmitted energy wavelength R = target range
Radar Fundamentals For the WSR-88D, the only variables that are not fixed are returned power (Pr), reflectivity (Z), attenuation factor (La), and range (R). The fixed variables are combined to create a new term which we will refer to as the radar constant, Cr. By combining the fixed variables into a radar constant, the previoussimplifies into where Cr is the radar constant. Solving for Z, the above equation becomes By knowing the power returned which the radar can easily measure, the above equation indirectly estimates target reflectivity.
Radar Fundamentals Range-normalized values of reflectivity, Z, can range over many orders of magnitude. To compress this large range of values for operational use, Z is displayed in decibels of Z, that is, dBZ. Converting Z to dBZ is simply done by using For example, if Z = 4000 mm6m-3, then dBZ = 10(log10 4000) » 10 x 3.6 = 36 dBZ.
Radar Fundamentals Due to the WSR-88D’s increased sensitivity, reflectivities as low as -32 dBZ can be detected in clear air mode near the RDA. How can there be such a thing as a negative dBZ? If 0 < Z < 1, log10Z < 0 and thus dBZ < 0. Very low dBZ values indicate the presence of extremely small sized particles (e.g., dust, haze, smoke). The WSR-88D can also detect reflectivity values as high as 95 dBZ. As an example, a one cubic meter volume containing just one 38.3 mm (~1.50 inch) diameter water-coated hailstone would yield a reflectivity value of approximately 95 dBZ. However, giant hail frequently occurs with reflectivities less than 70 dBZ. This is a good indication that such large targets do not meet the Rayleigh approximation
Radar Fundamentals If PRT is the time from the beginning of one pulse to the beginning of the next pulse, and t is the time actually spent transmitting, then PRT-t = t is the listening period. For example, if the WSR-88D is operating for 1.57 µsec and using a PRT of 1000 µsec (0.001 s or 1 millisecond), then the listening period is t = PRT- t = 1000 - 1.57 µsec = 998.43 µsec (or 0.99843 millisecond). As a result, for each hour the radar is active at this PRT, only about 5.7 seconds is spent transmitting. This means that 99.843% of the time the WSR-88D is listening for signal returns. In long pulse, the radar transmits 17.1 seconds every hour, spending 99.525% of its time listening.
Radar Fundamentals SMART-R (http://www.nssl.noaa.gov/smartradars/) is a collaborative radar meteorology research program. Two mobile 5-cm Doppler radars are used to study convective and mesoscale atmospheric processes to help improve forecasts of significant weather events such as flash floods, hurricanes and tornadoes.
WSR-88D Radar Products Base Reflectivity is one of the basic quantities that a Doppler radar (like NEXRAD) measures. Base Reflectivity basically corresponds to the amount of radiation that is scattered or reflected back to the radar by whatever targets are located in the radar beam at a given location (units are in dBZ). These targets can be hydrometeors (snow, rain drops, hail, cloud drops or ice particles) or other targets (dust, smoke, birds, airplanes, insects). The colors on the Base Reflectivity product correspond to the intensity of the radiation that was received by the radar antenna from a given location.
WSR-88D Radar Products Like Base Reflectivity, Base Velocity is a base product measured by the radar. Base Velocity is the average radial velocity of the targets in the radar beam at a given location. Radial velocity is the component of the target's motion that is along the direction of the radar beam. Positive values (warm colors) denote out-bound velocities that are directed away from the radar. Negative values (cool colors) are in-bound velocities that are directed towards the radar.
WSR-88D Radar Products Composite Reflectivity is the maximum base reflectivity value that occurs in a given vertical column in the radar umbrella. NEXRAD scans in several pre-defined "volume coverage patterns (VCPs), where the radar makes a 360-degree horizontal sweep with the radar antenna tilted at a given angle above the horizontal, then changes the elevation angle, and completes another 360-degree sweep, and so on. Composite reflectivity gives a plan view of the most intense portions of thunderstorms, and can be compared with Base Reflectivity to help determine the 3-D structure of a thunderstorm.
WSR-88D Radar Products The Rainfall Accumulation products attempt to estimate the amount of rainfall that has fallen in a given area under the radar's umbrella. NEXRAD does this by making certain assumptions about the number and kind of raindrops it detects. There are certain limitations involved with radar estimation of rainfall, which is a subject of current meteorological research, and there are plans to improve the way that NEXRAD produces its rainfall estimates. A given rainfall product should generally be compared with a product from another radar or with rain gage reports, if they're available.
WSR-88D Radar Products Storm-Relative Radial Velocity is Base Velocity with the average motion of all storm centroids subtracted out. Storm-Relative Radial Velocity can be useful in finding mesocyclones or other circulation patterns.
WSR-88D Radar Products Vertically Integrated Liquid, or VIL, is a calculation that converts a column of reflectivity into its liquid water equivalent. However, it turns out that VIL is seasonally and geographically correlated to hail size.
WSR-88D Radar Products The VAD Wind Profile is a time series of estimate of the horizontal wind at specific heights above the radar. It is useful in diagnosing the locations and structure of fronts, the movement of moisture from the Gulf of Mexico, and other meteorological phenomena.
