# Shared Flashcard Set

## Details

CDC 1W051A: Self-Test Questions
63
Meteorology
Professional
10/09/2012

Term
 What is electromagnetism?
Definition
 Magnetism developed by an electrical current passing through a wire coiled around a metal bar that induces a magnetic field.
Term
 If electromagnetic waves were visible and moving, describe what pattern they would have?
Definition
 Like a series of sine waves.
Term
 What is the effect of wavelength on attenuation?
Definition
 Attenuation has a greater affect on a shorter wavelength and weakens the energy of the beam.
Term
 How are frequency and wavelength related?
Definition
 Inversely, because wave speed is constant.
Term
 What does the attenuation of radar waves depend on?
Definition
 The wavelength of the emitted waves and the size and composition of the particles encountered.
Term
 Explain why absorbed energy that is later re-radiated back to the antenna is not recognized:
Definition
 Energy is absorbed and reradiated at a different wavelength. This energy that is different and unrecognizable to the radar the energy that was originally transmitted.
Term
 Name two ways to measure pulse length:
Definition
 In units of time it takes the radar to send one pulse (expressed in microseconds) or in units of distance (horizontal) from the front edge to the back edge of the pulse (expressed in meters)
Term
 What does the amount of energy transmitted by the radar depend on?
Definition
 The duration or length of the pulse.
Term
 Define pulse repetition frequency (PRF):
Definition
 The rate at which the pulses are transmitted in a unit of time.
Term
 What is the effect of pulse length and PRF on radar range?
Definition
 Pulse length affects the minimum range of the set, because the transmitter must be shut off by the time the reflected signal returns. Pulse repetition frequency affects the maximum range. The signal must have time to return to the set before the next pulse is transmitted.
Term
 Measured from the leading edge to the back edge of the pulse:
Definition
 Pulse length.
Term
 The rate at which pulses are transmitted:
Definition
 Pulse repetition frequency (PRF).
Term
 The period of waiting for a returned pulse:
Definition
 Listening time.
Term
 Is limited by how often pulses are broadcast:
Definition
 Maximum unambiguous range.
Term
 Occurs when energy is received from an old pulse after transmission of the next pulse:
Definition
 Range folding.
Term
 What happens if an antenna moves at too great an angle between pulses?
Definition
 The number of pulses returned per target will be too low, and some regions may not be probed for targets.
Term
 Explain range folding:
Definition
 Range folding occurs when energy is received from an old pulse after transmission of the next pulse.
Term
 Where can the maximum power density of the radar beam be found?
Definition
 Along the centerline of the beam.
Term
 What occurs when a target occupies only a small portion of the beam, thus hiding or altering the true characteristics of the target during display?
Definition
 Partial beam filling.
Term
 Explain the term “below beam effects”
Definition
 Missed targets below the beam.
Term
 What is beam blockage the result of?
Definition
 The radar beam striking large obstructions near the antenna site.
Term
 How far should two targets be separated, in order to be displayed as separate targets?
Definition
 By more than one beam width.
Term
 What is the ability of the radar to distinguish between two targets at the same direction but at different ranges called?
Definition
 Range resolution.
Term
 How does the cone of silence affect the viewing of meteorological targets that move into it?
Definition
 Meteorological targets appear to weaken and disappear as they move into the cone of silence.
Term
 Normally, what does the radar beam do with respect to height as it moves away from the antenna?
Definition
 Increases.
Term
 List the two types of anomalous propagation (AP) and briefly describe the atmospheric conditions that cause each to occur.
Definition
 Superrefraction—which occurs when warm, dry air overlies cool, moist air, as in an inversion. The result isthat the radar beam bends below its normal path. Subrefraction—occurs when water vapor contentincreases and temperature decreases with height. The result is that the radar beam travels above its normal path.
Term
 When the radar beam is being refracted below its normal path, what is this process called?
Definition
 Superrefraction.
Term
 What is the straightening of the radar beam upward called?
Definition
 Subrefraction.
Term
 What effect does superrefraction have on the displayed echo heights on the radar?
Definition
 Displayed echo heights are overestimated.
Term
 What effect does subrefraction have on the displayed echo height on the radar?
Definition
 Displayed echo heights are underestimated.
Term
 List five assumptions that the basic radar equation makes?
Definition
 The particles are small, homogeneous spheres whose diameters are much smaller than the radar’s wavelength (Rayleigh scattering) The particles are spread uniformly throughout the contributing region (sample volume) Precipitation throughout the sample volume is the same (all rain or all snow—no mixed precipitation) The main lobe of the antenna beam pattern is adequately described by mathematical notation. Attenuation and multiple scattering are negligible.
Term
 The reflectivity measurement received for each sample volume is simply a measurement of what?
Definition
 Power, expressed in watts.
Term
 The strength of a thunderstorm return increases from 49dBZ to 52dBZ. By how much has the strength increased?
Definition
 100 percent.
Term
 How does the use of the decibel system, to compare returned radar energy, allow increases or decreases to be expressed?
Definition
 Logarithmically.
Term
 What is the best resolution for base reflectivity product and how are the sample bins averaged?
Definition
 The .54nm reflectivity product. Average of four successive .13nm bins.
Term
 Describe how the 1.1nm base reflectivity product is displayed:
Definition
 Displays the highest of every two .54nm bins.
Term
 Describe how the 2.2nm base reflectivity data is displayed:
Definition
 Displays the highest of four .54nm bins.
Term
 What are the maximum degrees of phase shift in electromagnetic energy that can be related to a correct velocity?
Definition
 179º or less than 180º.
Term
 At how many degrees of phase shift do velocities become ambiguous?
Definition
 Greater than or equal to 180°.
Term
 If radar energy strikes a stationary target, what kind of phase shift is experienced?
Definition
 The electromagnetic energy experiences no phase shift.
Term
 If a target moving toward the radar causes the energy to be backscattered, will it be at a higher or lower frequency?
Definition
 Higher.
Term
 What is the Doppler shift?
Definition
 The change in position of the backscattered wave as the radar interprets it to be moving is called the Doppler shift.
Term
 What is meant by the term radar coherency?
Definition
 It is when a radar produces a pulse at the same frequency as the preceding ones and can remember the specific frequency to determine the frequency shift, and therefore, the radial velocity.
Term
 The ability of the radar to compare the frequency of each new pulse with that of the preceding pulse is known as what?
Definition
 Pulsed-pair processing.
Term
 By convention, how are velocities toward the radar expressed?
Definition
 Negative values.
Term
 By convention, how are velocities away from the radar expressed?
Definition
 Positive values.
Term
 The image below represents a single dipole exhibiting a frequency shift; use it to complete the following question. [image]  If the antenna is at point B and facing directly toward E, how is the radar detecting the target velocity?
Definition
 It shows the target as stationary because the motion is perpendicular to the beam; therefore, the target’s velocity is depicted as zero.
Term
 The image below represents a single dipole exhibiting a frequency shift; use it to complete the following question. [image] If the antenna is at point A and facing directly toward D, how is the radar detecting the target velocity?
Definition
 The motion is directly away from the beam. Therefore, the full component or full velocity is detected.
Term
 The image below represents a single dipole exhibiting a frequency shift; use it to complete the following question. [image] If the antenna is at point F and facing directly toward C, how is the radar detecting the target velocity?
Definition
 The motion is neither parallel nor perpendicular to beam, therefore only part of the target’s velocity is detected.
Term
 The image below represents a single dipole exhibiting a frequency shift; use it to complete the following question. [image] If the antenna is at point D and facing directly toward A, how is the radar detecting the target velocity?
Definition
 The motion is directly toward the beam. Therefore, the full component or full velocity is detected.
Term
 If the environmental wind is from 240° at 30kt and the antenna is pointing toward 180°, what is the velocity the radar detects?
Definition
 Using the equation Vr = Va (cos θ) Since θ = 60° (the difference between 240° and 180°) and Va = 30 knots Then: Vr = 30kt (cos 60°) Vr = 30kt (0.5)  Vr= 15kt Since the motion is toward the radar it is expressed as a negative, therefore the answer is –15kt.
Term
 Define unambiguous velocity:
Definition
 The limit to the speeds the WSR–88D can measure without error.
Term
 In radar terms, what are the speeds that exceed the maximum unambiguous velocity called?
Definition
 Aliased velocities.
Term
 What limits velocity detection?
Definition
Term
 How does increasing the PRF affect the chances of aliasing?
Definition
 Increasing the PRF increases the Nyquist co-interval.
Term
 Define Doppler dilemma:
Definition
 It is the tradeoff where an increase in PRF increases the maximum unambiguous velocity but decreases the maximum unambiguous range.
Term
 What is the Doppler dilemma a compromise between?
Definition
 The maximum range and the maximum velocity.
Term
 A PRF of 4,000 pulses per second results in what kind of velocity detection and range?
Definition
 Since 4,000 pulses per second is a high PRF, velocity detection will be high, but the range will be very short.
Term
 What is the square of the standard deviation of the distribution known as?
Definition
 Variance.
Term
 The sample volume represents the smallest amount of the atmosphere that can be instantaneously sampled by the radar. Why is this?
Definition
 The energy returned from particles separated by less than the sample volume’s dimensions will be averaged together as a single return.
Term
 Use the diagram below to answer the following question. [image] Which of the diagrams best shows a precipitation echo with large particles and high internal shear?
Definition
 Figure C, since large particles reflect more power back to the radar there will be more area under the curve.
Term
 What is most likely occurring when you observe low base reflectivity values with high spectrum width values?
Definition
 Early stages of convective development.
Term
 What is most likely the most likely cause of uniform spectrum width values with isolated areas of higher values?
Definition
 Stratiform precipitation with embedded convection.
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