Met 312- Radar Study Guide
Describe sources of clutter (when/where they occur).
(Ground) Clutter: semi- permanent (mountains, buildings) and transient (airplanes, vehicles)
Compute the approximate beam width given an antenna's diameter and the radar's wavelength. Know why it isn't practical to have an extremely narrow beam width
Beam width is proportional to 1/antenna size Beam width in radians = 1.22*wavelength/ Diameter Beam width in degrees 70*wavelength/Diameter It is zone shaped so there would be a very narrow portion that gets sampled.
Be able to identify sources of error for the dual pol products.
CC: degradation with range or at edge of precip with weak returns (noisy CC values greater than 1.0). Non uniform beam filling, result of heavy precip oriented along a radial (drop of CC in this area). Range folding in batch cuts. ZDR: mie scattering, there is bias towards large hydrometeors. Depolarization happens when the pulse is scattered back to the radar in it's opposite polar orientation so it show's up as a radial spike of high and low ZDR. KDP: Most be more than 0.90 CC which can lead to empty spots. Noisy with low returns, low reflectivity at long ranges or weak reflectivity on edges of storms. Non uniform beam filling which can make the CC less than 0.90
Describe the characteristics of a radar beam including: center beam side lobes beam width half power points height of beam with distance effects of partial beam filling
Center Beam: between the two half power points. Side Lobes: some energy is diffracted off the edge of the antenna, resulting in many spikes extending from the radar dish at angles to the radar beam. Beam Width: distance between the two half power points. Half Power points: Point where there is a 50% reduction in the radar's transmitted energy. Height of beam with distance: Beam gets longer (higher) with distance. Effects of partial beam filling: distant targets occupy less of an expanded beam. Thunderstorms may seem to be together when they are not. The thunderstorm that is closer will seem more powerful because it fills up more of the beam when it is really just closer and the same strength.
Clear Air Mode
Clear air mode updates images every ten minutes. Slowest antenna rotation. More sensitive.
Be able to define the 3 different dual pol products (CC, ZDR, KDP)
Correlation Coefficient- the measure of how similarly the horizontally and vertically polarized pulses are behaving within a pulse volume (0.2-1.05). Less than 0.8 is non meteorological, 0.8-0.97 is non uniform met (mixed precip), and 0.97-1 is uniform (like all rain). ZDR (differential reflectivity)- difference between the horizontal and vertical reflectivity facts in dBz. Usually -7.9 - +7.9. ZDR=ZH -ZV. Primary usage of ZDR is to provide an estimate of the median rain drop size. Sphere is around zero, if the ZDR is more than zero it is vertically oriented, if it is less than zero it is horizontally oriented. KDP- the range derivative of the differential phase shift along a radial between the horizontal and vertical pulse phases. -2 - 10 degrees/km. Heaviest rain will have the highest positive KDP (big drops and high concentration). Non met is noisy. Best used to detect areas of heavy rain.
Describe the meaning of the Gain of an antenna. Be able to compute the Gain of an antenna given information about power at a point from the radar in question and an isotropic radar
Gain of the antenna is the ratio of power that is received at a specific point in space (usually the center of the beam axis) with the reflector in place (p1) to the power that would be received at the same point from an isotropic antenna. Gain represents the capacity of the antenna (p2) to concentrate energy (including losses) (typical value = around 1000). How much power you gain by focusing the energy. g = p1/p2 g is gain of the antenna. G has units of decibels. Typical Gains for WX radars are 20-40 dB. G= 10log10 (p1/p2)
Be able to interpret the 3 dual pol products given a radar image (high or low values and what do they mean?) Be able to identify areas of heavy rainfall given KDP Be able to identify areas with hail given Z and ZDR Be able to identify melt layer given CC
High values mean... CC: Low CC means that is not meteorological, high values mean it is uniform meteoritical. ZDR: Higher ZDR is heavier rain and low means hail KDP: High KDP indicates where heavy rainfall is. Low means hail or snow and ice High reflectivity with low ZDR means hail and high reflectivity and high ZDR means large rain drops Heavy rainfall on KDP: consists of high values Hail on ZDR: low ZDR but Z would be high Melt Layer on CC: melt layer is the top of the bright band
Identify a second trip echo on a reflectivity image and explain what causes their appearance.
It is an echo from a given pulse that is not received until after transmission of the next pulse. Second trip echoes conserve azimuth angle, are weaker, and sometimes are associated with strange Doppler velocities. Eliminate it by changing listening time, use a different PRF every 2-3 pulses.
Be able to interpret Doppler radial velocity into actual wind velocity and direction with height.
It is the velocity of air towards or away from the radar so it is not the exact velocity. Parallel winds are not picked up. Positive values are for targets moving away from the radar. Negative values are for targets moving toward the radar.
Given the data display from a wind profiler be able to identify wind direction and speed with height for specific times.
It is time vs height. Be able to read wind barbs for magnitude and direction.
Given an image, be able to identify the type of display system used and its attributes.
Knowing if it is a PPI and knowing what the image means- the different heights and how that relates to azimuth.
Describe the characteristics of the WSR-88D radar used by the NWS.
Looks at 14 different elevations every five minutes. Transmits at 750,000 Watts. The angle between the two half points is 1 degree. Is an S band, average is 10cm. Only does the volume scans. There are modes like precipitation mode and clear air mode. Page 39 in textbook for more details.
Know what causes areas to have high spectrum width.
Means there is turbulence. Associated with tornadoes or a gust front.
Derived Products:
Melting Layer Detection Algorithm using ZDR and CC data. Quantitative Precipitation Estimation uses ZR relationship for rainfall rate based on the estimated precip type.
Compute pulse repetition frequency and determine the Maximum Unambiguous Range, and Maximum Unambiguous Velocity. Be able to explain the compromise between these 2 variables based on PRF.
PRF = c/2r Max unambiguous range = c/(zPRF) Vmax = (wavelength*PRF)/4
Describe scanning strategies for both PPI and RHI displays as well as VCPs
Plan Position Indicator: constant elevation angle, varies azimuth angle. The returns can be mapped on a horizontal plane. Can rotate 360 degrees. Range Height Indicator: constant azimuth angle, varies elevation angle. Rotated from horizontal to directly up at the sky overhead. VCPs: the radar makes multiple 360 degree scans of the atmosphere, sampling a set of increasing elevation angles.
Precipitation Mode
Precip mode updates images every 5-6 minutes. Less sensitive. Meant to see higher in the atmosphere.
Compute target distance from radar given a round trip delay time.
R=ct/2 Distance= speed of light * pulse's round trip time / 2
Given a reflectivity scan, be able to interpret a bright band and give a reason for it having such high reflectivity values.
Relatively high equivalent reflectivity that usually appears as an elevated layer at the height where falling ice particles begin to melt and thus become water coated.
Know the wavelength of S (10 cm), C (5 cm), X (3 cm), and K (8 mm) band radars.
S: 7.5-15 cm C: 4-7.5 cm X: 2.5-4 cm K: About 1
Define attenuation and know how the wavelength of a radar affects the attenuation of its beam.
Storms and precipitation close to the radar degrade the radar energy before it reaches storms further from the radar. Smaller wavelength radar beams attenuate more rapidly than long wavelength radar.
Explain in which cases and why refraction, subrefraction, superrefraction and ducting would occur: be able to identify clutter and AP on a reflectivity image
Subrefraction: happens with the decrease of density with height being more than normal. The beam bends less than normal and climbs too high. Refraction: normal atmospheric density naturally decreases in increasing elevation. Superrefraction: happens when there is a temperature inversion. If the decrease in density with height is less than normal.
Be able to identify the parts of the Radar equation.
T - pulse duration PT- power transmitted G- gain Theta- horizontal arc length Phi - vertical arc length Lamduh - wavelength r- distance from radar to target Pr- power received K- how the radiation is scattered Z = (1024ln(2)/cpi^3)* (lamduh^2/(PT*T*G^2*Theta*Phi))*(r^2*Pr/absltK^2)
Describe the function of each main component of the radar system.
The antenna: takes and inputs the EM waves from the transmitter and introduces them into the propagation medium (normally the atmosphere) Transmitter generates the EM wave. Receiver gets the information from the antenna. Indicator presents and interprets information gathered.
Describe the 3 pieces of information the radar gathers to determine target location.
The time it takes for a pulse to be transmitted and come back. Shift in phase (shape, position, form) is tracked and how it changes gives meaning to the target location.
Be able to describe the limitations of using VIL.
VIL is the integration of reflectivity within a column of air. A higher VIL means there is more precipitation in a column of air. The values are seasonally and regionally dependent. The value is also dependent upon the cloud physics and synoptic considerations for that particular day. VILs are less accurate for highly tilted storms. VIL is underestimated for tilted updrafts since the hail core tilts with respect to a vertical column of reflectivity. VIL value may be contaminated by non-precipitation echoes. Values within 20 miles of radar will be underestimated since part of the storm will be in the radar's cone of silence. Values at the edge of the radar display are also underestimated since the higher elevation angles are overshooting the storms (lower half of storm is not being sampled). VIL values will be higher where all tilt angles are able to sample the entire precipitation / hail core of the storm. There is the cone of silence.
Given velocity data, be able to interpret veering, backing, divergence, convergence, and windspeed/direction with height for linear flow
Veering: clockwise, warm air advection. s shape Backing: counterclockwise, cold air advection. Backward s shape Divergence: signatures have max velocity aligned on the same radial with the inbound max closest towards the radar Convergence: signatures have max velocity aligned on the same radial with the outbound max closest towards the radar Windspeed/ direction with height for linear flow: linear like is it a steady increase/ decrease with height be able to know how the windspeed changes (color)
Be able to define wind profilers
Velocities of wind at specific heights and specific times.
Describe the meaning of a ZR relationship and be able to identify the equation used for the WSR-88D
WSR-88D uses Z= 300r^1.4 ZR relationship is determining precip in different situations. It is between the radar reflectivity factor and the rainfall rate.
Be able to compute EM wavelength given the frequency and vice versa.
velocity/frequency= wavelength Velocity/Wavelength= Frequency
