EumetCal Examples

Learning ObjectivesBasic Principles of Weather Radar Systems

• Describe the basic operation of a weather watch radar and the parameters important for a meteorologist

• Identify the wavelengths used in weather radars, and discuss the performance of weather radars as a function of these wavelengths.

• Outline the effect of various meteorological parameters on the propagation and attenuation of radar waves in the atmosphere

• List the factors determining the radar reflectivity of a rain target • Outline the use of radar for estimating rainfall intensity and the problems to

overcome in doing this • Discuss the effect of polarization on the detection of meteorological targets• Define the pulse repetition frequency and describe its effect on the

unambiguous detection of radar targets• Discuss the azimuth and range resolution problems in weather radars• Discuss the radar beam pattern for weather radars, and evaluate the effect of

side lobes on radar displays• Describe the principles of operation of Doppler Radar and the factors which

must be considered when interpreting it

Radar Principles Review Exercise Read the paragraph below and fill in the missing words. If the frequency of a given radar is 3*109 Hz, what is its wavelength (in cm)? Match common weather radar wavelengths with their designated band (X, C, S, K): 5.4cm 10.3 cm 3.2cmThe pulse length of a radar is 1.2 km, or 4 microseconds. What is the range resolution of this radar set (in meters)? If the pulse length of a radar is 4.0 km, what is the best range resolution you can expect (in km)? What is the unambiguous range of a radar whose pulse length = 2 microseconds and PRF = 250 pulse per second (in km)? About how much of the broadcast energy, in percent, is contained in the half-power radar beam? A radar features a parabolic reflector with a diameter = 12 feet and an angular beam width = 2 degrees. What is the approximate wavelength of this radar in cm?

Side lobeswww.eumetcal.org/courses/mod/resource/view.php?r=642&frameset=ims&page=18

Side lobesA radar antenna focuses transmitted energy in a narrow, conical-shaped beam (see Figure 18). The angular width of the

radar beam is defined as that region of transmitted energy that is bounded by one-half (-3 dB) the maximum power. Around 80% of the energy is contained within the main beam, with the sidelobes generally being more than 1000 times less intense in a high quality antenna (unlike in the p-theta graph).

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Figure 18: Distribution of power on the radar at angle 0° Right →

Figure 19: Distribution of power with azimuth and elevation angles The maximum power lies along the beam centreline and decreases outward. Note that the pattern is actually 3

dimensional (see Figure 19). Note that the beamwidth is proportional to the wavelength, but inversely proportional to the antenna diameter. This means that a one degree beamwidth C band radar will have an antenna half the diameter of an S-band radar of a similar beamwidth. Similarly even though an S band radar may have the same antenna diameter as a C band radar, its beamwidth will be twice as much. Ideally we want a radar with a small beamwidth and sidelobes very much below the main lobe. That means that an antenna built to high quality specifications (ie as close to parabolic as possible), with a narrow beamwidth for better resolution is desired. The cost of a radar is largely driven by the size of its antenna; the bigger the antenna, the bigger the cost due to the more robust the gears, the bigger the radome etc etc.

Due to diffraction experienced by the electromagnetic energy at the edge of the parabolic reflector, only about 80 percent of the energy broadcast by a weather radar is contained in the half-power radar beam. Most of the remaining 20 percent is broadcast in other directions (some energy outside the half-power points also travels in the direction the antenna is pointing).

Side lobesThe radar beam in Figure 20 (the half-power beam) originates from the transmitting site, and the solid line indicates the distance (in all directions) at which

some given power density value would be observed. Of course, the distance is greatest along the centreline, where the largest concentration of broadcast energy is found. Notice, however, that “spikes” exist in other directions as well, indicating that there are preferred directions for the remaining energy to travel. These “spikes” extend outward only a short distance and contain very low power densities compared to the half-power radar beam.

Figure 20: Directional power density pattern

Nearly all the energy not contained in the half-power beam tends to travel in these other preferred directions, as shown in Figure 21, and comprises the

side lobes, which are presently an unavoidable and detrimental part of any weather radar system. The beam pattern presented in Figure 21 is representative of weather radar that employ parabolic reflectors, but each radar has its own unique beam

pattern. The centreline axis corresponds to zero degrees, where the reference value of power density (0 dB) is found. Moving to the right away from zero degrees, the first side lobe is found near 20 degrees, the second near 25 degrees, and so on, to the final one at 1800 degrees (opposite to the direction of broadcast). Note that the highest power density associated with any side lobe is at least 30 dB below that associated with the half-power radar beam.

Figure 21: Power density as a function of the angle of the antenna from the centerline Since the radar beam is three-dimensional, the side lobe pattern is also. Again, the pattern in the vertical for a weather radar with a parabolic reflector

would be as indicated in Figure 21, but with angles form the centreline measured vertically. Since side lobes contain much lower power densities than the half-power radar beam, the great majority of “side-effects” occur at close range. Since energy

returned to the receiver produces a displayed target in the pointing direction of the antenna, side lobe return can produce multiple displays of the same target (usually only at close range). Side lobe return accounts for the solid character of the display pattern from non-precipitating targets at close range, usually called ground clutter. This, of course, makes it very difficult at times to separate precipitation from non-weather targets within the ground clutter pattern. Predictable, unique ground clutter patterns exist for each radar site.

When measurements of storm tops are made by elevating the beam, convective cells which are highly reflective and/or at close range may return enough side lobe energy to be detected – even when the beam has cleared the storm top. The result is a narrow, elongated signature extending on the RHI directly above the storm core of highest indicated intensity. This feature has been called the “hail spike”, but is not always the direct result of the presence of hail. Rather, it represents the interaction of side lobe energy with a highly reflective storm core.

Rapid Cyclogenesis Case Study

• Define rapid cyclogenesis• To diagnose the precursor of cyclogenesis using

conceptual models.• Recognise signals within model fields that indicate

the likelihood of rapid cyclogenesis.• To diagnose development using real time satellite

and observational data.