presented by:
A.MAHENDRA CHARY
T.SRI MALLIKA

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“Nature can conquer man but man cannot conquer nature” goes the conventional saying.
But with the development of science, man can certainly able to predict adverse weather conditions to overcome huge losses of men and resources. Meteorology is the branch of science that has helped us out in this regard. Role of radar in this aspect has proved vital in many cases. In this paper, we describe how RADAR can be used in detecting the natural calamities.
1. INTRODUCTION
Radar (Radio Detection and Ranging) is basically a means of gathering information about distant objects or targets by sending electromagnetic waves to them and analyzing reflected waves or echo signals. Radar reflectivity is related to precipitation rate, total amount of precipitation falling on region over a fixed period of time can be determined by analyzing the reflectivity field. The targets to radar in this aspects or mainly raindrops however, various objects on ground varying from huge buildings to small air born insects also influence this criterion various calamities and extreme conditions like tornadoes, hurricanes, flash floods, snow storms can be predicted remarkably well by the radar and the following paper makes the brief yet effective contribution over this issue.
The radar creates an electromagnetic energy pulse which is focused by an antenna and transmitted through the atmosphere. Objects in the path of this electromagnetic pulse, called targets, scatter the electromagnetic energy. Some of that energy is scattered back toward the radar.
The receiving antenna (which is normally also the transmitting antenna) gathers this back-scattered radiation and feeds it to a device called a receiver.
2. SCATTERING OF A RADAR PULSE------ BY A TARGET TO THE RECIEVER
When a pulse encounters a target...
It is scattered in all directions. Of interest is the signal component received back at the radar. This signal is typically much weaker than the original sent from the transmitter and is called the "return signal".
The larger the target, the stronger the scattered signal.
The more targets there are to scatter the pulse, the stronger the return will be because the return signals from each target combine to produce a stronger signal. This means that many large raindrops will produce a stronger return than a few small raindrops. The quantity that radar measures is the returned power which, with knowledge of other radar characteristics, is converted to a quantity called the reflectivity factor, or more simply, the "reflectivity".
The magnitude of the reflectivity is related to the number and size of the drops encountered by the electromagnetic pulse. For this reason, high reflectivity generally implies heavy precipitation while low reflectivity implies lighter precipitation. Plots of the radar reflectivity, typically using colors to depict its magnitude, show both the location and intensity of precipitation. Extremely high reflectivities often indicate hail.
3. EFFECTS OF WAVELENGTH -----ON THE ABILITY TO DETECT AN OBJECT
The factors which govern the choice of a wavelength to be used in particular radar include its sensitivity, which is its ability to detect weak targets at long range, the radar's ability to resolve small features, the types of targets to be studied, and the effects of the intervening atmosphere on the transmitted energy. Other factors also must be considered such as the radar's size, weight and cost. Most weather radars have wavelengths that range between 0.8 centimeters (cm) and 10.0 cm. Generally short wavelengths mean smaller and less expensive equipment.
Short wavelength radars are more effective in detecting small particles such as cloud droplets and drizzle drops. However, the short wavelength electromagnetic energy is also partially absorbed by these same particles (a process called attenuation). This makes it difficult to accurately measure the intensity of back-scattered energy for more distant targets that lie beyond the range of closer targets.
The main advantage of using longer wavelengths is that absorption by the intervening particles is drastically reduced. This means that a distant thunderstorm behind a closer thunderstorm will appear on the radar screen with its proper intensity. Since detecting severe weather is one of the most important missions of operational radars, such as the National Weather Service's WSR-88D Doppler radars, these radars typically use a long wavelength.
4. EFFECT OF THE EARTH’S CURVATURE
One must account for the curvature of the earth when determining the altitude of a target.
Distant targets, which are close to the ground, cannot be seen by a radar because they will be below the horizon. (Fig 1)
The height of a distant target that is above the horizon will be underestimated if the curvature of the earth is not taken into account. For example, the height of the target on the figure below would be underestimated as "h" rather than the actual height "H".
A second effect, called refraction, also affects the path the electromagnetic energy will take as it propagates through the atmosphere. Normally, because the atmosphere's density decreases rapidly with height, the radar beam will be deflected downward, much like light passing through a glass prism. In extreme cases, where temperature increases with height and dry air overlays warm air, (a condition often found along coastlines), the beam can bend down dramatically and even strike the ground. Meteorologists call this effect "anomalous propagation". Both the curvature of the earth and normal atmospheric refraction must be accounted for when determining the position of a target.
5. CLEAR AIR RETURNS -- INSECTS AND TURBULENCE
When radar transmits energy, part of it may be intercepted by targets on the ground, such as buildings, trees, cars, or other objects. The return signal from these objects is called "ground clutter". Ground clutter interferes with the detection of meteorological targets, such as raindrops, because ground targets are large and typically produce high reflectivity. Ground clutter can result even if the main radar beam is above ground targets because part of the energy radiated from the antenna is emitted off the beam axis in what are known as "side lobes". Back-scattered energy from the side lobes is interpreted by the radar processor to come from the main lobe, so ground targets hit by one of the side lobes appear to a radar user in the same relative position in the main lobe.
Ground clutter is usually the worst within about 20 kilometers of the radar site where the beam is still close to the earth's surface. Farther from the site, the beam is higher due to both its elevation angle and curvature of the earth away from the radar site. Ground clutter is easily identified with a Doppler radar because the radial velocity measured by the Doppler radar will approximately be zero since none of the ground targets are moving with respect to the radar. The radial velocity is not exactly zero because moving targets within the beam, such as birds, bugs, or even raindrops, also contribute to the total power return to the radar.
6. THE EFFECT OF BUGS :
Insects present large targets to radar and they are always present during the warmer seasons. This actually is beneficial to meteorologists. Doppler radars require targets to determine the motion of the air. Outside of regions where precipitation is falling, there would be no targets if there were no insects. Airborne insects turn out to be very good tracers of air motion since, on average, they blow along with the wind. The returns from insects allow meteorologists to see air motions outside the storm circulation, which in many cases is important for predicting where new storms are likely to occur.
Turbulence provides another way in which electromagnetic energy from radar can be back-scattered. Turbulence is associated with variations in density in the atmosphere. When variations in density occur on a scale of half the wavelength of the radar, energy is scattered through a process called diffraction. Radar echoes in a clear atmosphere will be more common on days when the lower atmosphere is unstable, as when there are thermals present, or when the wind increases rapidly with height just above the ground, so that there is mechanical turbulence.