INTRODUCTION TO RADARS

Radar term is the abbreviation of RAdio Detecting And Ranging, i.e. finding and positioning a target and determining the distance between the target and the source by using radio frequency. This term was first used by the U.S. Navy in 1940 and adopted universally in 1943. It was originally called Radio Direction Finding (R.D.F.) in England. We can say that, everything for radar started with the discovering of radio frequencies, and invention of some sub components, e.g. electronic devices, resulted invention and developing of radar systems. The history of radar includes the various practical and theoretical discoveries of the 18th, 19th and early 20th centuries that paved the way for the use of radio as means of communication. Although the development of radar as a stand-alone technology did not occur until World War II, the basic principle of radar detection is almost as old as the subject of electromagnetism itself. Some of the major milestones of radar history are as follows:

1842 It was described by Christian Andreas Doppler that the sound waves from a source coming closer to a standing person have a higher frequency while the sound waves from a source going away from a standing person have a lower frequency. That approach is valid for radio waves, too. In other words, observed frequency of light and sound waves was affected by the relative motion of the source and the detector. This phenomenon became known as the Doppler Effect.

1860 Electric and magnetic fields were discovered by Michael Faraday.

1864 Mathematical equations of electromagnetism were determined by James Clark Maxwell. Maxwell set forth the theory of light must be accepted as an electromagnetic wave. Electromagnetic field and wave were put forth consideration by Maxwell.

1886 Theories of Maxwell were experimentally tested and similarity between radio and light waves was demonstrated by Heinrich Hertz.

1888 Electromagnetic waves set forth by Maxwell were discovered by Heinrich Hertz. He showed that radio waves could be reflected by metallic and dielectric bodies.

1900 Radar concept was documented by Nicola Tesla as .Exactly as the sound, so an electrical wave is reflected ... we may determine the relative position or course of a moving object such as a vessel... or its speed."

1922 Detection of ships by radio waves and radio communication between continents was demonstrated by Guglielmo Marconi.

1922 A wooden ship was detected by using CW radar by Albert Hoyt Taylor and Leo C.Young.

1925 The first application of the pulse technique was used to measure distance by G. Breit and M. Truve.

1940 Microwaves were started to be used for long-range detection.

1947 The first weather radar was installed in Washington D.C. on February 14.

1950 Radars were put into operation for the detection and tracking of weather phenomena such as thunderstorms and cyclones.

1990s A dramatic upgrade to radars came in with the Doppler radar.

RADAR TYPES

Radars may be classified in several ways due to the criteria of the classification, e.g. receiving and transmitting type, purpose of the use, operating frequency band, signal emitting type (pulse-CW), polarization type. It is also possible to make sub classifications under the main classification of radars. So major types of radars have been denominated as monostatic, bistatic, pulse, continuous (CW), Doppler, non-Doppler, weather radar, air surveillance radar, mobile radar, stationary radar, X-Band, L-Band, C-Band, S-Band, K-Band, single polarization radars, polarimetric radars, etc. Although our main concern is Doppler weather radars which will be studied in detail,

Al these Radars can be classified into two Types of Radar Detectors:

1. Pulse radar; 2.Continuous-wave radar

Brief explanation of major types of the radars are also given below:

Monostatic Radars

Monostatic radars use a common or adjacent antennas for transmission and reception, where the radars receiving antenna is in relationship to its transmitting antenna. Most radar system are use a single antenna for both transmitting and receiving; the received signal must come back to the same place it left in order to be received. This kind of radar is monostatic radar. Doppler weather radars are monostatic radars.

Bistatic Radars

Bistatic radars have two antennas. Sometimes these are side by side but sometimes the transmitter and its antenna at one location and the receiver and its antenna at another. In this kind of radar the transmitting radar system aims at a particular place in the sky where a cloud or other target is located. The signal from this point is scattered or reradiated in many directions, much of being in a generally forward direction. Such receiving systems may also be called passive radar systems.

Air Surveillance Radars (ASR)

The ASR system consists of two subsystems: primary surveillance radar and secondarysurveillance radar. The primary surveillance radar uses a continually rotating antenna mounted on a tower to transmit electromagnetic waves, which reflect from the surface ofaircraft up to 60 nautical miles from the radar. The radar system measures the time required for the radar echo to return and the direction of the signal. From this data, the system can measure the distance of the aircraft from the radar antenna and the azimuth or direction of the aircraft from the antenna. The primary radar also provides data on six levels of rainfall intensity. The primary radar operates in the range of 2700 to 2900 MHz.The secondary radar, also called as the beacon radar, uses a second radar antenna attached to the top of the primary radar antenna to transmit and receive aircraft data such as barometric altitude, identification code and emergency conditions. Military and commercial aircraft have transponders that automatically respond to a signal from the secondary radar with an identification code and altitude.

Synthetic Aperture Radars (SAR)

SAR is being used in air and space-borne systems for remote sensing. The inherent high resolution of this radar type is achieved by a very small beam width which in turn is generated by an effective long antenna, namely by signal-processing means rather by the actual use of a long physical antenna. This is done by moving a single radiating line of elements mounted e.g. in an aircraft and storing the received signals to form the target picture afterwards by signal processing. The resulting radar images look like photos because of the high resolution. Instead of moving radar relatively to a stationary target, it is possible to generate an image by moving the object relative to stationary radar. This method is called Inverse SAR (ISAR) or range Doppler imaging.

Continuous Wave (CW) Radars

The CW transmitter generates continuously unmodulated RF waves of constant frequency which pass the antenna and travel through the space until they are reflected by an object. The isolator shall prevent any direct leakage of the transmitter energy into the receiver and thus avoid the saturation or desensitisation of the receiver which must amplify the small signals received by the antenna. The CW radar can only detect the presence of a reflected object and its direction but it cannot extract range for there are no convenient time marks in which to measure the time interval. Therefore this radar is used mainly to extract the speed of moving objects. The principle used is the Doppler Effect.

FM-CW Radars

The inability of simple CW radar to measure range is related to the relatively narrow spectrum (bandwidth) of its transmitted waveform. Some sort of timing mark must be applied to the CW carrier if range is to be measured. The timing mark permits the time of transmission and the time of return to be recognised. The sharper or more distinct the mark, the more accurate is the measurement of the transit time. But the more distinct the timing mark, the broader will be the transmitted spectrum. Therefore a certain spectrum width must be transmitted if transit time or range is to be measured.The spectrum of a CW transmission can be broadened by the application of modulation, either by modulating the amplitude, the frequency or the phase. An example of the amplitude modulation is the pulse radar.

Pulse Radars

Pulse radar is primary radar which transmits a high-frequency impulsive signal of high power. After this a longer break in which the echoes can be received follows before a new transmitted signal is sent out. Direction, distance and sometimes if necessary the altitude of the target can be determined from the measured antenna position and propagation time of the pulse-signal. Weather radars are pulse radars.

Doppler Radars

Conventional radars use MTI in order to remove clutter as explained above. This processing system is used almost entirely to eliminate unwanted clutter from the background, selecting as targets only those objects which move with some minimum velocity relative to the radar or to the fixed background. A more advanced type of system is the pulse Doppler radar, defined as a pulsed radar system which utilises the Doppler Effect for obtaining information about the target, such as the target’s velocity and amplitude and not to use it for clutter rejection purposes only.

Basics principles of Pulsed (Wind Profiler) radar- Antenna Basics- radar signal processing

A simplified block diagram of the Wind profiler radar is shown in the diagram. The units to the left of the vertical dashed line can be located inside a small laboratory building or trailer. The antenna and the wind profiler transmit/receive unit shown to the right of the dashed line in diagram are located in the field. The wind profiler

transmit/receive unit is mounted directly below the microstrip antenna. Both transmitted and received signals are converted to an intermediate frequency (IF) of 50 MHz (for VHF wind profiler) to achieve lower transmission cable losses between the field and the laboratory/trailer. The 50 Mhz IF was chosen to correspond to conventional wind profiler frequency. In more general applications, any IF in the range of 30 to 70 MHz.

Fig. Block Diagram of wind profiler radar

Antenna:

Antennas are one of the most important components of a radar. At this point I feel a little like TV weatherman Willard Scott. Every time he broadcasts from a different city, he says that” this is my favorite city in the whole country”. Now that we are ready to discuss antennas, I have to say that antennas are one of my favorite parts of a radar!

The antenna is the device which sends the radar’s signal into the atmosphere. Most antennas used with radars are directional; that is, they focus the energy into a particular direction and not in other directions. One of the great advantages of radar is its ability to determine the direction of a target from the radar. It is the ability of a radar’s antenna to aim energy in one direction that makes it possible to locate targets in space.

An antenna that sends radiation equally in all directions is called an isotropic antenna. It can be compared to the light from a candle. A candle’s light is approximately the same brightness in all directions, except, of course, directly below the candle. For weather radar, transmitting a signal equally in all directions would usually not be very useful. Instead, radars are more like flashlights. Flashlights put a shiny reflector behind the light bulb to direct the light in a specific direction.

Weather radar usually have both an antenna and a reflector. The real antenna is the radiating element which transmits the radar signal into the atmosphere toward the reflector that then reflects and directs the signal away from the radar. Most weather radar use a feed horn as the true antenna although some use dipoles or other radiating elements.

The shape of the reflector determines the shape of the antenna beam pattern. Most for the meteorological radars have reflectors which are parabolic in cross- section and circular when viewed from the front or back; naturally, they are called circular parabolic reflections. The beam pattern formed by a circular parabolic reflector is conical and usually quite narrow, typically 1° in width for the better it is able to direct the signal and the narrower the beam of the antenna.

But are other kinds of antennas used for meteorological radar. Fig 1.5 shows a Cassegrainfeed antenna. In this design, the actual antenna is located at the end of the tube coming out of the center of the main parabolic reflector. The signal is aimed at a hyperbolic-shaped subreflector. The signal then reflects back to the main parabolic reflector and then out into space. The received signal follows the reciprocal path from space to the main reflector to the subreflector feedhorn to the receiver. One of the advantages of the Cassegtain feed is that they usually have better sidelobes for a given size antenna. The antenna beam pattern shown in Fig. 1.9 is from a Cassegrain antenna.

There are a number of things we need to know about an antenna. One is the wavelength it is desgned for. The radar transmitter determines this parameter, the antenna must match the transmitter’s wavelength.

A second parameter of interest is the size of the reflector. For circular parabolic reflectors, this is its diameter. Antenna of weather radars range from a small a foot to as much as 30 ft in diameter.

Another measure of importance t radar antennas is the gain of the antenna. The gain of an antenna is the ratio of the power that is received at a specific point in space (on the center of the beam axis, i.e., at the point where the maximum power exists) with the radar reflector in place to the power that would be received at the same point from an isotropic antenna. This is a unitless ratio since it is one power divided by another power and units cancel. In equation from, gain is defined as

g = (p1/p2) -------------------------------------------------- (1.1)

where p1 is the power on the beam axis with the antenna and p2 is the power from an isotropic antenna at the same point.

Usually antenna gain is measured logarithmically in decibels ( see Appendix A for a more complete discussion of logarithmic units). A power ratio in decibels is defined as

P = 10 log10 (p1/p2)

Where both powers p1, and p2 are measured in the same units, p is the logarithmic power ratio in decibels, and “log10 ()” represents the logarithm to the base 10 of the term in parentheses.

Since antenna gain g is actually a power ratio, w can thus write it in logarithmic form as

G = 10log10 (p1/p2) ------------------------------------------------------------- (1.2)

Where gain G has units of decibels. Typically antenna gains for meteorological radars range from 20 to 45 dB. The gain of an isotropic radiator would be 0 db (i.e., p1 = p2, so p1/p2 = 1).

Another important parameter of an antenna is its beamwidth. The beamwidth of antenna is defined as the angular width of the antenna beam measured from the point where the power is exactly half what it is at the same range on the center of the beam axis. Figure 1.6 illustrates the beamwidth of an antenna.

Antenna gain and antenna beamwidth are related. One expression that can be used to calculate one from the other is ( Battan, 1973)

g = π2k2 / θφ------------------------------------------------------------------ (1.3)

where θ and φ are the horizontal and vertical beamwidths of the antenna, respectively, and both are measured in radians. K2 depends upon the kind and shape of antenna. For circular reflectors, k=1. For circular reflectors, the horizontal and vertical beamwidths would be equal, giving

g = π2/θ2 -------------------------------------------------------------------- (1.4)

For example, for an antenna with a 1° beamwidth, the gain would be

= 3200

π2

g= ------------- { 1o π /180o }2

= 32400

Or, in logarithmic units,

G = 45.1 dB

Notice that gain is independent of wavelength. Any circular parabolic radar antenna with a 1° beamwidth would have the same antenna gain at any frequency, according to Eq. 1.4.

The shape of the mainlobe is often approximated the form

g = g0 exp( -2θ2/θ02)-------------------------------------------- (1.5)

where g is the gain at any arbitrary angle θ from the center of the mainlobe axis, θ0 is the beamwidth of the mainlobe, and g0 is the maximum gain on the beam axis. If g0 is set to 1, then g is the relative gain of the Gaussian beam pattern.

As mentioned, gain frequently expressed logarithmically. One equation for doing this is (Dovik and Zrnic, 1993)

G = -l6 ln (2)(θ/θ0) -----------------------------------------------(1.6)

Where G is the relative logarithmic gain (in decibels) at any angular distance θ from the beam axis, and θ0 is the beamwidth of the pattern. By multiplying the right side of Eq.1.6 by the maximum gain on the direction of θ.

According to Eq. 1.6, a radar antenna’s mainlobe beam pattern (measured in decibels) decrease in magnitude approximately, proportional to the square of the angular distance from the beam axis. If mainlobes really are Gaussian in shape, the decrease would be exactly as the square of the angular distance would be exactly as the square of the angular distance. Actual antennas differ slightly from this, however. For example, the National centre for Atmospheric Research CP2 S-band radar antenna had a mainlobe that decreased approximately to the 2.2 power of angular distance from the mainlobe.

Much as we would like to believe it, antennas are not perfect devices. The ideal antenna would direct all of the radar’s energy into a single direction and none of it would go anywhere else. This is physically impossible. Even flashlights do not do this job perfectly. While most light from a flashlight does go in some preferred direction, some of the light can be seen well off to sides of the brightest spot. Further, the illumination of the brightest spot is seldom uniform.

Real radar antennas are much like this. They will have a bright spot (called the mainlobe), but they will also transmit and receive energy off to the side of the mainlobe

in what are called sidelobes. Further, the sidelobes exist in all directions away from the mainlobe and are different from one direction to another. One difference between radar antennas and flashlights is that some of the radar energy can actually go directly behind the antenna, forming a “blacklobe”.

The top-hat antenna gain pattern shown in Fig.1.6 is for a perfect antenna while the Gaussian antenna beam pattern is reasonable approximation to the mainlobe of real antennas. But in either case, the pattern on Fig. 1.6, has no sidelobes whatsoever. Real antennas have sidelobes, and sometimes very strong sidelines. Let’s examine some sidelobes from a couple of radar antennas. When we examine antenna beam patterns, we usually only do so in one direction at a time, either in azimuth or in elevation. Figure 1.7 shows the antenna beam pattern in the horizontal direction for the AN/CPS-9 X-band antenna used by the Air Force and others during the 1950’s and ‘60’s (Donaldson, 1964). This pattern is a smoothed, idealized fit to the real pattern. It shows that the simple, single lobe pattern of Fig.1.6 is not a good approximation only near the center of the mainlobe. It does not represent the sidelobes at all!

Now let’s look at the measured antenna beam pattern form a real antenna. Figure 1.8 shows part of the antenna beam pattern for the FL2 radar, an S-band (10-cm wavelength) radar operated by MIT Lincoln Laboratory (see Appendix D). This pattern was measured by placing a calibrated signal generator at some distance from the radar antenna and scanning the antenna slowly in azimuth through almost a full circle while receiving and recording the signal. The antenna was aimed directly at the signal generator. Nearby sidelobes are also shown as moderately strong but narrow “spikes” on the pattern. Near 1200 either side of the mainlobe are regions of stronger sidelobes. Notice that some power was even detected when the antenna was aimed in the opposite direction from the signal generator (i.e., at an azimuth of 1800 ).

Figure 1.7 Modeled one-way antenna beam pattern for the CPS-9 X-band antenna, showing the mainlobe and the first three sidelobes. This antenna has a mainlobe with a 1° beamwidth. From Donation, 1964. (A) shows the beam pattern as

function of gain and angle. (B) shows the pattern in polar coordinate (10 –dB contours). It shows that the silobes are really quite close to the mainlobe of the beam pattern.

As complex as the antenna beam pattern is that is shown in Fig. 1.8, it does not really portray the complexity of a complete antenna beam pattern. The pattern shown is, after all, a single slice through what is really a two- dimensional pattern. As an example of the complexity of a real pattern in both azimuth and elevation, Fig. 1.9 shows the antenna beam pattern for the CP2 X-band Cassegrain-feed antenna of the National Center for Atmospheric Research (Rinehart and Frush, 1083). This pattern was obtained by transporting the antenna to the antenna range of the National Bureau of Standards in Boulder, Colorado, an expensive and time-consuming activity. As can be seen in the figure, there are a number of sidelobes encircling the mainlobe, with each successive sidelobe ring generally being of weaker strength. Note that this pattern only covers 100 of azimuth and elevation, 50 either side of the center of the mainlobe. A complete antenna beam pattern ±180 0 in azimuth and elevation around the mainlobe would be even more complex.

Figure 1.8 Antenna beam pattern of the FL2 S-band radar. This pattern was taken using vertical polarization and is through the center of the beam axis at 0° elevation angle . (A) shows the pattern in terms of gain and angle while (B) shows the pattern in polar coordinates (10-dB contours).

Figure 1.9 Antenna beam pattern of the NCAR CP2 X-band antenna. The elevation and azimuth angles extend about 5° either side of the mainlobe (0.1° per interval for both elevation and azimuth). The horizontal cotours are at 6-dB intervals. From Rinehart and Frush, 1983.

Signal Processing:

Any wave from no matter how complex, can be thought of as the summation of a large number (perhaps an infinite number) of sine wave of different amplitudes and phases. The variation of amplitude of this sine wave as a function of frequency is called

a spectrum. One objective of signal processing is to determine the spectral content of a given wave from. In the case of wind profiling, the wave from in question is the time – varying echo strength (pulse to pulse) for a given range gate. The result of the signal processing is a spectrum showing the Doppler shifted echo from which the radial velocity (and other quantities)can be computed.

Discrete Fourier Transform

The transformation of a wave form from the time to the frequency domain is known as a Fourier transformation. In particular, if the wave from is discrete (that is, sampled at regular intervals) the process is known as the Discrete Fourier Transform (DFT). There are several numerical schemes or algorithms for computing the DFT. One rapid method commonly used in digital computers is known as the Fast Fourier Transform (FFT). This is the method often used in wind profiling. No attempt will be made in this Appendix to lay out the foundation of Fourier analysis, but some of the basic relationship relating to it will be given.

If the input to a DFT is a single discrete wave from, the output will be a frequency spectrum showing the Doppler shift, but it will not be possible to know if the Doppler shift is positive or negative. To remove this ambiguity, the echoes from each pulse transmitted are sampled twice for each range gate: one sample is taken from the direct input signal and the other is taken from the input signal that has been delayed one-quarter wavelength (90°phase difference). These signals are known as the in-phase and quadrature components (or the sine and co-sine components). It is these phase difference that determine whether a Doppler shift is positive or negative.

For each range gate, the discrete samples are spaced apart in time at the pulse repetition period (PRP). The inverse of this is the pulse repetition frequency (PRF), the rate at which the samples are taken. The resulting discrete waveform (for each height) is the sum of many sine curves of various amplitudes and frequencies. In order to be able to determine any one of these sine curves, it must be sampled at least twice per wavelength. Thus, the highest possible Doppler frequency that can be detected is half the PRF. As discussed in the following section, several consecutive samples are normally averaged together before the FFT is calculated. Thus, the effective sampling rate is lower. If NCOH is the number of samples averaged , then the maximum frequency detectable is

FN=PRF

2 (NCOH )= 1

2 (NCOH )(PRP)

This maximum frequency is sometimes called the Nyquist frequency. If the incoming signal actually had a more rapid variation than this frequency, this effective sampling rate would not detect it properly: that is the frequency would appear lower than

it really is. This referred to as frequency aliasing (or, velocity aliasing, if the frequencies have been converted to radial velocities). This condition is effectively illustrated in movies depicting rotating, spoked wheels in which the wheel sometimes appears to rotate backward: in this case, the frame rate of the film is not fast enough to capture the true motion of the wheel, and the visual sequence of adjacent spokes overrides the true movement of individual spokes.

A fixed number (NFFT) of these averaged values – actually pairs of values, as both the in – phase and quadrature components must be averaged separately- are then used as input to the FFT. This number should be large enough to allow the spectrum to be defined adequately but small enough to keep the number of computations manageable for the computer that is to be used. NFFT is usually 2n, where n is an integer, as this greatly speeds the computations. Tycho profilers use NFFT = 256. The output of the FFT computation is a pair of number sets (called the real and imaginary components), each having NFFT points. These are then squared and summed point for point to yield the power spectrum, which also has NFFT points. Figure 8 in Chapter 2 shows an example of a power spectrum.

The total time required to gather the data for the FFT calculation is thus (NFFT) (NCOH) (PRP). The inverse of this is the frequency resolution of the power spectrum, Δƒ:

Δ f= 1(NFFT ) (NCOH )(PRP)

Equations (A.26) and (A.27) can be converted to their equivalents in velocity using the Doppler frequency – velocity relation (f = -2 v/ג ) to get the maximum unaliased velocity:

V max=λ

4 (NCOH )(PRP)(A.28)

and the velocity resolution in the spectrum

ΔV= λ2(NFFT )(NCOH )(PRP)

The negative sign is not used in (A.28) and (A.29) as Vmax and ΔV refer to limits, which can be positive or negative, rather than a measured velocity.

To illustrate the order of magnitude of these values, consider a 400 MHz (75 cm) profiler using NCOH = 50, NFFT= 256, and PRP=125 µ s. This yields

f N=1

(2)(50)(125×10−6)=80Hz

V max=(80)(0.75)/2=30m /s

Δ f= 1(50 ) (256 ) (125×10−6 )

=0.625Hz

ΔV=(0.625)(0.75)

2=0.23m /s

The choice of NCOH and NFFT are based in part on the expected V max and the desiredΔV . Note that for a different operating frequency, different values of NCOH and perhaps NFFT should be used.

Averaging:

The averaging of several consecutive samples for each value used in the FFT is called time- domain averaging or coherent averaging. This process has several effects. First, it reduces the total noise power by removing the high frequency components from the signal. Removing these components has the possible disadvantage of introducing velocity aliasing, but with the proper choice of NCOH this condition can be avoided. The principal advantage of coherent averaging is the substantial reduction in the number of computations required to produce a spectrum. If no coherent averaging were done, but the same spectral resolution were required, the value of NFFT would be NCOH times larger, and the number of calculations required for the FFT would increase by the factor (NCOH)[In (NCOH)(NFFT)]/[In (NFFT)].If NCOH=50 and NFFT=256, this increase would be a factor of 85, which is substantial.

The other type of averaging used in wind profiling is the averaging of consecutive power spectra, point for point in the frequency domain. This is referred to as spectral averaging or incoherent averaging. This improves the detectability of the spectral peak by smoothing out the noise “floor” and making the peak better defined. The maximum height of useful data is thereby in creased. The disadvantage of incoherent averaging is the loss of time resolution. For many applications this is no problem since one wind profile every few minutes is adequate. Without spectral averaging it is possible to get a profile every few seconds. The total integration time in one mode is thus

Integration time =(NCOH ) (NFFT ) (NINCOH ) (PRP )

Where NINCOH is the number of spectra averaged together.

In the operation of a wind pro filer it is common to operate in more than one mode. For the altitude range close to ground level, short pulses (yielding good

height resolution) are used, and longer pulses (though producing poorer resolution) are used to reach to higher altitudes. The total dwell time for one beam is then the sum of the integration times of the modes used. It is common to integrate for about one minute per mode, so that the dwell time per beam is about two minutes. For a three beam system, the time to acquire the data for a complete wind profile is thus

about six minutes; for a five beam system it would about 10 minutes. Figure A-4 is a summary of the times typically required for all the processing steps. (The numbers

shown are for Tycho's Model 400 wind profiler, vertical beam, and high mode.)

Radar Equation:

Let us consider a transmitter which radiates isotropically. The power will spread equally in all directions, and, at a range r, the power per unit area on the surface of a sphere of radius r is Pt /4πr2 , where Pt is the power during the brief period that the transmitter is operating. In practice, radar sets employ antennas which are highly directional, in order to concentrate the power into a narrow beam. The ratio of power per unit area along the axis of the radar beam to the isotropic value is a measure of the gain, G. of the antenna. It should be noted that, if the radar set uses one antenna for transmission and another for reception, the value of G would have to be replaced by G t , the gain of the transmitting antenna.

If there is a target at range r with a cross-sectional area A t , it will intercept an amount of power Pσ , given by

Pσ = ( PtGAt )/4r2 ------------------------------------------- ( 1.7)

If it is assumed that the target does not absorb any power, but reradiates it all isotropically, the power intercepted by the radar antenna, Pr , will be given by

Pr = (Pt GAt Ae )/(4)2 r4 ----------------------------------- (1.8)Where Ae is the effective cross section of the antenna. It can also be written Ae = Ap , where Ap is the apertural or cross-sectional area of the antenna and is the antenna efficiency whose value depends on the efficiency of the feed and the antenna illumination.

It was shown by silver (1951) from theoretical considerations that Ae = Gλ2

/ 4 . Also , when an antenna employs a circular, paraboloidal reflector the gain is given approximately by G = 8 Ap / 3λ2 . note that in this instance Ae and G in equation (1.8) , one obtains

Pr = Pt G2 λ2 At / (4)3 r4 ≈ (Pt Ap2At)/9λ2 r4 ----------------------------- (1.9)

It was assumed earlier that the target had a cross section A t and that it scattered isotropically. In practice, there are no targets- certainly no meteorological ones-which scatter isotropically. Nevertheless, it has been found convenient to introduce a function , σ , called the “back scattering cross section,” which is defined as “ the area intercepting that amount of power, which if scattered isotropically , would return to the receiver an amount of power equal to that actually received.” The function σ may also be defined as “ the area which, when multiplied by the incident intensity, gives the total power radiated by an isotropic source which radiates the same power in the backward direction as the scatterer.” This can be written

σPi = 4r2 S, -----------------------------------------------(1.10)

where Pi is the power per unit area incident on the target at a range r, and S is the backscattered power per unit area at the antenna.

For a single scatterer, equation (1.9) becomes

Pr = PtG2 λ2σi / (4)3 r4 -------------------------------------------------(1.11)This equation is perfectly general up to this point and can be applied

whether the target be the moon, an airplane, or a raindrop. The problem for the meteorologist is to determine the form of σi for the meteorological elements he wishes to detect.

In equation (1.11) σi is given a the backscattering cross section of a single scatterer. In practice, the radar beam illuminates a large group of scatterers – e.g., raindrops – at the same time; the number is equal to that within a volume defined by the beam widths and pulse length of the radar set. In this case, it is necessary to consider the power reflected from a large number of drops. The backscattered signal voltage from a volume of randomly distributed scatterers is the sum of the signals scattered by each of the scatterers., with the phase of each signal taken into account. It has been found to vary from one reflected pulse to the next because of the movement of the particles with respect to one another. After a period of the order of 10-2 sec, a random array of scattering particles changes into an essentially independent one. If the received power is averaged over a large number of independent arrays, one can write

----------------------------------------------- (1.12)

Where the summation is carried out over the entire volume Vm from which power is scattered back to the receiver at any instant.

If the particles are uniformly distributed throughout Vm , the total backscattering cross section can be written as the backscattering per unit volume multiplied by Vm . the quantity Vm is given approximately by

-------------------------------- (1.13)

Where θ and ø are the horizontal and vertical widths of the beam in radians.It should be noted that, in calculating the volume illuminated, a depth of

h/2 is used instead of h. this is done because the equations involved refer only to

the scattered power which returns to the radar receiver at the same instant of time. The power backscattered by particles at a range (r + h/2) from the front of the outgoing pulse of length h will arrive at the radar antenna at the same time as the power backscattered by particles at range r from the rear of the outgoing pulse of length h.

By substituting in equation (1.12) , one obtains

---------------------------------- (1.14)The factor vol σi represents a summation of σ over a unit volume. It is called the “ radar reflectivity,” designated by the symbol η, and is commonly expressed in units of centimeters square per cubic meter or per centimeter.

It should be noted that in the derivation of equation (1.14) it was assumed that, across the radar beam between the half-power points, the transmitted power per unit area has the same value. Obviously, this is not the case. The transmitted power is maximum along the beam axis and decreases to half the maximum value at the angles corresponding to half the beam widths. When the factor G was introduced into equation (1.7), the assumed constant value of transmitted power was taken as the value along the beam axis. Clearly, the implicit assumptions about the character of the beam will lead to an overestimate of the power backscattered to a radar set.

Types of Radar Scattering theory:

Scattering from Refractive irregularities:

Because the radio refractive index of air, n, is so nearly equal to one, it is normally expressed in terms of N, which is defined as

N=(n-1)x 106

The quantity N is simply the variable part of n and has units that are referred to

as “N units”. Bean and Dutton (1966) found the N can be expressed as

N= 77.6T

(p+ 4810Te

)

Where p is the pressure of the (dry) air in millibars (mb), e is the vapor pressure in mb,

T is the temperature in kelvins. At sea level, N is typically between 250 and 400. It is the

variations in N over short distances (one-half the radar wavelength) that are of particular

interest.

Even through the value of N depends upon the variables P,T, and e, the

variations in N depend primarily on variations in T and e. The variations in P are

generally unimportant as they are associated with large-scale, slow-moving weather

patterns or high velocity acoustic waves, neither of which give variations relevant to

wind profiling. On the other hand, small scale variations in T and e are caused by

turbulence and typically have lifetimes of few seconds, long enough to scatter

thousands of pulses.

The variations in n over short distances are normally expressed in terms of D, the

refractive index structure function, as

Dn(1)= <[n(r→

+ l→

) – n(r→

)]2>

Where the angular brackets denote time average, r→

is the position vector of any

selected point, and l→

is the displacement vector of any another nearby point from the

first. The difference in n is squared to assure that positive quantities always result.

Atmospheric turbulence is generated at scales of tens to hundreds of meters by wind

shear and convection (up drafts, down drafts, etc.). These large eddies are thought to

break up into smaller eddies, which break up into still smaller scales by viscous heating.

The energy is thus input at the large scale sizes and dissipated at the small sizes. The

scale sizes between these, where the turbulence energy is “just passing through” is

referred to as the inertial sub range. It has been shown that the inertial sub range the

dependence of D upon l can be expressed explicitly.

Dn(1)=Cn2l→2/3

Where Cn2, the “constant of proportionality”, is known as the refractive index structure

parameter or refractive index turbulence parameter. It does not depend upon l. Therefore, within a given sampling volume at a given time, measurements of Cn

2 at different scales should be the given time, measurements of Cn

2 at different scales should be the same, as long as the scales are within the sub range. Therefore, Cn

2 is one of the standard parameters used in discussing turbulence.

In the inertial subrange, the radar reflectivity is proportional to Cn2 and is given by

Ƞ = 0.38 Cn2 λ-1/3 ……………………………..(24)

Where λ is the radar operating wavelength. By combining equations 888888 and eq(24) we see that a measurement of Pr can be converted into Cn

2. The wind profiler is therefore also a Cn

2 profiler.

The height range of useful returns depends on the operating frequency and degree of turbulence. Obviously, the stronger the turbulence, the stronger the echo. In addition, viscous forces are height dependent, and the dividing line between the inertial and viscous sub ranges is therefore also height dependent. There are fewer small scale than large scale eddies at the greater heights and therefore the higher frequency profiles cannot “see” as high as the lower frequency profilers. (It must be pointed out, however, that most wind profilers are height-limited by their power-aperture product; as a result, few profilers are powerful enough to reach the height corresponding to the dividing line between the inertial and viscous subranges.)

SCATTERING FROM HYDROMETERS:-

Over the years, radar meteorology has been largely study of radar reflections from hydrometeors. See Battan (1973) and Gossard and Strauch (1983) for details. The reflectivity of hydrometeors depends upon their size distribution, density, dielectric properties, shape and orientation, and the radar’s wavelength. For a single particle, the reflectivity has been shown to be.

Ƞ = (factor) d6

λ4

Where d is the diameter of the particle and (factors) depends upon the dielectric constant of the particle, its shape, and its orientation. The volume being probed by the radar pulse contains large numbers of such particles of various sizes, and so equation (A,25) must be integrated over the size distribution to find the total particle reflectivity.

Note the equation's dependence on wavelength. Typical meteorological radars operate at wavelengths on the order of 5 to 10 cm, whereas wind profilers operate typically near wavelengths of 75 cm and 6 m. Therefore, hydrometeors are some 104 times less reflective to the wind profilers than to conventional weather radars.

It should also be noted that liquid water (rain drops, melting snow, water coated icy particles) are considerably more reflective than solid ice at radar wavelengths.

The reflectivity of clear air varies as λ-1/3, whereas that of hydrometeors varies as λ -4• Weather radars are therefore very sensitive to precipitation and very insensitive to the clear air. The reverse is true for the VHF wind profilers. There is no distinct cut-off in wavelength between the two types of radar as this sensitivity depends strongly on atmospheric conditions. Observations show, however, that wind profilers operating at 400 MHz and above do detect rain, whereas those at 50 MHz detect rain only when it is heavy.

Precipitation, when detected, can be a problem in some circumstances. Clearly, if one is interested in the vertical motion of the air, precipitation is a problem, as the vertical velocity measured is that of the precipitation, and not the air. Likewise, Cn2 cannot be determined in the presence of detected precipitation. On the other hand, if the horizontal wind is the main interest, precipitation may help, as it presents a larger target than the clear air alone. The horizontal motion of light precipitation is the same as that of the air (i.e., the wind) and as long as the precipitation is the same in the three beams (which is usually the case) the horizontal wind calculations will be correct. Heavy precipitation may not follow the wind, which would make accurate wind velocity measurements impossible.

Wind Vector calculations:

In the diagram to the left, i, j, k and r represent unit vectors in the East, North, vertical and radial directions, respectively. For a wind-profiler radar beam directed at a zenith angle of θ and at an azimuth angle of φ, the radial component of the wind vector, vR, i.e. that along the beam pointing direction, is related to the zonal, u (i.e. towards East), meriodonal, v (i.e. towards North), and vertical, w, components through the following expression:

In order to determine the full three dimensional wind vector, observations must be made in a minimum of 3 non-coplanar beam directions.

A typical sequence includes observations made in the vertical direction and at an off-vertical angle (of around 17° for boundary-layer wind-profilers, or at 6° in the case of the NERC MST Radar) in two orthogonal azimuths; this is known as the Doppler Beam Swinging (DBS) technique. Since the the azimuths of the off-vertical beams are not, in general, aligned with the cardinal directions, it is useful to consider the problem in the frame of reference of a vertical plane along φ; the component of the horizontal wind vector in this plane, vH(φ), is given by:

The equation for the radial component of velocity simplifies to:

In the case of boundary-layer wind-profilers, which typically make observations in only three beam pointing directions, the vertical velocity is determined from the Doppler shift of the vertically directed beam, and the horizontal component, in each azimuth, is determined through the relationship

The NERC MST Radar typically makes observations at in the vertical direction and at 6° off-vertical in four different azimuths, each one separated by 90° from the next; this allows each component of velocity to be derived from three different combinations of the radial velocities. In addition to the combination described above for 3-beam observations, the radial velocities for off-vertical beams with opposite azimuths can either be added or subtracted to derive the vertical or horizontal components of velocity, respectively:

Wind Profiler Applications [Aviation, Tropical Cyclone, Thunderstorm, Meteorological (Synoptic and Mesoscale) and Environmental]

The wind profiler represents the newest technology making use of the recent developments of computers and radiotechnology. Its indisputable merits are the automatic, continuous, all-weather operation, and high accuracy; its main disadvantage is the relatively high investment cost of the larger systems. The wind profiler is the only means to provide wind observations from remote, unattended sites and continuous profiling with a proper position fix to high altitudes. These features make-it suitable for synoptic scale and mesoscale observations.

The principal advantages are: + Continuous profiling + low-operating costs and low life-cycle cost+ Vertical range 100 m - 18 km + Fast profiling + Three-dimensional profiles + Fairly immune to weather conditions + Remote, unattended operation + Independence + Proper position fix The disadvantages are: - Relatively high investment costs (except for the smaller systems) - Poor mobility, except for the smaller systems (which have limited performance) - Active electromagnetic system

Aviation:

The aviation industry stands to reap substantial benefits from the wind profiler. One important area will be flight planning. Today, a commercial aircraft must carry enough fuel to be able to stay aloft for a period substantially longer than the time required to simply fly to its destination. Extra fuel is required to make unexpected deviations around storms or remain "stacked" above an airport while other planes, also delayed by weather, are landing. With a reasonably dense network of wind profilers in place providing continuous data, pilots will be able to file more accurate flight plans. The amount of fuel required for a flight will thus be reduced. It has been estimated (Carlson and Sundararaman, 1982) that the commercial aviation industry could save $100 to 300 million per year in fuel, in the United States alone, through flight planning that used a network of wind profilers.

Flight Safety:-

Another benefit is increased flight safety and comfort for crew and passengers. The wind profiler not only measures the wind, it also measures the degree of atmospheric turbulence. With this information, pilots could plan flights not only to maximize fuel efficiency but also to avoid regions of enhanced turbulence will reduce air sickness, spilled coffee, and other similar annoyances. The avoidance of severe turbulence will reduce accidents and save lives.

One of the most serious weather problems in aviation is that of strong wind shear and microbursts, especially in the vicinity of airports. Wind shear was a factor in some 40% of the fatalities in U.S. commercial passenger traffic over the last 21 years. Although the wind profiler is not well suited for detecting microbursts (the microburst would have to be very near the site), it has the potential of detecting the conditions that occur before the onset of microbursts. Locating wind profilers at or near airports will facilitate this application.

Take-off and landing operations on aircraft carriers and other ships is greatly influenced by winds. A low level, high resolution version of the system, in which the motion of the ship is allowed for, is under development. It will be of great utility in these applications. Wind profilers on ships will also be of direct use in tactical weather intelligence and weather forecasting.

Petroleum companies can use the wind profiler since helicopters are an important mode of transportation to and from oil rigs in the ocean. Wind information is very important for take-off and landing. Measurements should be made as close to the rig as possible. The antenna for the radar could be embedded into the landing pads on oil rigs and thus wind information would be obtained at the rig itself. Tycho's low level, high resolution version of the system is well suited for such applications.

Additional synoptic and sub-synoptic data improve the weather services that

are critical for off-shore operations. More accurate and more frequently updated forecasts of winds and sea state can be made.

A detailed knowledge of the vertical structure of the wind is needed for launching missiles and satellites. Both the accuracy of the launch and the mechanical stresses on structural members of the rocket are affected by the wind. For this application a "super" wind profiler that measures to great altitudes should be located as close to the launch pad as is practical. In the case of the Space Shuttle, there is no second chance for a landing. A small wind profiler at the landing site would provide an extra margin of safety.

Mesoscale phenomena are still poorly understood. Measurements of many kinds, including winds, are needed. Some of the areas needing study are: the lifetimes of mesoscale convective systems (MCS) – how they form and dissipate; the interactions of MCSs with other weather systems, both on a larger and smaller scale; and the motions of these systems - why some move steadily while others remain relatively motionless. Wind profilers can play a significant role in this research and, indeed, are already starting to do so.

Atmospheric turbulence and eddy diffusion studies can be aided by wind profilers. The radar echo power and width of the spectral peak are both a measure of the degree of turbulence. Thermal and particle diffusion rates are, in turn, related to the degree of turbulence. These studies are still in their infancy and we can expect to see substantial progress in the years to come.

Tropical cyclone:

A tropical cyclone is a storm system characterized by a large low-pressure center and numerous thunderstorms that produce strong winds and heavy rain. Tropical cyclones strengthen when water evaporated from the ocean is released as the saturated air rises, resulting in condensation of water vapor contained in the moist air. They are fueled by a different heat mechanism than other cyclonic windstorms such as nor'easters, European windstorms, and polar lows. The characteristic that separates tropical cyclones from other cyclonic systems is that any height in the atmosphere, the center of a tropical cyclone will be warmer than its surrounds; a phenomenon called "warm core" storm systems.

The term "tropical" refers to both the geographic origin of these systems, which form almost exclusively in tropical regions of the globe, and their formation in maritime tropical air masses. The term "cyclone" refers to such storms' cyclonic nature, with counterclockwise rotation in the Northern Hemisphere and clockwise rotation in the Southern Hemisphere. The opposite direction of spin is a result of the Coriolis force. Depending on its location and strength, a tropical cyclone is referred to by names such as hurricane, typhoon, tropical storm, cyclonic storm, tropical depression, and simply cyclone.

While tropical cyclones can produce extremely powerful winds and torrential rain, they are also able to produce high waves and damaging storm surge as well as spawning tornadoes. They develop over large bodies of warm water, and lose their strength if they move over land due to increased surface friction and loss of the warm ocean as an energy source. This is why coastal regions can receive significant damage from a tropical cyclone, while inland regions are relatively safe from receiving strong winds. Heavy rains, however, can produce significant flooding inland, and storm surges can produce extensive coastal flooding up to 40 kilometres (25 mi) from the coastline. Although their effects on human populations can be devastating, tropical cyclones can also relieve drought conditions. They also carry heat and energy away from the tropics and transport it toward temperate latitudes, which makes them an important part of the global atmospheric circulation mechanism. As a result, tropical cyclones help to maintain equilibrium in the Earth's troposphere, and to maintain a relatively stable and warm temperature worldwide.

Many tropical cyclones develop when the atmospheric conditions around a weak disturbance in the atmosphere are favorable. The background environment is modulated by climatological cycles and patterns such as the Madden-Julian oscillation, El Niño-Southern Oscillation, and the Atlantic multidecadal oscillation. Others form when other types of cyclones acquire tropical characteristics. Tropical systems are then moved by steering winds in the troposphere; if the conditions remain favorable, the tropical disturbance intensifies, and can even develop an eye. On the other end of the spectrum, if the conditions around the system deteriorate or the tropical cyclone makes landfall, the system weakens and eventually dissipates. It is not possible to artificially induce the dissipation of these systems with current technology.

Physical structure

Fig: Structure of a tropical cyclone

All tropical cyclones are areas of low atmospheric pressure in the Earth's atmosphere. The pressures recorded at the centers of tropical cyclones are among the lowest that occur on Earth's surface at sea level. Tropical cyclones are characterized and driven by the release of large

amounts of latent heat of condensation, which occurs when moist air is carried upwards and its water vapor condenses. This heat is distributed vertically around the center of the storm. Thus, at any given altitude (except close to the surface, where water temperature dictates air temperature) the environment inside the cyclone is warmer than its outer surroundings.

Eye and center

A strong tropical cyclone will harbor an area of sinking air at the center of circulation. If this area is strong enough, it can develop into a large "eye". Weather in

the eye is normally calm and free of clouds, although the sea may be extremely violent. The eye is normally circular in shape, and may range in size from 3 kilometres (1.9 mi) to 370 kilometres (230 mi) in diameter. Intense, mature tropical cyclones can sometimes exhibit an outward curving of the eyewall's top, making it resemble a football stadium; this phenomenon is thus sometimes referred to as the stadium effect.

There are other features that either surround the eye, or cover it. The central dense overcast is the concentrated area of strong thunderstorm activity near the center of a tropical cyclone;[7] in weaker tropical cyclones, the CDO may cover the center completely.[8] The eyewall is a circle of strong thunderstorms that surrounds the eye; here is where the greatest wind speeds are found, where clouds reach the highest, and precipitation is the heaviest. The heaviest wind damage occurs where a tropical cyclone's eyewall passes over land.[3] Eyewall replacement cycles occur naturally in intense tropical cyclones. When cyclones reach peak intensity they usually have an eyewall and radius of maximum winds that contract to a very small size, around 10 kilometres (6.2 mi) to 25 kilometres (16 mi). Outer rainbands can organize into an outer ring of thunderstorms that slowly moves inward and robs the inner eyewall of its needed moisture and angular momentum. When the inner eyewall weakens, the tropical cyclone weakens (in other words, the maximum sustained winds weaken and the central pressure rises.) The outer eyewall replaces the inner one completely at the end of the cycle. The storm can be of the same intensity as it was previously or even stronger after the eyewall replacement cycle finishes. The storm may strengthen again as it builds a new outer ring for the next eyewall replacement.

Size descriptions of tropical cyclones:

ROCI TypeLess than 2 degrees latitude Very small/midget2 to 3 degrees of latitude Small3 to 6 degrees of latitude Medium/Average6 to 8 degrees of latitude Large anti-dwarfOver 8 degrees of latitude Very large

Size:

One measure of the size of a tropical cyclone is determined by measuring the distance from its center of circulation to its outermost closed isobar, also known as its ROCI. If the radius is less than two degrees of latitude or 222 kilometres (138 mi), then the cyclone is "very small" or a "midget". A radius between 3 and 6 latitude degrees or 333 kilometres (207 mi) to 670 kilometres (420 mi) are considered "average-sized". "Very large" tropical cyclones have a radius of greater than 8 degrees or 888 kilometres (552 mi). Use of this measure has objectively determined that tropical cyclones in the northwest Pacific Ocean are the largest on earth on average, with Atlantic tropical cyclones roughly half their size. Other methods of determining a tropical cyclone's size include measuring the radius of gale force winds and measuring the radius at which its relative vorticity field decreases to 1×10−5 s−1 from its center.

Formation

Main article: Tropical cyclogenesis

Map of the cumulative tracks of all tropical cyclones during the 1985–2005 time period. The Pacific Ocean west of the International Date Line sees more tropical cyclones than any other basin, while there is almost no activity in the Atlantic Ocean south of the Equator.

Map of all tropical cyclone tracks from 1945 to 2006. Equal-area projection.

Worldwide, tropical cyclone activity peaks in late summer, when the difference between temperatures aloft and sea surface temperatures is the greatest. However, each particular basin has its own seasonal patterns. On a worldwide scale, May is the least active month, while September is the most active while November is the only month with all the tropical cyclone basins active.

Thunderstorm:

Figure 1: Hail stone measuring 21 centimeters in diameter. (Source: NOAA Photo Library - National Severe Storms Laboratory).

A thunderstorm forms when moist, unstable air is lifted vertically into the atmosphere. Lifting of this air results in condensation and the release of latent heat. The process to initiate vertical lifting can be caused by:

1. Unequal warming of the surface of the Earth. 2. Orographic lifting due to topographic obstruction of air flow. 3. Dynamic lifting because of the presence of a frontal zone.

Figure 2: Multiple lightning strikes from a thunderstorm occurring at night. (Source: NOAA Photo Library - National Severe Storms Laboratory).

Immediately after lifting begins, the rising parcel of warm moist air begins to cool because of adiabatic expansion. At a certain elevation the dew point is reached resulting in condensation and the

formation of a cumulus cloud. For the cumulus cloud to form into a thunderstorm, continued uplift must occur in an unstable atmosphere. With the vertical extension of the air parcel, the cumulus cloud grows into a cumulonimbus cloud. Cumulonimbus clouds can reach heights of 20 kilometers above the Earth's surface. Severe weather associated with some these clouds includes hail, strong winds, thunder, lightning, intense rain, and tornadoes.

Generally, two types of thunderstorms are common:

Figure 3: Developing thunderstorm cloud at the cumulus stage. (Source: PhysicalGeography.net)

1. Air mass thunderstorms which occur in the mid-latitudes in summer and at the equator all year long.

2. Thunderstorms associated with mid-latitude cyclone cold fronts or dry lines. This type of thunderstorm often has severe weather

associated with it.

The most common type of thunderstorm is the air mass storm. Air mass thunderstorms normally develop in late afternoon hours when surface heating produces the maximum number of convection currents in the atmosphere. The life cycle of these weather events has three distinct stages. The first stage of air mass thunderstorm development is called the cumulus stage (Figure 3). In this stage, parcels of warm humid air rise and cool to form clusters of puffy white cumulus clouds. The clouds are the result of condensation and deposition which releases large quantities of latent heat. The added heat energy keeps the air inside the cloud warmer than the air around it. The cloud continues to develop as long as more humid air is added to it from below. Updrafts

dominate the circulation patterns within the cloud.

Figure 4: Mature thunderstorm cloud with typical anvil shaped cloud. (Source: PhysicalGeography.net)

The mature thunderstorm begins to decrease in intensity and enters the dissipating stage after about half an hour. Air currents within the convective storm are now mainly downdrafts as the supply of warm moist air from the lower atmosphere is depleted. Within about 1 hour, the storm is finished and precipitation has stopped.

Figure 5: Downdrafts from this mature thunderstorm are moving air and rain from the cloud to the ground surface (Photo © 2004 David Jenkins; Source: PhysicalGeography.net)

Thunderstorms form from the equator to as far north as Alaska. They occur most commonly in the tropics were convectional heating of moist surface air occurs year round. Many tropical land-based

locations experience over 100 thunderstorm days per year. Thunderstorm formation over tropical oceans is less frequent because these surfaces do not warm rapidly. Outside the tropics, thunderstorm formation is more seasonal occurring in those months where heating is most intense.

Figure 6: Average number of thunderstorm days per year in the United States. (Source: Oklahoma Climatological Survey).

Figure 6 describes the annual average number of thunderstorm days across the United States. According to this map, the greatest incidence of thunderstorms occurs in the southeast and in parts of Colorado, Arizona, and New Mexico. This

particular spatial distribution suggests that extreme solar heating is not the only requirement for thunderstorm formation. Another important prerequisite is the availability of warm moist air. In the United States, the Gulf of Mexico supplies adjacent continental areas with moist maritime tropical air masses. These air masses are relatively unstable, quickly forming cumulonimbus clouds when surface heating is intense. The secondary maximum found in Colorado and New Mexico is due to another climatic factor. All of these areas are mountainous. Mountain slopes in these areas that face the sun absorb more direct solar radiation and become relatively warmer creating strong updrafts that form into cumulus clouds. If the differential heating is also supplemented by winds from the east, the cumulus clouds are further enhanced by orographic lifting and become thunderstorms. Notice that only a few thunderstorms occur along the west coast of the United States and Canada. This region is frequently under the influence of cooler maritime air masses that suppress convectional uplift over land.

Severe Thunderstorms

Most thunderstorms are of the variety described above. However, some can form into more severe storms if the conditions exist to enhance and prolong the mature stage of development. Severe thunderstorms are defined as convective storms with frequent lighting, accompanied by local wind gusts of 97 kilometers per hour, or hail that is 2 centimeters in diameter or larger. Severe thunderstorms can also have tornadoes.

Figure 7: Model of the major features and circulation patterns associated with a severe thunderstorm. (Source: Oklahoma Climatological Survey).

In most severe thunderstorms, the movement of the storm, in roughly an easterly direction, can refresh the storm's supply of warm humid air. With

a continual supply of latent heat energy, the updrafts and downdrafts within the storm become balanced and the storm maintains itself indefinitely. Movement of the severe storm is usually caused by the presence of a mid-latitude cyclone cold front or a dry line some 100 to 300 kilometers ahead of a cold front. In the spring and early summer, frontal cyclones are common weather events that move from west to east in the mid-latitudes. At the same time, the ground surface in the mid-latitudes is receiving elevated levels of insolation which creates ideal conditions for air mass thunderstorm formation. When the cold front or dry line of a frontal cyclone comes in contact with this warm air, it pushes it like a bulldozer both horizontally and vertically. If this air has a high humidity and extends some distance to the east, the movement of the mid-latitude cyclone enhances vertical uplift in the storm and keeps the thunderstorms supplied with moisture and energy. Thus, the mid-latitude cyclone converts air mass thunderstorms into severe thunderstorms that last for many hours. Severe thunderstorms dissipate only when no more warm moist air is encountered. This condition occurs several hours after nightfall when the atmosphere begins to cool off.

Figure 7 illustrates the features associated with a severe thunderstorm. This storm would be moving from left to right because of the motion associated with a mid-latitude cyclone. The upper-level dry air wind is generated from the mid-latitude cyclone. It causes the tilting of vertical air currents within the storm so that the updrafts move up and over the downdrafts. The green arrows represent the updrafts which are created as warm moist air is forced into the front of the storm. At the back end of the cloud, the updrafts swing around and become downdrafts (blue arrows). The leading edge of the downdrafts produces a gust front near the surface. As the gust front passes, the wind on the surface shifts and becomes strong with gusts exceeding 100 kilometers per hour, temperatures become cold, and the surface pressure rises. Warm moist air that rises over the gust front may form a roll cloud. These clouds are especially prevalent when an inversion exists near the base of the thunderstorm.

Some severe thunderstorms develop a strong vertical updraft, commonly known as a mesocyclone. Mesocyclones measure about 3 to 10 kilometers across and extend from the storm's base to its top. They are also found in the southwest quadrant of the storm. In some cases, mesocyclones can overshoot the top of the storm and form a cloud dome (Figure 7). About half of all mesocyclones spawn tornadoes. When a tornado occurs, the mesocyclone lengthens vertically, constricts, and spirals down to the ground surface. Scientists speculate that mesocyclones form when strong horizontal upper air winds interact with normally occurring updrafts. The shearing effect of this interaction forces the horizontal wind to flow upward intensifying the updraft.

METEOROLOGICAL APPLICATIONS:-

Weather forecasting has improved steadily over the years, but further advances depend upon the availability of significantly more weather data. Data are required at greater resolution in both time and space. This is particularly true for upper-air data, which are largely limited to twice daily balloon soundings and to satellite observations of clouds and temperatures. The wind profiler offers a means of greatly expanding our data base and thus improving forecasts for both synoptic scale and mesoscale phenomena.

SYNOPTIC SCALE:-

Weather systems exist on many scales, from global to local. The largest are the synoptic-scale systems, which extend from a few hundred to several thousand kilometers in size and have lifetimes from several days to a few weeks. These systems include planetary waves, jet streams, extratropical cyclones, and the major weather fronts. Week-to-week changes in weather are normally attributed to synoptic-scale weather systems.

The existing network of balloon-based, upper-air stations in the northern hemisphere has provided enough data to reasonably define synoptic-scale weather events. In the southern hemisphere synoptic-scale weather systems are not as well defined. Upper-air data with a greater resolution in time and space would greatly facilitate both manual and numerical forecasting. Because the wind profiler radar is completely automatic, it can operate at unmanned, remote sites. As it is also economical to operate, it is ideal for filling in the gaps in the synoptic network and replacing the wind-only (PILOT) balloon observations made at certain upper-air stations.

MESOSCALE:-

The mesoscale encompasses systems that extend from a few kilometers to several hundred kilometers in size and have lifetimes from several minutes to a few days. Most significant weather is associated with mesoscale phenomena: thunderstorms, tornadoes, and tropical cyclones are all examples of mesoscale weather systems.

If a mesoscale convective system remains stationary, devastating floods can occur. If it moves steadily, beneficial precipitation often results. It is critical, therefore, to be able to forecast the movements of these systems accurately. Because existing upper-air sounding stations are spaced hundreds of kilometers apart and gather data only cnce every twelve hours, mesoscale systems are often missed. As a result forecasts are often inaccurate. Improvements in weather forecasts clearly depend upon observations being made more frequently and with a closer spacing. The wind profilers offer significant advantages over an extensive balloon launching program because they measure wind profiles much more economically and accurately than the common radiosonde.

COMMERCIAL WEATHER SERVICES:-

Weather reports and forecasts provided by the government are general, as they must serve a wide spectrum of users. In the USA, specialized forecasts for specific needs are normally left to the private sector. Some industries, such as aviation and television, employ their own in-house staff to provide these services. Other industries don't find this practical, so they turn to commercial weather services companies. One industry needing specialized forecasts is agriculture, where the most important factors are precipitation and temperature. Detailed knowledge of the wind is needed to forecast these factors accurately. In forest management it is important to be able to predict the motions of forest fires. The need for wind information is critical. In these areas and many others, the wind pro filer can play an important role.

ENVIRONMENTAL APPLICATIONS:-

The trajectories and dispersion of air-borne pollutants are largely controlled by the wind and turbulence. There are a wide variety of sources for air-borne pollutants - power plants (both fossil fuel burning and nuclear), chemical factories, forest fires, automobile exhaust, and many others. In time of war, potentially very serious pollutants are Nuclear/Biological/Chemical (NBC) agents used in battles.

Wind profilers would be extremely useful in these situations. They are needed for basic research studies on the dispersion and transport of pollutants, such as acid rain studies. They would be crucially important in emergency situations, such as the accidental release of radioactive particles from a nuclear power plant or in NBC

warfare. With a knowledge of the trajectories and diffusion rates of such noxious materials, defense forces could take evasive or protective action. Similarly, civilian populations could be taken to safety and unnecessary evacuations could be avoided.