Radar Meteorology Chapter 1 Introduction
Radar is an acronym for a electro-magnetic wave Radio near Detection 1 Ghz, And Ranging. A back a
radar is an electro-magnetic device capable of transmitting receiver reflection from a target and based on the characteristics of the returned signal determine things about the target. Radars have become indispensable in several major fields of research and in commerce. The Federal Aviation Agency (FAA) makes extensive use of radars not only to track aircraft, but to make sure landings and take-offs are uneventful. Meteorologist use radars to track severe weather and to estimate the amount of rainfall. Radar meteorology means many things to many people. Depending on what your research interests is your definition may be very different from mine. As a working definition I will use the following: Definition: Radar meteorology is the study of the
atmosphere using radar as a tool. Radar Meteorology is not a true branch of meteorology
because it is use by several true branches of meteorology, such as cloud physics and severe storms, as a tool for that particular branch. Radar meteorology is also not a branch of radio meteorology; Radio meteorology is the study of how electro-magnetic waves travel through the atmosphere. As such radio meteorology deal with refraction, reflection and propagation meteorology. of electro-magnetic waves. Although these concepts are very important they are not the core of radar
Radar is a remote sensing tool in that it is not in contact with radar the is object an and it is sensing in the Radar it measures modifies the the characteristics of the atmosphere from a distance. Further, active then it sensor measures samples appears a the to that atmosphere forecast distance forecast. atmospheres from very response. a close
Radar is not a prognosticator, i.e. it does not make a rather and Radar atmosphere make of a there is accurate locating
identifying, measuring and then displaying the atmosphere and what is in it. Radar is useful because of the following characteristics: 1. Radar scans a three-dimensional volume and can be
pointed any where in space. The scale of the smallest volume is meso-. 2. Continuous scanning in space Typically with 5 minutes between scans of the same volume. 3. Reasonable resolution. For a typical 2 sec pulse at 100 nm the volume is about 5 km x 5 km x 600m 4. Total variability of the atmosphere can be measured, i.e. Radar can measure all the components of the total derivative. 5. Radar can make in-storm measurements 6. Radar can measure the actual severity of the storm, since Ze is a measure of the number of hydrometers per cubic unit. -> 8
7. Radar, if coherent, can measure the three components of the wind. Thus from the meteorologists point of view a radar provides a large number of advantages over any other tool designed to look at the structure of severe storms and clouds. Much of what we know about the inner workings of thunderstorms and other precipitating cloud systems come from radar. Radar uses an antenna producing a narrow beam of energy to scan a volume of space until a reflection is obtained. The direction the antenna is pointing and the time interval between and the transmission of the and reception determine determine the the location of the reflection in space. Further the strength polarization reflection characteristics of the target.
A Short History of Radar
The history of radar is very closely linked to the history of radio. The very definition of RADAR suggests its origin in radio. The beginnings of radar meteorology are also closely linked to the Second World War. During the 1930's while experimenting with radio communications, it was noted radio communications signal strength between to points on the river varied when a ship passed between the two radios. Once it was noted that such variations occurred, the physics of this variation came under intense scrutiny. One of the key inventions that made radar possible was that of the magnetron transmitter tube. Up to this point one of the major problems was creating a vacuum tube that could handle the tremendous amounts of power necessary to make radar
range. Before the magnetron tube no method of creating a tube to amplify the signal to these levels was possible. In addition the magnetron tube allowed microwaves to be used, reducing the size of the antenna necessary to resolve the interesting features. The earliest uses of radars in meteorology came about just after the Second World War when military surplus radars became available. The thunderstorm project conducted by Byers and Braham in 1947 used these surplus military the radars to of study radars the in life cycle and the structure of thunderstorms. Although research Meteorologist understood (Weather Advances in modern electronics were quickly introduced into the radar. When transistors they were were first also introduced introduced and the SPOL into into communications dramatically also support equipment usefulness meteorology, weather didn't take advantage of this tool until the WSR-57
radars allowing the size of the support equipment to be reduced. Digital the since electronics size the of NCAR computers radar radar and was significantly equipment. reduced Even
designed (the radar fits in four SeaTainer), these advances have allowed the radar to be reduced to the point where the radar can be made mobile, such as Josh Wurman's Doppler on Wheels (DOW). One of the biggest advances in radar technology was the introduction of the Doppler radar. The original WSR-57 radar was not phase coherent radar. That is the phase of the transmitted signal could not be determined and was not
the same from pulse to pulse. By insuring that the phase of the transmitted signal was always the same, the phase of the returned signal could be compared to the phase of the transmitted signal. Any phase shift in the returned signal has to be due to the movement of the targets. The amount of the phase shift can determine the direction and speed of the target relative to the beam. In the case of a thunderstorm with billions of targets each with a different bin. This velocity, provides the the radar is a provides means of of the information on the relative number of targets within each velocity cloud. Another major advance has been the determination of the polarization of the return signal. Just as with a Doppler radar, the polarization of the transmitted signal and returned signal were not recorded or compared. By saving this information the shape, size and kind of particles in the volume can be determined. The latest development has been in the form of phased array antennas. it is Normally meteorological and radar has a the parabolic received dish as an antenna. The parabolic dish focuses the beam as being transmitted concentrates signal. Using an array of dipole antennas distributed in a rectangular array and adjusting when the transmitted power reaches each dipole I can direct the beam in different directions. This is the technique that is use with the profiler arrays. The profiler is nothing more than a vertically pointing phased array radar that is pointed in researcher determining the relative drop-size distribution
three directions (North, West and Vertical) by changing the time at which each dipole receives the transmitted signal. Phased array radars are also in common use in the military for ships.
Types of Radars
Radars come in many forms depending on the use that the radar. As with anything, the type of radar can be broken down into a number of different types. The first major subcategory is based on the kind of antenna used. If the transmitters antenna and the receivers antenna are the same antenna the radar is mono-static. This is the most common type of radar in use by meteorologists. A bi-static radar is one where the antenna for the transmitter and receiver
Figure 1.1 NCAR's SPOL radar are different. The normal case is where I have a single
transmit antenna and many different receivers. This is the case with NCAR's SPOL radar. This radar can be setup as a mono-static system or it can be setup as a bi-static array,
Figure 1.2 NCAR’s bi-static Antenna where the series of receive only facilities are distributed around the primary SPOL. The bi-static Radar Network (BINET) consists of several
where the series of receive only facilities are distributed around the primary SPOL.
Network from a
link. In addition, its small size and low power consumption allow operation small-integrated
Figure 1.2 NCAR’s bi-static Antenna generator unit. A wireless data link eliminates difficult installation and hookup even in the most remote locations in the world. because The of BINET the system ships in the same sea containers along with the S-Pol radar. Site preparation is minimal bi-static receiver's autonomous operation. Shipboard operation is also possible provided the ship be located 5 to 100 Km from the S-Pol transmitter.
Figure 1.3 NCAR Electra with the tail mounted ELDORA radar Data is assimilated and processed by a host at the S-Pol site. Because can be data from each Doppler, cell full and is collected vector in simultaneously, retrieval multiple wind
radar coordinates. This allows unprecedented detail of the local meteorological physical processes in real time. A third possibility is that the radar is mounted ona ship or aircraft. The aircraft radar presents you need an at interesting least two case for the application of radar. Normally to recover the horizontal and vertical winds, Doppler placed such that the angle between the two radars beams is 90. This often very difficult with ground based radars. With an aircraft housed radar the aircraft can fly close to the region of interest. By mounting two antennas, one pointing fore and one pointing aft, as the radar flies by the target first the fore radar will sense the target and then the aft radar. By combing the fore and aft signals I can recover the three-dimensional wind field s described in chapter 8. NCAR’s used Electra in aircraft has the most ELDORA notably aircraft radar mounted in the tail (see Fig. 1.3) and has been extensively several projects VORTEX.
Use Cloud Detection Storm Detection Storm Detection Storm Detection
Band K X C S
Wavelength 0.75 ->2.4 cm 2.4 -> 3.75 cm 3.75 -> 7.5 cm 7.5 -> 15 cm
Frequency 40 -> 12.5 GHz 8 -> 12.5 GHz 4 -> 8 2 -> 4 GHz
Example 0.86cm AN/TPQ-11 3.2cm AN/CPS-9 AN/FPS-77 WSR-57 and NEXRAD
Table 1.1 Radar Wavelengths Versus Band The second set of qualifiers indicates how the signal is transmitted. There are two ways that this can work. For a Continuous Wave (CW) radar the radar transmits a signal continuously. The return signal is also a continuous wave. Thus the only way that the transmitted and received signals can be differentiated is if the return is somehow different from the transmitted signal. The most common technique is to measure the Doppler shift due to the target moving. This disturbs that phase angle of he continuous wave. This phase shift is the Doppler effect and is a measure of the speed of the target. Although the first Doppler radars in meteorology were CW radars, all most all of the current meteorological radars are the second variety; the pulsed radar. A pulsed radar transmits a signal for a short time and then listens for a longer period for the return signal. The pulse duration determines the minimum resolvable feature along the beam and the listen time determines the maximum usable and range of the radar. the The fact that radar I is must the stop most transmitting to listen for the return has both advantages disadvantages. Since pulsed
common type in meteorology we will spend the most amount of time talking about this kind of radar. The third sub-category is defined by the wavelength of the radar. This type of categorization goes beyond those used in meteorology and into those used by the FAA, Police departments and the military. There are four major kinds of radars wavelengths in use by the meteorological community. These are given in the table 1.1. There are some important issues with the selection of the wavelength for the radar. The problem is the choice of wavelengths places some limitations on what can be seen and what cannot. The K band radars are very limited in use since they detect everything including the smallest cloud droplets, dust insects, bird etc. This means we don't see much farther than the leading edge of the storm. X band radars also have significant attenuation problems due to attenuation by raindrops. This wavelength is primarily the type of radar used by cloud physicists. S band radars don't have attenuation problems, but the radar becomes so large and heavy that it is expensive to build and operate. C band radars are the compromise radars.