RADAR. MIT Radiation Laboratory made similar observations in the early 1940 s (U.S. Air Corps meteorologists receiving radar training at MIT in 1943

Transcription

1 Radar Meteorology

2 RADAR RAdio Detection And Ranging Has its roots in radio In 1934, after a plane disrupted radio communication, the idea for using pulses of energy for target detection was born Developed during WWII for military purposes Brittan's initial goal was a Death Ray Used to track warships and airplanes Radar operators annoyed at weather always getting in the way Proliferated after WWII Feb 20, cm (S-band) radar used to track rain showers in England (Ligda) Possibility of such observations was predicted by Ryde (1941) MIT Radiation Laboratory made similar observations in the early 1940 s (U.S. Air Corps meteorologists receiving radar training at MIT in 1943 First operational weather radar, Panama, 1943 Science of radar meteorology born from WWII research

3 Frequency (GHz) Radar Wavelengths Wavelength (cm) Band W K X C S L PRF determines the maximum useable range of a radar R max = c/(2 PRF) The pulse repetition time (PRT) is the inverse of the PRF PRF = 1000 Hz PRT = 1 ms W and K band radars are cloud radars X, C, S and L band radars are precipitation radars

4 Electromagnetic Spectrum Microwave portion of the electromagnetic spectrum is 300 MHz to 300 GHz, or wavelengths from 1 meter to 1 mm. Immediately above this portion of the spectrum is the infrared region. Rayleigh and Mie scatter apply to precipitation and cloud particles in the microwave spectrum. Bragg scatter can also be observed, typically prior to the point where Rayleigh targets are produced Radar applications extend to VHF frequencies as well, 30 to 300 MHz, for profiler applications. Here Rayleigh scatter from precipitation and Bragg scatter from clear air are both important Bragg scatter is associated with scatter by turbulent fluctuations of the index of refraction caused by temperature and moisture fluctuations

5 Scattering Theoretical work (Mie scattering theory) in the late 1940 s showed that weather clutter arose from the scattering of electromagnetic radiation by precipitation particles (resonant interaction between propagating EM wave and a dielectric such as water and ice). Today modern radars can not only detect hydrometeors (both precipitation and cloud particles), but clear air targets such as insects and large aerosol particles, as well as changes in the index of refraction, the latter caused by turbulent motions in the atmosphere.

6 Common Radar References Battan (1973) Radar Observation of the Atmosphere Doviak and Zrnic (1984) Doppler Radar and Weather Observations. Academic Press, Second Edition (1992) Skolnik (1980) Radar Systems, McGraw Hill, Second Edition Atlas, Radar in Meteorology, AMS (Battan Memorial volume), 1990 Journal of Atmospheric and Oceanic Technology, AMS Radar Meteorology Conferences, Preprint Volumes, AMS

7 Functions of Radar Systems Detect wx target Locate wx target Measure reflectivity Identify rotation Display distributions of wx targets

8 Basic Characteristics of Radar Systems Remote sensing Active Sensor (Transmits EM wave) Observing tool with exceptional nowcasting abilities Continuous sensing Reasonable Resolution (~5min, < 1000 m) Ability to sense total 3D structure and variability of target Coherent observation (velocity) of in-storm motion field (Doppler Only)

9 RADAR-Radio Detection and Ranging Radar is the art of detecting by means of radio echoes the presence of objects, determining their direction and range, recognizing their characteristics and employing the data this obtained. Object refers to meteorological targets such as raindrops, hailstones, cloud ice and liquid particles and snowflakes. For the purpose of clear air detection, insects are considered the objects. Birds also are readily detected and hence are a great interest. (Radar Ornithology) Radar is based on the propagation of electromagnetic waves through the atmosphere, a non-vacuum. EM waves propagate at the speed of light in a vacuum, c=2.998 x 10 8 m/s. Propagation speed in a non-vacuum determines the index of refraction, n = c/ν where ν is the wave speed

10 Radar Systems Early radar systems were known as non-coherent radars. Unable to measure the difference in phase between the outgoing radar pulse and the returning pulse. Particle velocity is one contributor to this difference in phase. (Doppler shift). Capable of measuring return power only. Coherent radars provided detection of the phase difference between the outgoing and returning pulses. The time rate of change of this phase (phase out - phase in )/delta t provides a measure of the Doppler shift frequency Directly related to the radial velocity of the target. More advanced radars provide either frequency agility or polarization agility. Frequency agility--transmit and receive multiple frequencies (near in frequency to each other) to increase sampling rate Polarization agility--alternatively transmit either horizontal or vertical polarization to provide wealth of information on particle phase, shape and orientation Hurricane Carla (9/10/1961)- Galveston, TX Arrow indicates Tornado location

11 Simple Radar System antenna transmitter 10 6 W T/R switch display receiver W

12 Radar System All radars have the same 4 general components Transmitter--produces high power pulses at desired frequency. Pulses may be 1 microsecond in duration. Older systems had a continuous transmitter and required a separate antenna Receiver--detects, amplifies and converts (digitizes) received voltages from each pulse as a function of range Designed to detect very weak signals, on the order of to watts. Transmitted power is typically 10 6 watts, peak power. Antenna--(1) radiates transmitted power in narrow beam for maximum gain ; (2) receives backscattered signal from targets They often rotate from between 10 and 70 degrees per second about a vertical axis The antenna will do a complete circle at one elevation angle (tilt) and then increase a few degrees and do another Volume Scan T/R switch--switches antenna between transmitter and receiver at high rate, typically once every millisecond

13 Radar Signals Because of the need to deal with a large range of signal powers, we express power in terms of decibel units P(dB) = 10 log 10 (P 1 /P 0 ) where P 1 is the received power in watts, and P 0 is a reference power of 1 milliwatt (10-3 watts). Since the power is referenced to a milliwatt, we often write P(dB) as P (dbm). The minimum detectable signal (MDS) is the minimum signal power that can be detected above the noise level of the system. Highly sensitive radars can detect signal levels as low as -115 dbm. The dynamic range of the radar system is the range of powers that can be detected. The very weakest signals can be detected above the noise level, whereas the strongest signals (strong storms at close range) saturate the receiver. Radar systems typically use step attenuators to lower the power return from nearby strong storms, by decreasing the return power by a known amount prior to detection by the receiver.

14 Radar Systems Pulsed coherent radars typically emit EM radiation in the form of short pulses, typically 1 microsecond in duration. Provides a pulse length of 300 m, which reduces to 150 m in practice due to sampling considerations This length defines the along beam distance over which returns from a spectrum of hydrometeors are acquired. The angular dimensions of the pulse perpendicular to the radial dimension are defined by the beamwidth characteristics of the antenna The product of the pulse length and the pulse area defines the pulse volume. For a 1 degree angular beamwidth, pulse volumes are typically on the order of a few km 3. 1 degree beamwidth (pulse volume is range dependent) Pulses are emitted at a high rate, typically 1000 pulses per second, which defines the pulse repetition frequency, PRF.

15 Doppler effect Electromagnetic radiation experiences a Doppler effect. For relative speeds, u, that are small compared with the speed of light, c, the frequency of EM waves shifted by Doppler effect: f ' u = f (1 ± ) c the plus sign for source and receiver are approaching the minus sign for source and receiver are receding

16 EM Spectrum EM waves can be generated in different frequency bands: radio, microwave, infrared, visible, ultraviolet, x-rays, gamma rays Note that the visible portion of the spectrum is relatively narrow. The boundaries between various bands of the spectrum are not sharp, but instead are somewhat arbitrary.

17 Polarization When light is polarized, the electric field always points in the same direction. A beam of light that is: (a) polarized in the vertical direction: The electric field points in the vertical direction. (b) unpolarized: Superposition of many beams, approximately parallel, but each with random polarization. Every atom in the filament of an incandescent bulb radiates a separate wave with random phase and random polarization.

18 A Polarizing material will only allow the passage of that component of the electric field parallel to the polarization direction of the material Polarization

19 Polarization A Polarizing material will only allow the passage of that component of the electric field parallel to the polarization direction of the material Unpolarized light is the superposition of many waves, each with random polarization direction θ, relative to fixed axis of polarizer Average attenuation is <cos 2 θ> = 1/2

20 The WSR-88D

21 WSR-88D Characteristics Data Types: Reflectivity Radial Velocity Spectrum Width Data Resolution: Beam Width = 0.93 Bin resolution = 250m (1km for Reflectivity) Maximum reflectivity range = 230km Without unfolding Maximum unambiguous velocity range = 150km Less for higher unambiguous velocities

22 WSR-88D Operation Modes Scanning modes currently available: VCP-11 = 14 elevation / 5 minute volume VCP-21 = 9 elevation / 6 minute volume VCP-31&32 = 4 elevation / 10 minute volume VCP 12 = 17 elevation / 4 minute volume VCP 121 =20 rotations (9 elev) / 5 minutes VCP-31 & 32 are clear-air scanning strategies designed for non-convective events VCP-21 is a precipitation mode designed for observing stratoform precipitation events VCP-11 is a severe weather mode designed to observe intense convective events

23 VCP elevations slices (like VCP 11) Performs scans in a very fast 4 minutes 6 seconds. Elevation tilt increases in VCP 12 range from 0.4 to 0.9 up to 4 (instead of 1 elevation tilt increments seen in all other VCP's) The radar beams overlap the each over. Provides a denser vertical sampling at lower elevation angles Quicker update cycle Better vertical definition of storms Improved detection capability of radars impacted by terrain blockage Better rainfall and snowfall estimates

24 VCP related Definitions Split Cut Scans an elevation slice two or more times, using a different PRF for each full scan Accurately places targets in range using a low PRF and to collect accurate velocity data using a high PRF For the lowest elevations scans (all elevations below 1.65 ) where efficient clutter suppression is required and velocity range folding is likely, all VCPs employ the split cut technique using a Contiguous Surveillance (CS) scan followed by one or more Contiguous Doppler (CD) scans Contiguous Surveillance (CS) is a constant low PRF (long Rmax) employed for the entire 360 scan to determine proper target location and intensity (dbz) No range unfolding technique is applied since all target locations are considered to be unambiguous or correct Contiguous Doppler (CD) is a constant high PRF (short Rmax and high Vmax) employed for the entire 360 scan to accurately determine "1st guess" velocity and spectrum width information A result of the high PRF (short Rmax) is that multiple trip echoes can occur and, therefore, a range unfolding technique using data from the CS scan must be applied

25 VCP related Definitions Batch Mode (B): Used in the middle angles of most VCPs Alternates low and high PRFs on each radial for one full rotation at each elevation angle The radar starts transmitting pulses using a low PRF (long Rmax) to obtain target intensity and location information, then, before the antenna completes a 1 sweep, the transmitter quickly switches to high PRF (high Vmax) to obtain more accurate velocity information Done for each radial until a full 360 scan has been completed The two data sets resulting from the different PRFs are combined to resolve range ambiguity. Used at elevation slices between 1.8 and 6.5 Contiguous Doppler X (CDX) (or contiguous Doppler with no range unfolding) Combines a high PRF and a rapid antenna rotation rate to obtain all base data in the higher elevation slices (>7 ) No range-unfolding algorithm is applied to the data At these higher elevation angles, range folded echoes are highly unlikely Example: at 7.5, the radar beam is already at ~ 50,000 feet at 62 nm range (the shortest CD Rmax)) CDX is employed at all elevation slices >7 in VCPs 11, 12, 21 and 121, and above 3 in VCP 31 (CDX is not employed in VCP 32)

26 VCP related Definitions Multiple PRF Dealiasing Algorithm Group: Used in VCP 121 Provides rapid volume sampling updates to monitor changing radar echo patterns while at the same time significantly reducing the amount of range folded (50-70% reduction) and incorrectly dealiased velocity data To accomplish the dealiasing goal, VCP 121 uses multiple CD rotations at the lower elevation scans For example, for the 0.5 and 1.5 elevations, the Split Cut technique includes one CS scan and three CD scans using 3 different PRFs Similarly, two PRFs are used at 2.4 and 3.3 to range unfold and velocity dealias

27 VCP 121 Addresses velocity aliasing or the ability of the radar to determine wind velocity and problems caused by "second trip echoes". With the same nine elevation tilts as VCP 21, VCP 121 completes 20 rotations in five minutes. Makes several elevations scans at the same elevation tilt but at varied PRFs Minimizes "range folding" Low PRF: (longer time between transmission of pulses) Signal travels farther to more distant objects and reduces second trip echoes Ability to determine velocity is greatly reduced High PRF: (less listening time between pulses) Greatly improve the radar's ability to determine velocity Increases the number of second or third trip echoes. Doppler dilemma

28 Range (km) VCP-21

29

30

31 Example images

32 WSR-88D Data Level-II data Full resolution base data Reflectivity, Velocity, and Spectrum Width No derived products Compressed formats (1-2 MB per VS) *raw *ldm *Z *bz2 CRAFT NSSL NCDC CAPS Uncompressed format (8-12 MB per VS) *ridds all Level-III data Post processed data (lower resolution) VIL, ET, Composite Reflectivity etc

33 Important Algorithms Storm Cell Identification and Tracking Algorithm (SCIT) Identifies Storm-cells based on 3-D reflectivity data Mesocyclone Detection Algorithm (MDA) Identifies mesocyclone vortices from radial velocity data using a Rankine vortex model Tornado Detection Algorithm (TDA) Identifies TVSs base on gate-to-gate shear from radial velocity data Hail Detection Algorithm (HDA) Provides probability of severe hail as well as maximum expected hail size

34 Example MDA Output

35 Components of the WSR 88D Three major functional components: 1. RDA- the RDA (Radar Data Acquisition) component of the system acquires and processes the raw data. Consists of the antenna, pedestal, radome, tower, transmitter, receiver, minicomputer, and signal processor. 2. The RPG (Radar Product Generator) is where the algorithms compute all of the derived fields. All derived fields, or products, are generated from the base fields (reflectivity, radial velocity, and spectrum width). The RPG generates the level III data that is widely available through the NEXRAD Information Dissemination Service (NIDS). 3. The PUP (Principle User Processor) is where the data from the RPG is sent. The PUP is basically a workstation that allows the user to display and manipulate all the products.

36 88D Components

37 Types of Radars Monostatic Single antenna (Modern WSR-88D) Bistatic Two antennas Antennas must be aimed at the same region but can be quite some distance apart Used in military for detecting aircraft or missiles at ranges of several thousand km

38 Types of Radars Continuous Wave (CW) Received signal is also continuous Only way to detect anything is if signal is different Occurs when target is moving Shift in the frequency proportional to the speed of the target relative to the radar Police radar Pulsed radar Transmits a short pulse and waits for a return echo Modern radars

39 Doppler radar Types of Radar Measures velocity of a target sort of Based on Doppler Shift The frequency increases as the object emitting sound moves closer and decreases as it moves away Doppler radar compares the received frequencies with the transmitted frequencies and deduces the velocity

40 Terminal Doppler Weather Radar (TDWR) Employed at airports to detect wind shear Microbursts Gust Fronts Smaller Wavelength Types of Radar

41 Beam Power Structure Side Lobe Energy ½ Power Point Max Power at center of beam ½ Power Point Side Lobes cause most of the clutter in close proximity to the radar The Radar Beam is defined by the half power points

42 .96 Degree Beam Resolution Radar resolution with respect to beam width / range D = Beam Width D If R = 60 NM 120 NM 180 NM 240 NM D = 1 NM 2 NM 3 NM 4 NM

43 Azimuth Resolution Considerations Rotational couplet identification can be affected by azimuth resolution. As the diagram shows, the closer a rotation is to the radar the more likely it will be identified correctly. If the rotation is smaller than the 1 0 beam width (possible at long ranges) then the rotation will be diluted or averaged by all the velocities in that sample volume. This may cause the couplet to go unidentified until it gets closer to the radar. Enlarged image along a radial. Individual blocks represent one sample volume. This graphically shows the radar resolution. Weak inbound, weak outbound Azimuth 3 ' Strong inbound, strong outbound ' Rotation too small to be resolved ' Azimuth 2 Azimuth 1 ' Stronger inbound than outbound Range 0 (example) 120 nm

44 Pulse Repetition Frequency- PRF PRF controls the Max Radar Range and Max Unambiguous Velocities PRF is the number of pulses per second transmitted by a radar

45 Range and Pulses r=range to target c=speed of light T = interpulse period Typically about 1 millisecond Ru=cT/2 (maximum unambiguous range) τ = pulse duration Typically about 1 microsecond so 1/T = PRF => Pulse Repetition Frequency

46 Range Folding Ru = unambiguous range = ct/2 but T = 1/PRF so Ru = c/(2*prf) A typical value for the unambiguous range is about 150 km. For the 88-D, it can be as large as 460 km. If target is at r < Ru, it will be displayed at its correct location, if it is r > Ru, it will not If Ru=150km and the storm is at 130 km, it will be displayed correctly. If it is at 170 km, it will be positioned at r=20 km Range Folding Range folded echoes are called second trip echoes

47 Second trip echoes Second Trip Echo Detection Conserve azimuth angle Are weaker Sometimes are associated with strange Doppler velocities How do we eliminate second trip echoes? Change listening time (PRF) Use a different PRF every 2-3 pulses, if the echo moves, it is bogus!

48 Second Trip Echoes

49 Some Nomenclature Name Symbol Units Typical values transmitted frequency f t MHz, Ghz Mhz wavelength pulse duration l τ cm microsec 3-10cm 1 microsec Pulse length h m m (h=ct) pulse repetition frequency PRF sec sec -1 Interpulse period T millisec 1 millisec peak transmitted power P t MW 1 MW average power P avg kw 1 kw (Pavg = P t t PRF) received power P r mw 10-6 mw

50 Display Systems Two types PPI Plan Position Indicator What 88D commonly uses Range, Azimuth display at constant elevation angle Good surveillance scan Good in operational setting

51 RHI Range Height Indicator Constant azimuth Range, Elevation Display Good for examining vertical structure of storms 88D does not perform RHI scans Constructs RHI from PPI Scans Display Systems

52 db Scale for Reflectivity db, or log scale is used for the radar reflectivity field since the returned power to the radar can vary over 8 orders of magnitude!!!! db= 10 log(x) P(dB)=10 log (P1/P2) The receiver takes the log of the signal (in base 10) For meteorological radars, typical db units used are: x=p r =10-13 W WRT 1 milliwatt (0dBm=1mW) Z (echo intensity) MDS=Minimum detectable signal (x=10-13 W)~-110 dbm where Z = radar reflectivity (more on this later) the dbz unit is used extensively and varies over a range from -20 to 70 dbz In the old days, dbz was turned into VIP (Video Integrator Processor) levels

53 Video Integrator Processor Levels VIP 1 (18-30 dbz) - Light precipitation VIP 2 (30-38 dbz) - Light to moderate VIP 3 (38-44 dbz) - Moderate to heavy VIP 4 (44-50 dbz) - Heavy rain VIP 5 (50-57 dbz) - Very heavy rain; hail possible VIP 6 (>57 dbz ) - Very heavy rain and hail; large hail possible.

54 Radar Hardware A radar system consists of several subsystems

55 Radar Hardware Transmitter Source of the EM radiation emitted by the radar Generates high frequency signal which leaves the radar s antenna and goes out into the atmosphere Magnetron tube for generating microwaves Invented in 1939 Small size and high power generating capability Generated signals > 500 kw The electrons spiral away from the hot cathode due to the orientation of the magnetic field As the electrons pass the openings to the cavities, they whistle, generating the microwave frequency of the magnetron The signal is then radiated into the waveguide

56 Radar Hardware Transmitter Klystron transmitters Bigger and bulkier True amplifiers Easier to control waveform of the transmitted pulses More powerful than Magnetrons (2+MW) Output signals are purer frequencies Makes measuring target motion easier Used in WSR-88D

57 Radar Hardware Klystron Transmitter A klystron looks and works something like an organ pipe. In an organ pipe: Blowing into the organ pipe produces a flow of air. Flowing air excites vibrations in the cavity of the whistle. The vibrations flow into the surrounding air as sound wave In a klystron: An electron gun produces a flow of electrons (1). The bunching cavities (2) regulate the speed of the electrons so that they arrive in bunches at the output cavity. The bunches of electrons excite microwaves in the output cavity (3) of the klystron. The microwaves flow into the waveguide (4). The electrons are absorbed in the beam stop (5).

58 Radar Hardware Modulator Switches transmitter on and off Provides correct waveform for the transmitted pulse When to transmit and what duration Also stores up energy between pulses

59 Radar Hardware Master Clock (Computer) Interface between the operator and the radar Converts human choices into radar actions Also controls processing of received data Main functions Defines the PRF (Cycles per second; Hz) Range from 200 Hz (older) to 3000 (newer) Hz Duration of transmitted signal Pulse duration (τ =Tao) if measured in time.1 to 10 µs Pulse length (h) if measured in length Distance = speed x time (h=cτ)

60 Radar Hardware Waveguide Conductor connecting the transmitter and antenna Electricity wire Radio and higher frequencies Coaxial cable Microwave radiation Waveguide Rectangular metal tube whose dimensions are defined by the wavelength of the radiation being carried Bends signals around corners Magnetic Field Electric Field

61 Radar Hardware Antenna Sends the radar s signal into the atmosphere Directional Ability to focus energy (WX radar) Isotropic Emits energy in all directions equally Weather radars have both an antenna and a reflector Circular parabolic reflector that results in a conical radar beam approximately 1 degree in width Cassegrain Antenna Have better side-lobes for antenna size

62 Cassegrain Antenna Sub-reflector Main Reflector

63 Radar Hardware Antenna Designed for specific wavelength Determined by the transmitter Antenna must match the transmitter Side lobe implications Size of the reflector (diameter for WX radar) ~1ft 30ft (WSR-88D 28 ft)