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Radar
 
Education Centre
 
General Meteorology : Air Masses -  Cold Front -  Fog -  High Pressure -  Humidity -  Introduction to Weather -  Large Thunderstorms -  lightning -  Low-Pressure -  Meteorology -  Occlusion Fronts -  Rain -  Sea Breaze & Land Breaze -  Temperature -  The Water Cycle -  Tornadoes -  Warm Front -  Wind
Clouds : Cirrus -  Clouds -  Cumulus -  Cumulonimbus - 
Radar : General -  Radar Technology -  Attenuation in the atmosphere -  Velocity measurements -  Sources of error -  Optimizing radar characteristics -  Radar installation -  Precipitation measurements
  1. General

    1. The Weather Radar
    2. Radar characteristics, terms, and units
    3. Meteorological applications
    4. Meteorological products
    5. Radar accuracy requirements

    This chapter is an elementary discussion of meteorological microwave radars - the weather radar - used mostly to observe hydrometeors in the atmosphere. It places particular emphasis on the technical and operational characteristics that must be considered when planning, developing and operating radars and radar networks in support of meteorological and hydrological services. It is supported by a substantial list of references. It also briefly mentions the high frequency radar systems used for observation of the ocean surface. Radars used for vertical profiles are discussed in Chapter 5.

    1. The weather radar

       Meteorological radars are capable of detecting precipitation and variations of the refractive index in the atmosphere that may be generated by local variations of temperature or humidity. Radar echoes may also be produced from airplanes, dust, birds or insects. This chapter deals with radars in common operational usage around the world. The meteorological radars having characteristics best suited for atmospheric observation and investigation transmit electromagnetic pulses in the 3-10 GHz frequency range (10-3 cm wavelength, respectively). They are designed for detecting and mapping areas of precipitation, measuring their intensity and motion, and perhaps their type. Higher frequencies are used to detect smaller hydrometeors, such as cloud or even fog droplets. Although this has valuable applications in cloud physics research, these frequencies are generally not used in operational forecasting because of excessive attenuation of the radar signal by the intervening medium. At lower frequencies, radars are capable of detecting variations of the refractive index of clear air, and they are used for wind profiling. They may detect precipitation, but their scanning capabilities are limited by the size of the antenna required to achieve effective resolution.

      The returned signal from the transmitted pulse encountering a weather target, called an echo, has an amplitude, a phase and a polarization. Most operational radars worldwide are still limited to analysis of the amplitude feature that is related to the size distribution and numbers of particles in the (pulse) volume illuminated by the radar hewn. The amplitude is used to determine a parameter called the reflectivity factor (Z) to estimate the mass of precipitation per unit volume or the intensity of precipitation through the use of empirical relations. A primary application is thus to detect, map, and estimate the precipitation at ground level instantaneously, nearly continuously and over large areas.

      Doppler radars have the capability of determining the phase difference between the transmitted and received pulse. The difference is a measure of the mean Doppler velocity of the particles - the reflectivity weighted average of the radial components of the displacement velocities of the hydrometeors in the pulse volume. The Doppler spectrum width is a measure of the spatial variability of the velocities and provides some indication of the wind shear and turbulence. Doppler radars offer a significant new dimension to weather radar observation and most new systems contain this capability.

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    2. Radar characteristics, terms, and units

       The selection of the radar characteristics, and consideration of the climate and of the application is important for determining the acceptable accuracy of measurements for precipitation estimation (Tables 9.1, 9.2 and 9.3).

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    3. Meteorological applications

      Radar observations have been found most useful for:

      (a) Severe weather detection, tracking and warning;
      (b) Surveillance of synoptic and mesoscale weather systems;
      (c) Estimation of precipitation amounts.

      The radar characteristics of any one radar will not be ideal for all applications. The selection criteria of a radar system are usually optimized to meet several applications but can also be specified to best meet a specific application of major importance. The choices of wavelength, beamwidth, pulse length, and pulse repetition frequencies (PRFS) have particular consequences. Users should therefore consider carefully the applications and climatology before determining the radar specifications.

      SEVERE WEATHER DETECTION AND WARNING

      A radar is the only realistic surface-based means of monitoring severe weather over a wide area. Radar echo intensities, area and patterns can be used to identify areas of severe weather. These storms include thunderstorms with probable hail and damaging winds. Doppler radars which can identify and provide a measure of

      Radar frequency bands

      * Most common weather radar bands
      Radarband Frequency Wave Length Nominal
      UHF 300-1 000 MHz 1-0.3 m 70 cm
      L 1 000-2 000 MHz 0.34.15 m 20 cm
      S* 2 000-4 000 MHz 15-7.5 cm 10 cm
      C* 4 000-8 000 MHz 7.5-3.75 cm 5 cm
      X* 8 000-12 500 MHz 3.75-2.4 cm 3 cm
      Ku 12.5-18 GHz 2.4-1.66 cm 1.5 cm
      K 18-26.5 GHz 1.66--i.13 cm 1.25 cm
      Ka 26.5-40 GHz 1. 13@.75 cm 0.86 cm
      w 94 GHz 0.30 cm 0.30 cm

      intense winds associated with gust fronts, downbursts and tornadoes add a new dimension. The nominal range of coverage is about 200 km, which is sufficient for local short-range forecasting and warning. Radar networks are used to extend the coverage (Browning, et al., 1982). Effective interpretation requires alert and welltrained personnel to provide effective warnings at present and until automated algorithms and storm models have been developed for the local areas.

      SURVEILLANCE OF SYNOPTIC AND MESOSCALE SYSTEMS

      Radars can provide a nearly continuous monitor of weather related to synoptic and mesoscale storms over a large area (say a range of 220 km, area 125 000 km2) if unimpeded by hills. Due to ground clutter at short ranges and the Earth's curvature, the maximum practical range

      Physical radar parameters and units

      Symbol Parameter Units
      C Speed of light m s-i
      f Transmitted frequency Hz
      Fd Doppler frequency shift Hz
      Pr Received power mW or dBm
      Pt Transmitted power kW
      PRF Pulse repetition frequency Hz
      T Pulse repetition time (=I/PRF) Ms
      Ω Antenna rotation rate deg s- I or rpm
      λ Transmitted wavelength cm
      ø Azimuth angle deg
      θ Beamwidth between half power points deg
      τ Pulse width μ
      γ Elevation angle deg

      for weather observation is about 200 km. Over large water areas, other means of observation are often not available or possible. Networks can extend the coverage and may be cost effective. Radars provide a good description of the precipitation. In regions where very heavy and extensive precipitation is common, the selection of a 10-cm wavelength may be warranted. In other areas, such as mid-latitudes, 5-cm radars may be quite effective at much less cost. The 3-cm wavelength suffers from too much attenuation in precipitation to be very effective except for very light rain or snow situations. Narrower beamwidths provide better resolution of patterns and greater effectiveness at longer range.

      PRECIPITATION ESTIMATION

      Radars have a long history of use in estimating the intensity and thereby the amount and distribution of precipitation with a good resolution in time and space. Most studies have been associated with rainfall but snow measurements can also be made with appropriate allowances for target composition. Ground-level precipitation estimates from typical radar systems are made for areas of typically 2 km2, successively for 5-10 minute periods using low elevation plan position indicator (PPI) scans with beamwidths of 1°. The radar estimates have been found to compare with spot precipitation gauge measurements within a factor of two. Gauge and radar measurements are both estimates of a continually varying parameter. The gauge samples an extremely small area (100 CM2) while the radar integrates over a volume, on a much larger scale. The comparability may be enhanced by adjusting the radar estimates with gauge measurements.

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    4. Meteorological products

       A radar can be made to provide a variety of meteorological products to support various applications. The products that can be generated by a weather radar depend on the type of radar, its signal processing characteristics, and the associated radar control and analysis system. Most modern radars automatically perform a volume scan consisting of a number of full azimuth rotations of the antenna at several elevation angles. All raw polar data are stored in a threedimensional array, commonly called the volume database, which serves as the data source for further data processing and archiving. By means of application software a wide variety of meteorological products is generated and displayed as images on a high resolution colour display monitor. Grid or pixel values and conversion to x-y coordinates are computed using threedimensional interpolation techniques. For a typical Doppler weather radar, the displayed variables are reflectivity, rainfall rate, radial velocity, and spectrum width. Each image pixel represents the colour-coded value of a selected variable.

      The following is a list of presentation of measurements and products generated most of which are discussed in this chapter:

      (a) The plan position indicator (PPI) is a polar format display of a variable, obtained from a single full antenna rotation at one selected elevation. It is the classic radar display, used primarily for weather surveillance;

      (b) The range height indicator (RHI) is a display of a variable obtained from a single elevation sweep, typically from 0 to 90', at one azimuth. It is also a classic radar display that shows detailed crosssection structures and it is used for identifying severe storrns, hail, and the bright band;

      (c) The constant altitude plan position indicator (CAPPI) is a horizontal cross-section display of a variable at a specified altitude, produced by interpolation from the volume data. It is used for surveillance and for identification of severe storms. It is also useful for monitoring the weather at specific flight levels for air traffic applications;

      (d) Vertical cross-section: This is a display of a variable above a user-defined surface vector (not necessarily through the radar). It is produced by interpolation from the volume data;

      (e) Column maximum: A display, in plan, of the maximum value of a variable above each point of the area being observed;

      (f) Echo tops: A display, in plan, of the height of the highest occurrence of a selectable reflectivity contour, obtained by searching in the volume data. It is an indicator of severe weather and hail;

      (g) Vertically-integrated liquid (VIL) can be displayed, in plan, for any specified layer of the atmosphere. It is an indicator of the intensity of severe storms.

      In addition to these standard or basic displays, other products can be generated to meet the particular requirements of users for purposes such as hydrology, nowcasting, or aviation:

      (a) Precipitation-accumulation: An estimate of the precipitation accumulated over time at each point in the area observed;

      (b) Precipitation subcatchment totals: Area-integrated accumulated precipitation;

      (c) Velocity azimith display (VAD), sometimes called velocity volume processing (VVP), which is an estimate of the vertical profile of wind above the radar. It is computed from a single antenna rotation at a fixed elevation angle;

      (d) Storm tracking: A product from complex software to determine the tracks of storm cells and to predict future locations of storm centroids;

      (e) Wind shear: An estimate of the radial and tangential wind shear at a height specified by the user.

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    5. Radar accuracy requirements

       The accuracy requirements depend on the most important applications of the radar observations. Modern radars appropriately installed, calibrated and maintained are relatively stable and do not produce significant measurement errors. External factors, such as ground clutter effects, anomalous propagation, attenuation and propagation effects, beam effects, target composition particularly with variations and changes in the vertical, and rain rate-reflectivity relationship inadequacies, contribute most to the inaccuracy.

      Considering only errors attributable to the radar system, the measurable radar parameters can be detern-iined with an acceptable accuracy.

      Accuracy requirements

      Parameter Definition Acceptable accuracy*
      ø Azimuth angle 0.1°
      γ Elevation angle 0.1°
      Vr Mean Doppler velocity 0.25 m s-I
      z Reflectivity factor IDBZ
      σ Doppler spectrum width I m s-I

      * These figures are relative to a standard Gaussian spectrum with a variance smaller than 4 m2S-2. Velocity accuracy deteriorates when the spectrum Ividth grows, while reflectivity accuracy improves.

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