Weather Radar Notes

Weather Radar

History of Radar

  • 1935: Robert Watson-Watt, a meteorologist, developed the first practical radar system for tracking aircraft.
  • During wartime, weather echoes were initially considered a nuisance.
  • 1950s and 60s: First studies of storm dynamics and precipitation forecasting were conducted.
  • RADAR: Stands for RAdio Detection And Ranging.
  • Dee Weather Radar project (1966-1975): Real-time rainfall measurements were taken over the Dee River valley catchment in Wales.
  • 1971: A network of radars was proposed by P.J. Bulman and K.A. Browning.
  • North West Radar Project (1976-1984): The first unmanned, automatic radar system was established at Hameldon Hill, near Burnley.
  • Professors Chris Collier & Keith Browning researched supercells and sting jets.
  • 1985: 4 radars were in operation in the UK.
  • Mid 1990s: The number of radars increased to 12.
  • Today: There are 18 radars, with 15 operated by the Met Office.
  • UK Radar network resolutions: 1km, 2km, 5km.
  • 2018: Upgrade to Doppler and Dual-polarisation was completed.
  • NCAS radar was deployed near Inverness from January to July 2016 and near St Bees in Cumbria from October 2018 to December 2020.
  • An example rainfall composite is shown for "Storm Eowyn" on January 24th, 2025, displaying rainfall rates ranging from "No data" to 32.0+ mm/hr.

General Principles

  • Electromagnetic Spectrum:
    • High frequency to low frequency ranges from 10^{19} Hz to 10^5 Hz.
    • Wavelengths range from 0.0001 nm to 100 m.
  • Radar Bands:
    • S-band (NWS, USA): Frequency of 2-4 GHz, wavelength of 8-15 cm, antenna size of 8.5 m.
    • C-band (Met Office): Frequency of 4-8 GHz, wavelength of 4-8 cm, antenna size of 4.3 m.
    • X-band (NCAS): Frequency of 8-12 GHz, wavelength of 2.5-4 cm, antenna size of 2.4 m.
  • Polar Coordinates:
    • Speed (c) = Distance (2R) / Time (T)
    • c = 300,000,000 meters per second
  • Radar Scanning Pattern:
    • Scan angle is 0.5°.
  • Types of Radar Scans:
    • Plan Position Indicator (PPI)
    • Range Height Indicator (RHI) – primarily for research

Reflectivity, Rainrate, Velocity

  • The Radar Equation: Describes the relationship between transmitted power (Pt) and received power (Pr), the distance between the radar and targets (R), properties of the radar (C), and the target reflectivity (Z).

  • Radar Reflectivity (Z): Proportional to the number (N) and size (D^6) of the drops in the volume.

    • Z = D1^6 + D2^6 + D3^6 + … + Dn^6
    • Example: If D = 3mm, D^6 = 3 \times 3 \times 3 \times 3 \times 3 \times 3 = 729 mm^6, Volume (V) = 27mm^3
    • Example: If D = 1mm, D^6 = 1 \times 1 \times 1 \times 1 \times 1 \times 1 = 1 mm^6, Volume (V) = 1mm^3

  • dBZ (decibels of Z): dBZ = 10 \log_{10}(Z)

    • Z = 0.1 à dBZ = -10
    • Z = 1 à dBZ = 0
    • Z = 10 à dBZ = 10 (drizzle)
    • Z = 100 à dBZ = 20 (moderate rain)
    • Z = 10,000 à dBZ = 40 (heavy rain)

  • Reflectivity to Rainfall:

    • Z = N D^6
    • R = N D^3
    • Z = aR^b

  • Quantitative Precipitation Estimation (QPE) or Forecasting (QPF).

  • Z = reflectivity (dBZ), N = number of drops, D = size of drops, R = rainfall rate (mm/hr).

  • Doppler Velocity:

    • Christian Doppler (1803-53) was an Austrian mathematician and physicist.
    • Doppler radar can detect whether a target is moving towards or away from the radar location.
    • Doppler effect/shift: change in frequency due to a moving object.

  • Example of Tornado Detection using Radar Data

Dual Polarisation

  • Provides information on the size and shape of cloud and precipitation particles.

  • Examples include rain, snow, and hail.

  • Differential Reflectivity (ZDR): ZDR = 10 \log{10}(\frac{ZH}{Z_V})

    • If ZH > ZV, then ZDR > 0 dB.
    • If ZH = ZV, then ZDR = 0 dB.

  • Correlation Coefficient (RhoHV):

    • A measure of how similar the H and V pulses are.
    • Indicates the consistency of the shapes and sizes of targets within the radar beam.

  • Types of targets:
    * Low RhoHV
    * High RhoHV
    * Medium RhoHV

Radar Errors and Uncertainties

  • By identifying different targets using dual-polarisation variables, we can filter out noise/artifacts from good data.
  • Examples of target classification:
    * Noise
    * Insects
    * Rainfall

Summary

  • Radars transmit pulses of radio waves and measure the reflected power.
  • Power is related to reflectivity, which is converted to rainfall.
  • Radars are used to observe:
    • The development of clouds and precipitation.
    • The distribution and quantity of rainfall.
  • Doppler (radial) velocity tells about the wind field and helps identify features like tornadoes.
  • Dual-polarisation helps identify types of particles and leads to:
    • A better understanding of cloud processes.
    • More accurate estimates of rainfall.