Weather Radar SOEE: Lecture 10 Dr Lindsay Bennett

Weather Radar

SOEE: Lecture 10

Dr Lindsay Bennett

Who am I?
University of Leeds
PhD Meteorology
    2003-2008

  Post-Doctoral
Research Assistant
   2008-2011

                                Instrument Scientist
                      National Centre for Atmospheric Science

LECTURE PLAN

•   History of Radar
•   General Principles
•   Reflectivity, Rainrate, Velocity
•   Nowcasting
•   Dual Polarisation

History of UK Weather Radar
• 1935 Robert Watson-Watt, meteorologist by training,
  developed the first practical radar system to track aircraft

                                    RADAR=RAdio Detection And Ranging

 • During the war, weather
   echoes were initially
   considered a nuisance
 • 1950s and 60s, first studies
   of storm dynamics and
   precipitation forecasting

History of UK Weather Radar
• Dee Weather Radar project, 1966-1975
    – real time rainfall measurements over the
      Dee River valley catchment
• Network of radars proposed by
  P.J. Bulman and K.A. Browning in 1971

• North West Radar Project 1976-1984
    – first unmanned, automatic radar
      system at Hameldon Hill, near Burnley

UK Radar network
• 4 radars in 1985, 12 by
  the mid 1990s
• Today: 18 radars,
  15 operated by the
  Met Office

                            5km
                                  2km

                                  1km

Rainfall Composites

  Snow event in UK on 4th February

How does Radar work?

Sends out pulses of electromagnetic radiation in a narrow beam
Radiation scatters off targets, some returns to the radar
Radar listens for the return pulse
Targets can be weather or any physical object

Frequency of Weather Radar
       High frequency                                   Low frequency

1019 Hz                                                         105 Hz

   •      Weather radar frequencies are usually S,C and X band
   •      Range between f=2-12 GHz (109Hz) (λ=2-15cm), f ~ 1/λ
   •      Microwave energy peak power = 50-500 kW
   •      Microwave oven 2.45 GHz/ λ=12cm, 700 W

Weather Radar Scanning
• PPI – Plan Position Indicator
• RHI – Range Height Indicator

                   Z

                          X
          Y

              X         Met Office radars repeat a sequence of
                       several PPIs at different elevation angles
                                    every 5 minutes

Weather Radar Scanning
• For a radar to locate a target
  of interest (e.g. rain) 3 pieces
  of information are needed:
  range, azimuth and elevation
                                              Y
                        R
                                                  X

                  Speed=distance/time

                                              Z

   Power received  Reflectivity  Rainrate       X
   Target Velocity  Air motions

The Radar Equation
• The weather radar equation describes the
  relationship between:
   –    the transmitted (Pt) and received power (Pr)
   –   the properties of the radar (C)
   –   the properties of the targets (Z)
   –   the distance between the radar and the targets (R)

                               Pt CZ
                           Pr  2
                                R

Radar Reflectivity
• Z, Radar Reflectivity, is a function of target size
  (D6), number of drops (N) and has units of
  mm6/m3
            6   6
                                 D
  D=5mm, D =5 = 15,625
  D=0.5mm, D6=0.56 = 0.01          D
• dBZ (decibels of Z) = 10 log10(Z)
• 30 dBZ = 10 X 20 dBZ!!

Reflectivity – Hurricane Katrina

Estimating Precipitation with Radar
• Z = f (D6) Reflectivity Z (mm6/m3) is a function of the size of drops (D6)
   and the number of drops (N)
• R = f (D3) Rainfall rate R (mm/hr) is a function of D3, N and the fall
  speed of the drops (v)
• No direct relationship between Z and R
• If we assume a distribution of particles (i.e. number of drops in different
  size categories) we can relate Z to R
• Z=ARB R=(Z/A)1/B
    – Marshall and Palmer (1948) Z=200R1.6 A=200, B=1.6
    – hundreds of relationships: depends on rain type (convective, stratiform, mixed), season,
      location (tropics, mid-latitudes) and cloud type

• 30dBZ ~ 2.7mm hr-1
• 60dBZ ~ 205mm hr-1
• Quantitative Precipitation Estimation (QPE) or Forecasting (QPF)

Doppler Velocity
• Doppler effect or Doppler shift –
  change in frequency due to a
  moving object
• Austrian mathematician and
  physicist, Christian Doppler (1803-
  53)
• Doppler radar can detect whether
  a target is moving towards or
  away from the radar location
• Measure the change in the phase
  of the returned pulses
• Calculate Doppler velocity
• Met Office radars being upgraded
  to have Doppler capability

Tornado Signature
     Reflectivity (dBZ)                    Radial Velocity (m/s)

                                                               5km

-6                        34         Towards                       Away

                          Radar location

Clear Air Data
• Radar waves scatter off non-meteorological targets too
   – trees, buildings, power lines are known as “ground clutter”
   – insects act as “tracers” of the air motion

STORM                              STORM

                 INSECTS                             INSECTS

        Radial Velocity (m/s)               Reflectivity (dBZ)

Clear Air Data

                  Insects                     Insects

Radial Velocity (m/s)         Reflectivity (dBZ)

Nowcasting
• Very short-range forecasting
• Detailed analysis of current
  weather situation and forecasting
  up to 6 hours ahead
  – track radar echoes and extrapolate into
    future
  – combine high resolution numerical
    forecast models with observational
    data (satellite, sounding, surface)
  – UK Met Office Nimrod, Gandolf
    systems
  – USA – TITAN, Auto-Nowcaster

Radar Errors
• Ground clutter              • Strong winds
• Attenuation                 • Evaporation below beam
• Bright band                 • Mixed precipitation (ice and liquid)

                                   HAIL

                                          Ground Clutter

  WIND

            Rain Guage

Bright Band
Vertically-
 pointing
  radar

   Height
    (km)

                 Time (UTC)

Attenuation
• Reduction in signal due to scattering and absorption
• Worse for shorter wavelengths (X-band)

Dual-polarisation Radar

By comparing reflected power returns in different ways (ratios, correlations, etc.), it
 is possible to obtain information on the size, shape, and ice density of cloud and
                               precipitation particles.
     Can also correct for errors associated with attenuation and bright band

Differential Reflectivity
                 ZDR ~ log (ZH/ZV)

              Big drops ZH > ZV, ZDR > 0
                  Hail ZH=ZV, ZDR=0

Dual-polarisation Radar
•   http://www.wdtb.noaa.gov/courses/dualpol/animations/PhiDP_animation.html

• Horizontal and vertical waves are slowed down
  (attenuated) different amounts when propagating
  through precipitation
• Radar measures the difference in the phase
  (phase shift) of the H and V returned pulses
• Amount of phase change ~ amount of
  attenuation ~ intensity of rainfall
• Better estimates of rain rate

UK Radar Network

• Met Office started upgrade of
  the network
• Doppler by end of 2012
  (1-2 weeks downtime)
• Dual-polarisation by Autumn
  2014 (2-3 weeks downtime)