Radar Displays

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Radar Displays

• PPI - Plan position Indicator
• Maps the received signals on polar coordinates in
  plan view. The antenna scans 360 at fixed
  elevation angle. At every azimuth the voltage
  output of the receiver as a function of range is
  used to intensity-modulate a tube with polar
  coordinates (Rogers and Yau, 1989). This produces
  a plan view of the distribution of precipitation.
• Without careful calibration, PPI records are
  only useful to show the location and time
  occurrence of precipitation.

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Radar Displays - cont.

• RHI - Range Height Indicator
• This display is generated when the antenna scans
  in elevation with fixed azimuth, thereby showing
  the details of the vertical structure of
  precipitation.
• CAPPI - Constant Altitude PPI
• Azimuth and altitude are varied systematically to
  survey region surrounding the radar site.

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PPI
RHI
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HTI
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Precipitation Processes

• Areal extent and lifetime of precipitation are
  largely controlled by vertical motions
• Customary to classify precipitation according to
  the dominant mechanism for the vertical motion
• Two major classifications
• 1. Widespread, stratiform continuous precip.
  associated with large scale ascent
• 2. Localized, convective showery precip.
  associated with cumulus-scale convection
• The distinction between the two is not always
  sharp. However, it is usually possible to
  describe a pattern as either markedly uniform
  (stratiform) or non-uniform (convective).

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Example of a radar reflectivity pattern from a
stratiform precip. event . (Fig. 11.1, Rogers,
1979)
Stratiform Precipitation

• evolves relatively slowly with time
• usually associated with nimbostratus clouds
• radar reflectivity (Z) pattern is relatively
  uniform
• vertical air motion is weak
• gravitational settling determines the Z pattern

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Time-Height Pattern of Z and Doppler Velocity

• The contours I 65 and 95 correspond to 30 and 0
  dBz, respectively. (Fig. 11.2 Rogers, 1979 see
  Fig 12.5)

• Doppler Velocity is constant with time and
  decreases upward
• Z decreases upward and is constant with time.
• Results from small raindrops near top of echo
  slowly falling through the cloud growing by
  coalescence.

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Example of Radar Bright Band
Doppler Velocity m/sec
Reflectivity
Height above ground (km)
Doppler Velocity
Reflectivity Factor DMZ
PPI View at 8 elevation
From Rogers, 1979 see Rogers and Yau, Fig. 12.3

• Bright Band is located in a transition layer
  from snow to rain

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Bright Band Explanation

• Bright Band is located in a transition layer
  from snow to rain.
• Z increases because
• Dielectric constant of water exceeds that of ice
  by a factor of 4.Melting produces distorted wet
  snow flakes with higher reflectivities than those
  of spherical drops.
• Z decreases because
• Raindrops fall faster than snowflakes therefore
  their concentration in space is reduced, lowering
  Z.
• Note that the bright band is weak, diffuse or
  absent if there is convective overturning because
  mixing disrupts the stratification.

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Showers
Fig. 11.6 Rogers (1979)

• Echoes have lifetimes the order of an hour
• Patterns evolve rapidly
• Convection dominates the growth process
• the same horizontal and vertical dimensions

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Reflectivity Time-Height PatternWarm-Rain Shower
Intensity I DMZ 95 - I
Height (km)
Local standard time (hrs)

• Fig 11.7a, Rogers (1979) Fig. 12.7 Rogers and
  Yau (1989)

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Stratiform Precipitation - Summary

• Develop principally by the ice crystal process
• Last a long time - several hours
• Each level plays a distinct role
• Upper levels (T -20C) supplies ice crystals
• Mid-levels (T -15C) - rapid diffusional growth
• Low-levels - aggregation/accretion
• Useful approximation
• Stratiform rain is a steady state process in
  which cloud quantities may vary with height but
  are approximately constant with time at any given
  height.

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Convective Clouds - Summary

• High liquid water contents (wl)
• Coalescence process dominates
• Lifetimes are the order of 20 to 30 min
• Precipitation begins early in the developing
  cloud at low levels
• Initiation of precipitation may be by either the
  coalescence or ice-crystal process, but most
  growth is by accretion
• Useful approximation
• Showers are systems in which the cloud
  properties may vary with time, but they are
  (approximately) constant with height at any given
  time.

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Horizontal Scales and Structure of Precipitation

• Every precip area contains one or more of each
  smaller size
• RLM 2 RS RSM 2 RLM RC 2-10 RSM
• Main contribution to total rainfall comes from
  the small and large mesoscale areas